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Title
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Eng
A Study of the Suitability for the Continued Use of DDT for Malaria Control
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Date
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2003
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Creator
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Eng
Sharer, William R
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Subject
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Eng
Environmental Studies
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extracted text
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A STUDY OF THE SUITABLITY FOR THE CONTINUED USE OF DDT FOR
MALARIA CONTROL
by
Bill Sharer
A Thesis: Essay of Distinction
Submitted in partial fulfillment
Of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
June 2003
by
Bill Sharer
has been approved for
The Evergreen State College
by
Member of the Faculty
~A/2M3
Date
ABSTRACT
Malaria has plagued humankind throughout history. Human malaria is caused by four
species of parasite from the genus Plasmodium. The parasite is transmitted between
humans by mosquitoes from the genus Anophelines. It currently infects between 300 and
500 million people each year and kills approximately 1.5 million. The disease causes
extensive suffering and economic losses, mostly in third-world countries. The parasite is
treated using Quinine, made from bark of the Cinchona tree, or any of several
antimalarial drugs. The parasite has developed resistance to several of these treatments
around the globe. Dichlorodiphenyltrichloroethane; or DDT, was developed and used as
an insecticide against Anophelines mosquitoes. It was also used as a broad-spectrum
insecticide in agriculture as well. Extensive use of DDT greatly reduced incidence of
malaria worldwide, buts its use has diminished due to environmental concerns. DDT has
been shown to cause thinning of eggshells in birds and has been linked to endocrine
disruption and impaired development in birds and other animals. DDT is persistent in the
environment and has bioaccumulated throughout the world's food web. While most
humans have DDT residues in their bodies, it has not been shown to cause any human
disease. While DDT use has been reduced, malaria is reemerging in many areas of the
world. An attempt to ban its use worldwide was modified to allow its continued use for
malaria control. It remains an effective antimalarial tool in many areas. Malaria
transmission is likely to increase as the mean global temperatllfe rises. Although many
would like DDT to be completely banned, it should continue to be available as a tool in
malaria control.
1. Introduction
1
Summary of Disease to the Present.
1
Malaria in the United States
2
Human Costs of Malaria
3
Life Cycle
4
Historical Malaria Control Efforts
5
Reemergence of Malaria
7
Operation Roll-Back Malaria (RBM)
8
U. Adverse Effects of DDT on Wildlife
9
Eggshell Thinning in Birds
9
Lake Apopka Studies
10
IIJ Ubiquitous Nature of DDT
10
Deep Oceans. . .. . .. ... . .. . ... .. . .. . .. . ... .. . .. ..
.
12
IV. Human Levels
13
South Africa Data
14
InJia Data
16
,
17
Mexico Data
Brazil Data
18
\1 Discussion
18
Human Health Effects
19
Effects of Malaria Control after Cessation of Spraying
21
Cost Comparisons with Alternative Insecticides
24
25
Resistance to Pesticides and Drugs
Vector Resistance
25
Parasite Resistance
27
Implications from Global Warming
28
VI. Summary and Recommendations
30
References
33
Appendix A: Current Malaria Transmission Areas
Appendix B: Current Areas of Parasite Resistance
Appendix C: Projected Increase in Transmission Areas Assuming a Two-Degree
Fahrenheit Temperature Increase
111
Figure 1. Parasite Life Cycle
5
Figure 2. Parasite Incidence and House Spray Data from Pan American Health
21
Organization Data
Figure 3. Parasite Index for School Children from Four Villages in Madagascar
Highlands
24
Figure 4. Relationship Between Fever Deaths and Precipitation in the Punjab
Region, 1868 to 1908
29
Appendix A. Current Malaria Transmission Areas
Appendix B. Current Areas of Parasite Resistance
Appendix C. Projected Increase in Transmission Areas With a Two-Degree Fahrenheit
Temperature Increase
LIST OF TABLES
Table
Table
Table
Table
Table
Table
Table
1. Loss of Economic Growth in African Countries Due to Malaria
3
12
2. Organochlorine Levels in North Atlantic Cod
3. Organochlorine Levels in Atlantic Fish
13
4. Organochlorine Levels in Monterey Bay Fish
13
5. Blood Serum Levels
14
6. Liver Function Parameters
15
7. Changes in Alkaline Phosphates by Age for Exposed and Unexposed DDT
Workers
15
Table 8. Blood Serum Levels from Two Areas in Mexico
17
Table Y. Summary of DDT levels for Various Endemic Areas
19
Table 10. Parasite Survey Results for Schoolchildren in Madagascar Highlands before
and after OPID Program
23
Table 11. Abundance of Anophelines funestus in Villages of the Madagascar Highlands
before and after OPID Program
23
Table 12. 1990 Comparison of Insecticides, from World health Organization Data 25
Table 13. Mortality of Anopheles fluviatillis to HCH and DDT
26
IV
This work was only possible through the help and sacrifices of others. Thanks
and appreciation are given to Carolyn Dobbs, whose helpful comments were instrumental
in arriving at a finished document.
I would like to thank John Perkins, whose pesticides class inspired my interest in
and gave me the idea for this topic.
I wish to thank the members of my immediate study group: Scott, Donelle,
Chrissie, Anne, Anita, Cindy, and Diane. Our weekly Saturday meetings over the past
several months made these works possible.
I lastly want to thank my wife, Tina, and my stepdaughters, Mary and Jordan, for
their encouragement and tolerance for my never ending need for computer time and also
for the piles of documents and books in the living room. I also thank Travis and Callie
for being really cool dogs and keeping me company on those late computer nights.
v
In the 1950's, malaria was considered to be endemic in over 140 counhies.
Although this number has dropped to a little over 100 countries, more than 2.4 billion
people are still at risk (World Health Organization, 2000). The plimary reduction has
occurred as a result of in-house spraying using dichlorodipl::Ienyltrichloroethane, or
DDT, to repel the parasite's primary vector, various species of mosquitoes belonging
to the genus Anophelines (Tren and Bate, 2001). DDT was originally synthesized in
1874 by Otto Zeidler, but its insecticidal properties were not discovered until 1939
when Paul Mueller tested it as part of a screening study (Committee on the Future
Role of Pesticides in US Agriculture, National Research Council, NRC, 2000, p24).
It came into widespread use as a pesticide in the 1940's. Due to environmental
concerns, its use in the U.S. was abandoned in the 1970's, but it is still used in other
countnes, primarily for control of malaria. Because DDT is faIrly inexpensive and
the amount of exposure to humans is thought to be low and the reduced malaria
benefits are high, international debate about DDT's use continues (Longenecker et aI,
2001).
While many groups such as WWF, Greenpeace, and Physicians for Social
Responsibility have called for the outright ban of DDT worldwide, many health
professionals feel that continued use of DDT for malaria control is justified (Tren and
Bate, 2001). The purpose of this study is to determine if DDT should still be used for
malaria control, given its historical and present problems.
Summary of Disease to the Present
Malaria is a parasitic disease that has been with us throughout history. The
introduction of agriculture around 7000 B.C. led to larger populations of settled
people with more favorable conditions for malaria transmission. Human malaria IS
caused by four species of parasite of the genus Plasmodium. P. vivax is responsible
for the majority of infections, while P. jalciparum is the most deadly. P. malariae
causes the highest fevers (106 to 107 degrees Fahrenheit), but is rarely fatal. P. ovale
produces a tertian infection that occurs at night at regular intervals. It is possible to
be simultaneously infected by more that one species (Wargo, 1998,p 19). The
1
Anophelines (Reiter, 2001). Clinical malaria generally involves bouts of fever
followed by periods of remission. Diagnosis is generally done by visual inspection of
erythrocytes, or red blood cells (Institute of Medicine, 10M, 1991 p24). Depending
on the malarial species involved, the fevers tend to occur on either every third or
fourth day, which gave rise to the term tertian and quartan fevers (Reiter, 2001).
Although the majority of infections are caused by P. vivax, P. falciparum infections
are on the rise (10M, 1991). After relative control, malaria is reemerging as a health
issue in parts of the world. Between 300 and 500 million people are infected with the
parasite each year and over 1.5 million people die from the disease (Kondrachine and
Trigg, 1995), many of them being children and pregnant women. Causes of death
include cerebral edema, renal failure, pulmonary failure and anemia (Butler, 2000).
Appendix A contains a map showing the current infectious areas for the world. The
malaria problem is exacerbated by the parasite's ability to become resistant to
traditional treatment as well as the vector's ability to become resistant to vanous
pesticides (Mutabingwa et aI, 2001).
In areas of intense, but stable, malaria transmission, most of the adult
population has some protection from the disease through acquired immunity, but
pregnant women and young children are particularly at risk. In areas where malaria
transmission is less stable, all age groups may be vulnerable to the disease and
epidemics may occur. In Africa, 75% of the population lives in area of stable malmia
transmission while 18% of the population lives in areas of unstable malaria
transmission (WHO, 1995).
Malaria in the United States
Two species of malaria, P. vivax and P. Malariae, are believed to have been
introduced by the English settlers at Jamestown, Virginia. P. falciparum is believed
to have been introduced through the importation of African slaves sometime after that
in the early 1600's. The parasite moved westward throughout the North American
continent as settlement occurred. As many as 500,000 cases of malaria were reported
annually in the United States as recently as the early 1900's. (lOM, 1991, P 38).
2
Although this was well before any vector-control programs came about, factors such
as population shift from nlral to urban areas, improved drainage projects, and better
health and nutrition helped bring the disease under control. All 48 contiguous states
contain species of Anopheline mosquitoes that are capable of malaria transmission
(Zucker, 1996). The disease has been eradicated in the United States, and current
cases of malaria in this country are primarily imported from foreign visitors or
returning U.S. travelers and returning military personnel. The Centers for Disease
Control (CDC) estimates that it receives between 1000 and 1500 cases per year
(CDC, 2002).
Human Costs of Malaria
Beyond the human suffering and mortality attributed to malaria, the disease
inflicts heavy financial burdens on some of the world's poorest counties (Tren and
Bate,2001). School absenteeism attributed to malana can be as high as 28% m some
areas. The annual estimated direct and indirect cost of malaria is more than two
billion dollars for Africa alone (World Health Organization, 2000). Table 1 lists the
percentage of annual income lost between 1980 and 1995 for several African
countries.
Table 1. Loss of Economic Growth in African Countries Due to Malaria, 1980 to 1995 (Tren and
Bate, 2001).
Country
Percent
Country
Percent
Percent
Country
loss
Loss
Loss
Benin
17%
18%
Gabon
17%
Ni\?:er
Botswana
5%
Gambia
18%
18%
Nigeria
Burkina Faso
18%
Ghana
18%
18%
Rwanda
Burundi
18%
Guinea
14%
18%
Senegal
Bissau
Cameroon
18%
18%
17%
Kenya
Sierra
Leone
Central African
18%
18%
Madagascar
South Africa 1%
Rep.
Chad
17%
Malawi
18%
Togo
18%
18%
Con\?:o
17%
Mali
Zambia
18%
-
Congo, Dem. Rep.
17%
Mauritania
15%
18%
Zimbabwe
18%
Namibia
10%
Cote d'Ivorie
10%
Total
3
malaria. One study found that farm families with malaria cleared sixty percent less
land than families that were malaria free. Despite all that has been done, malaria's
effect on overall productivity has not been satisfactorily determined (Institute of
Medicme, 10M, p 238).
Life Cycle
The mosquito life cycle contains four stages: egg, larva, pupa, and adult. The
adults emerge from the egg sometime between seven and twenty days. Anophelines
females can survive up to one month in moderate temperatures and humidity, which
allows sufficient time for the malaria parasite to develop. During a human blood
meal, the mosquito either transfers the parasite to the human host, if it already carries
the parasite, or picks the parasite up from an already infected person and can then
transfer it to another person (Speilman and D' Antonio, 2001, P 168).
The parasite's life cycle is complex. It has a liver phase, a blood phase, and a
mosquito phase. It can reproduce either sexually or asexually, depending on what
phase of its life cycle it is in. It has three individual stages in the mosquito and two
stages in the human host. It has been divided by scientists into about a dozen steps.
Figure 1 shows the parasite's life cycle in its mosquito and human host (lOM,1991).
After the parasite has been introduced by the mosquito, sporozites invade liver
cells where they develop into schizonts, each of which contain between ten and thirty
thousand merozoites, which are released to the bloodstream and invade red blood
cells. Once inside these cells, each merozoite matures into schizonts each containing
8 to 32 merozoites, causing the cell to rupture, which then allows them to invade new
red blood cens (10M, 1991, P 25-28). It takes the parasite anywhere from 6 to 16
days to invade the red blood cells.
Once in the red blood cells, the parasite can easily be detected by visual
examination of blood smears using a microscope. It is at this point when infected
mdividuals develop the symptoms and pathologic characteristics ofthe disease. P.
vivax and P. ovale can remain dormant in the liver tissue as hypnozites, and can cause
relapse of the disease many months or even years later. In general, parasites
4
immune response or antimalarial drugs (Poser and Bruyn, 1999).
/
Development of Parasite in Mosquito
Human Skin
Development
Of Gametes
~.
r)
.- ·
.
•'.:-j')
.
g
~4.
~
...
~
-
.
@
4"@~
.
•
..
© -:-. . .
"'.
.~
.i:- :i"'• ..... ~
I'>·;'
•.
C • \. .
.
Infection of
Liver Cells
~~
Mature
Liver Schizont
Merozoites
Infection of
Red Blood Cells
Figure 1. !'arasite Life Cycle (Adapted from Sherman, 1998, p 9.)
Historical Malaria Control Efforts
South American Indians used bark from the Cinchona tree to treat early
malarial fevers. When the bark was brought back to Europe in the 1600's by
returning missionaries, it became the treatment of choice there as well. In 1820, the
alkaloid quinine was identified as the active ingredient (lOC, 1991, p39). Early
attempts at malaria control primarily consisted of the use of quinine. Quinine has
unpleasant side effects, which provided the impetus for continued research for better
treatments (Spei Iman and 0' Antonio, 2001, P 94).
5
environmental modifications such as drainage and landfills or by application of
insecticidal chemIcals (Reiter, 2001). After development of organic pesticides in the
20 th century, many countries turned to these faster and more economical control
methods, but these successes were short lived. Insecticide resistance occurred in
almost all cases (Carmichael, 1972).
In the developIng countries, post WWII malaria control policies have
traditionally been implemented and organized by the World Health Organization
(Baird, 2000). DDT was used in early eradication efforts for control of Anophelines
mosquitoes. It was heavily applied for agricultural purposes as well. It has become
pretty well established throughout the world. DDT has been identified as a persistent
organic pollutant (POP), a class of chemicals which have been linked to both cancer
and endocrine disruption, and which can adversely affect growth and development III
both animals and humans (Key et ai, 1998). It has been cited in EPA documents as a
probable human carcinogen (EPA, 2000), although this has never been proven
scientifically (Roberts et ai, 2000; Henderson, 2000). A recent agreement, signed by
several countries, allows continued use of DDT for malarial control throughout the
world (Maurice, 2001). DDT is credited with helping more than 1 billion people lIve
free of malaria (Turusov et ai, 2002).
During WWII, malaria was a major problem for the war effort on both SIdes.
At some points, up to two-thirds of the men in many fighting units were either sick
with the disease or recovering from it. The US military conducted extensive research
in an effort to bring malaria under control, and they were largely effective. A high
level of international cooperation devoted to eliminating endemic malaria existed in
the years following WWII. The United Nations Rehabilitation and Relief
Administration was formed in 1943, with the U.S. as the principal financial backer,
was active in many malaria control projects. Under the Interim World Health
Organization, the first Expert Committee on Malaria met in 1947, and in 1948 the
World Health Organization (WHO) was officially formed (IOC, 1991, pp 40-41).
A global malaria eradication campaign was adopted in May 1955, at the
Eighth World Health Assembly, in Mexico. Oddly, this global resolution was never
6
Assembly. The campaign consisted of widespread use of DDT for indoor and outdoor
spraying for mosquito control and antimalarial drugs to kill the parasite in humans.
Support for the campaign was not unanimous. Many public health officials from the
tropics agued that eradication should not be attempted in stable areas, where the
majority of the population had acquired immunity. They argued that if control
measures broke down, malaria would return and large numbers of people who would
normally have immunity would now be vulnerable to the disease (Packard, 1998).
Such an approach had been used to eradicate malaria from some countries by
the end of WWII (Tligg and Kondrachine, 1998). The program was successful in
many areas. India, which prior to 1945 reported 75 million new cases and 800,000
deaths each year, had by 1964 had an annual parasite index (API), which is the
number of cases per 1000 persons, of only 0.00098. This represents approximately
one case of malaria per 1 million people (Baird, 2000). By 1961, twenty percent of
people once plagued by the disease now lived in areas free of malaria (Speilman and
D' Antonio, 2001). Malaria was eradicated in the United States, Japan, Korea,
Taiwan, Spain, Italy, the Balkans, Greece, and northern Africa, all areas which had
been seasonally malarious though recent history (Baird, 2000).
Reemergence of Malaria
In 1969, after realizing that total malaria eradication was probably not
possible, WHO shifted its emphasis from malaria eradication to malaria control. As
part of this policy shift, the use of DDT was deemphasized and its use for in-house
spraying for mosquito control has been reduced as a result (Butler, 2000). At the
same time that in-house spray rates have been reduced, malaria rates have increased
in many parts of the world. In Brazil, malaria cases were reduced by 50% between
1960 and 1974 through the use of DDT and the drug chloroquine. Between 1974 and
1991 the number of cases rose almost tenfold (Gusmao, 1998). In Peru, malaria
cases dropped from 95,000 in 1944 to 1500in 1965. In 1997, Peru's hardest hit
district alone reported over 120,000 cases (Butler, 2000). It has also reappeared in
areas where it has previously been eradicated, including North and South Korea,
7
developed counties has increased, as well (Roberts et aI, 2000). Resistance to
antimalarial drugs has also been a major factor in worldwide malaria resurgence
(Reiter, 2001).
DDT was widely used as a pesticide beginning in the 1940's. Although no
longer used in the U.S., it is still used in other countries, primarily for control of
malaria. Because DDT is fairly inexpensive and the amount of exposure to humans is
thought to be low and the reduced malaria benefits are high, international debate
about DDT's continued use continues (Longenecker et aI, 2001). The chemical has
clearly been shown to be persistent and to concentrate in the lipid tissues of
organisms and has also been shown to be transported to upper latitudes via long-range
atmospheric transport and oceanic currents (Hargrave et aI, 2000). A shift away from
spraying strategies and DDT has substantially contributed to the deterioration of
vector control programs in the developing world (Baird, 2000). This shift has been
due more to social pressures than to resistance in the insect vectors (Coetzee and
Horne, 1999). The amount being used in this role is a small percentage of the amount
used when DDT was in widespread use for agriculture.
Operation Roll-Back Malaria
The World Health Organization recently launched their Roll Back Malaria
(RBM) program with the aim of reducing the global malaria burden 50% by the year
2010. Core elements of this program include rapid diagnosis near the home; the use
of insecticide-treated bednets and environmental measures for vector control;
preventative malaria treatment for pregnant women; improved epidemic awareness
and response; and research for new medicines, vaccines and insecticides
(Teklehaimanot et aI, 2001). The initial emphasis will be placed in the sub-Saharan
African region (WHO, 2000).
Alternatives to DDT are actively being researched, but in many cases DDT
represents the most effective strategy cUlTently available. Through widespread use
the chemical has become cosmopolitan in nature. Trace amounts of the chemical can
8
ocean, it is found in most plant and animal tissues, and it is also found in humans.
II.
ADVERSE EFFECTS OF DDT ON WILDLIFE
DDT has been studied extensively since it came into use as a pesticide in the
1940's. These studies have focused on both wildlife and human exposure. A strong
correlation has been made between DDT and abnormalities in wildlife. This was one
of the key points made by Rachel Carson in Silent Spring. The results of the human
studies are a lot less clear.
Numerous studies have been done on both wildlife and human exposure.
Warnings of wildlife damage were made as early as 1946. Tests performed in 1945
and 1946 showed that birds and fish could suffer DDT poisonmg by field
applications, but that the long-term effects were still unknown (Nelson and Surber,
1947).
Reproductive and developmental abnormalitIes observed in North American
gulls were supported by laboratory studies. Specifically, gulls injected with DDT at
concentrations found in wild gulls induced abnormalities similar to those observed in
the wild gulls which included skewed sex ratios, behavioral modifications, and
gonadal abnormalities (NRC 1999, p5).
In rat studies, immature females injected WIth DDT had a significant increase
in uterine wet weight, and newborn females injected with DDT showed an early
development of puberty and loss of fertility. DDT has also been shown to impair
learning in rats and locomotor abilIty in mice. The o,p' -DDT isomer has estrogenic
properties, while the DDT metabolite p,p '-ODE, has been shown to have anti
androgenic properties (NRC, 1999, P 121). Bioassays to determine DDT's
carcinogenicity in rats and mice found no correlation (NIH, 1978).
Eggshell Thinning in Birds
It was observed in the 1960'sand 1970's that many bird species populations
were declining in North America when individuals were unable to successfully
incubate eggs because the eggshells were abnormally thin. Many of these species
9
background levels in nature decreased over time. It has since been well established
that DDE played a significant role in the eggshell's thinning (Committee on
Hormonally Active Agents in the Environment, NRC, 1999, P165).
Adult animals can generally tolerate higher levels of pollution than their
offspring can. The eggshell thinning may have hidden the effects from other
pollutants by killing the embryo. Since the reduction in DDT use, more chicks have
been surviving and have been showing other abnormalities that have been linked to
dioxin, furans, and certain PCBs (Colborn et aI, 1997, pI55).
Lake Apopka Studies
In Lake Apopka, Florida, dicofol, a pesticide which was contaminated with up
to 15% DDT, was accidentally spilled in 1980. Alligator eggs collected between
1984 and 1985 contained DDT concentrations ranging from 3.4-7.6 ppm for the eggs
collected in 1984 and .89 to 29 ppm for the eggs collected in 1985. These levels are
above those required to reduce hatchling success and cause deformities. Juvenile
alligators hatched from eggs collected there have exhibited gonadal and genital
abnormalities as well as abnormal levels of testosterone in males and estradiol in
females. Alligator populations declined dramatically in the years following the spill
(Committee on Hormonally Active Agents in the Environment, NRC, 1999, p155
158).
III.
UBIQUITOUS NATURE OF DDT
In order to understand the total ubiquity of DDT on the planet, some basic
understanding of the physical, chemical, and biological processes involved is
required. No single cycle is extremely complex, but the cycles overlap and are
interconnected with each other. DDT is easily dispersed through runoff and
atmospheric transport. In the aquatic environment, the half-life of DDT can range
from a few days in high-energy environments, to more than 150 years (EPA, 2000).
Once released to the environment, chemicals like DDT and polychlorinated
biphenyls, or PCBs, are readily absorbed and accumulated in organisms throughout
10
(Rodricks, 1992).
The overall cycle is well illustrated in Our Stolen Future, where a story is told
about an imaginary molecule of another persistent organic pollutant (POP),
polychlorinated bIphenyl, or PCB-153. Its hypothetical journey and bioaccumulation
is described as it makes it way from inception at the Anniston, Alabama, plant in
1947, through many years and several trophic levels, to end up in a polar bear in the
northern Arctic (Colborn et aI, 1997).
An excellent study illustrating these bioaccumulative effects was undertaken
by George Woodwell, et aI, who identified DDT in seawaterat 50 parts per trillion
and followed DDT levels through the trophic web. DDT was found in plankton at 4C
parts per billion, in pickerel at 1 part per million, and in cormorants at 26 parts per
million. The overall increase in concentration from seawater to the cormorants was
by a factor of 520,000. Similar processes occur in terrestrial systems. Leaf residue
containing DDT transfers it to the soil, which is then biomagnified
In
earthworms,
which are then eaten by robins, etc. (Wargo, 1998, p 140-141).
Once in the ocean, the compound can be transferred to fresh water and
terrestrial food webs via migration of anadromous fish. In the Copper River area of
Alaska, pollutant transport was compared between lakes that receive migrating
anadromous fish and lakes that do not. DDT residues were shown to be transported
by Sockeye Salmon (Oncorhynchus nerka) and biomagnified in Arctic Graylings
(Thymallus articus). Graylings in the salmon-spawning lakes had significantly higher
levels of both DDT and PCBs than did graylings in the salmon-free lakes. PCB levels
were 1024 ppb in the Salmon-fed lakes and only 548 ppb in the Salmon-free lakes.
DDT levels were 239 ppb in the Salmon-fed lakes and only 41 ppb in the salmon-free
lakes (Ewald et aI, 1998).
PCB levels have also been well studied in Arctic marine mammals, and numerous
studies have identified them in body tissues of indigenous arctic peoples (Muir et aI,
2000; O'Hara et aI, 1999).
11
Deep Oceans
Looser ,et aI., (2000) studied three ocean regions, representing different
geographIc regions. The North Atlantic receives most of its river pollutants from the
eastern United States and from the Gulf of Mexico, which in tum receives pollutants
transported north by the Gulf Stream. Atmospheric input generally occurs from the
northern West Wind Belt, which collects most of the atmospheric emissions from the
United States and Canada.
The South Atlantic differs in that it receives much lower inputs from the
continents. It also receives wind input from the southern circumpolar West Wind
Drift. The water from the Amazon Rlver mainly flows northward to the Gulf of
Mexico via the Gulf Stream currents.
The third region selected was the deep-sea canyon associated with the
Monterey Bay Marine Sanctuary off the coast of California. This area receives
localized inputs from the Salinas River and is intensely used for agriculture.
In aJl areas the levels of contaminants were significantly different between the
surface-water and the deep-water fishes. While the levels between species and
individual fish will vary somewhat, some general trends are suggested. Table 2
shows pollutant levels in Cod Liver oil taken from North Atlantic cod from the years
1984 and 1993. The Levels of DDT and other toxins have significantly dropped since
1984.
Table 2. Organochlorine Levels (ppb) North Atlantic Cod 1984-1993 (Looser, 2000).
Pollutant
Total DDT
Total Chlordanes
Dieldrin
Total HCH
Cod Liver Oil-1984
1200
227
145
75
Cod Liver Oil #1-1993
30
18.5
5.8
1.7
Cod Liver Oil #2-199
33
19
7.5
n.d.
Table 3 shows levels of organochlorine residues for Atlantic fish, both
surface-dwelling and deep-sea-dwelling between the years 1994 and 1998. There
were no samples collected to represent the North Atlantic surface-dwelling fish, but
they were assumed by the authors to be similar to the South Atlantic surface-dwelling
fish. The OC levels are significantly higher in South Atlantic deep-sea fish than in
12
higher concentrations of OC' s than are their surface counterparts.
Table 3. Organochlorine Residues (ppb) Atlantic Fish (Looser, 2000).
Pollutant
North Atlantic
Deep-sea (Halibut)
South Atlantic
Surface (Snoek)
South Atlantic
Deep-sea (Kingclip)
Year
Total DDT
Total Chlord.
Dieldrin
Total Toxoph
Total HCH
1994
555
188
67
330
12
1994
63
1994
200
8
1998
220
120
44
155
11.4
n.d.
5.3
15
15
1998
78
7.5
5.0
15
1.0
1998
175
8.6
3.2
22
35
n.d.
22
98
Table 4 shows the results from the Monterey Bay area. Note that the
toxophene and chlordane levels are actually higher in the surface species. This is
probably due to recent pesticide inputs of these compounds relative to the
significantly older applications of DDT. Overall, these data suggest a similar
situation to other areas discussed previously.
Table 4. Organochlorine residues (ppb) Montery Bay Fish (Looser, 2000).
Sample
Petrale Sole
(surface)
Rockfish
(surface)
Dover Sole
(deep-sea)
Thornyhead
(deep-sea)
Brittle star
(deep-sea)
Year
Total DDT
Total ChI.
Dieldrin
Total Tox.
Total HCH
1995
1260
54
1995
2380
28
5.6
24
20
1995
2420
33
7.6
40
17
1995
3340
130
66
80
12
1.3
1994
1875
51
7.6
67
71
II
19
Although the levels between species and individual fish vary, the results
obtained suggest a trend of the deep-sea floor becoming the ultimate sink for many of
these compounds. The potential impacts for deep-sea fauna should not be
underestimated. (Looser, 2000).
Indeed, no living organism can be considered DDT-free. It is most easily
stored in fatty tissues, and would probably take from IOta 20 years to disappear if all
exposure were to suddenly cease (Turusov et aI, 2002).
IV.
HUMAN LEVELS
DDT residues have been shown in humans from several studies (Bouwman et
aI, 1991; van Wendel de Joode et aI, 2001; Ayotte et aI, 2001; Yanez et aI, 2002).
13
to the chemical revealed an average tissue level of 5.3 ppm DDT (Dunlap, 1981,
p249). This level increased to 15.6 ppm in 1956 and dropped to 3 ppm in 1980
(Turusov, 2000).
Human consumptIOn of top predators in food webs transfer OC's stored in
animal fat to humans. InUIt people living in the Arctic are far removed from
industrial activities, but their diets, which include large amounts of food denved from
marine mammals, have resulted in high levels of DDT metabolites in their fatty
tissues and breast milk (NRC, 1999, p92). The benefits of breast-feeding, howev~',
are cunently believed to outweigh the risk of chemical exposure, even at the hlgh end
of the exposure range (NRC, 1999).
South Africa Data
A study was conducted by Bouwman et ai, of two populations in Kwazulu.
One population was from an area where DDT has been sprayed annually in dwellings
for malaria control since 1976. The unexposed control group was from Port
Shepstone in southern Natal province where malaria does not occur, and no DDT has
been used for any purpose since 1996. Table 5 shows blood serum DDT levels and
Table 6 shows liver function parameters between the two groups.
Although the mean levels of total DDT in serum were 23 times higher than the
control group, liver function parameters between the two groups showed a normally
distributed range of values. Levels of gamma-glutamyl transferase (YGT) were twice
as high in the exposed group as the control, but both values fell within the normal
laboratory range.
Table 5. Blood Serum Levels (Bowman et ai, 1991)
Parameter
N
A~e
Males
Females
ODE
DOD
DDT
Total DDT
Exposed
71
22.4
33
38
103.4
21
37.3
1409
Control
77
28
20
57
595
015
.077
6.04
14
Parameter
N
AT
AP
YGT
TP
ALB
Aft
Exposed
63
45
332
30.8
836
448
61
Control
74
2.4
338
15.4
818
443
4.5
LNR (normal ranee)
0-25
73-207
8-38
60-80
38-48
0-29
In fact, liver function parameters varied more with age and alcohol
consumption. They were better predictors of liver levels than was exposure to the
pesticides (Bouwman et aI, 1991). Table 7 shows a comparison of alkaline
phosphates for the exposed and control groups, further divided into age groups. No
differences were found for AP or any other liver parameters for DDT or its
metabolites between the exposed and control groups (Bouwman, 1991).
Table 7. Changes in Alkaline Phosphates (AP) by age for exposed and unexposed DDT workers
(Bowman et aI, 1991).
Age
Exposed Group Control Group
480.5 (302-944) 4937 (212-810)
3-10
500.1 (211-678) 4079 (123-826)
11-20
124.8 (71-194)
181.5 (94-266)
21-30
158.1 (92-243)
107.0 (72-142)
31-40
114.3 (87-165)
150.5 (143-158)
41-50
219.5 (171-268) 164.0 (111-224)
51-60
173.8 (153-207) 182.7 (117-214)
61-70
Note: MInImum and maXimum values shown In parentheses.
Edward Laug first discovered in 1951 that breast milk from nursing U.S.
mothers contained an average level of 0.13 mg/I of DDT (Wargo, 1998, p 169). A
study conducted in the Kwazulu area of South Africa showed that in sprayed areas,
DDT makes its way into human breast milk and is subsequently passed on to nursing
infants. Nursing mothers in the DDT exposed area showed mean DDT levels of
574.89 ug/l in their milk and unexposed controls showed levels of22.07 ug/l. This
represents almost a 25-fold increase in DDT levels. The amount of DDT passed on to
the infants as a result ranged from 0.1 to 0.375mg/kg/day. The allowable daily intake
(ADI) for infants is only 0.005 mg/kg/day (Bouwman et aI, 1990a).
15
In India, where DDT and HCH have been used extensively for control of
vector-borne diseases, they have been detected in blood and adipose tissue. Twenty
samples of human milk were collected each from Bharat Heavy Electricals Limited
(BHEL) and Bahadrabad, both of which lie in the Hardwar District (Dua et aI, 1996).
The Bahadrabad area still uses lICH and DDT for malaria control, while the BHEL
area has used bioenvironmental methods and has not utilized insecticides since 1986.
Total DDT levels of human breast milk samples taken from the BHEL area ranged
from .002 to .085 mg/kg with a mean value of .021 mg/kg. Total DDT levels in
samples collected from Bahadrabad, where they still use pesticides, ranged from
0.020 to 0.503 mg/kg with a mean value of 0.145 mg/kg. Bovine milk samples alsc
showed elevated levels. The mean value for total DDT in Bovine milk from the
BHEL samples was 0.008 mg/kg while the mean value for the samples collected from
Bahadrabad was 0.029 mg/kg. This study shows a strong correlation between DDT
spraying for malaria control and DDT residues in human and bovine milk (Dua et ai,
1997).
Dua et al also measured DDT levels in soil, water, and whole blood in the
BHEL and Bahadrabad areas of Hardwar district. Fourteen soil samples were
collected from each area. Samples from the BHEL area showed total DDT levels
ranging from 0.0 to 9.60ug/kg with a mean value of3.68 ug/kg. The samples from
the Bahadrabad area ranged from 21.1 to 1833 ug/kg with a mean value of 270.5
ug/kg. Levels in the BHEL might be a result of aerial transport and long persistence
in the soil. Drinking water samples from BHEL showed no DDT detected while
samples collected from Bahadrabad ranged from .01 to .12 ug/I with a mean value of
0.07 ug/l. This is well below the maximum permissible limit of 1.0 ug/l established
by the World Health Organization (Dua et al, 1996).
Total DDT levels in whole blood samples from BHEL ranged from 0 to 25.0
ug/l with a mean value of 4.71 ug/I, while the samples from Bahadrabad ranged from
4.33 to 90.7 ug/l with a mean value of 38.1 ug/l. This study showed significant
differences in DDT levels for soil, water, and whole blood between the two study
areas (Dua et al, 1996).
16
Mexico Data
Yanez et al (2002), measured DDT levels in surface soil and blood serum in
two malarious areas in Mexico. One area, Chiapas, still used DDT for malaria
control, and the other area, Oaxaca, had switched to synthetic pyrethroids two years
earlier. In Chiapas, surface soil total DDT levels ranged from 3316 to 8229 ug/kg for
eight samples, two of which were indoor samples while 6 were from outdoor sources.
In Oaxaca, where DDT use had been stopped two years previously, surface soil total
DDT concentrations ranges from 140 to 2562 uglkg for six samples, three of which
were from indoor sources and three of which were from outdoor sources.
Blood serum levels are shown for both children and adults in both localities in
Table 8. DDT levels were SIgnificantly higher in Chiapas than they were in Oaxaca
for both children and adults. Since there was no DDT applied in Oaxaca, there are no
data for DDT sprayers In both regions, levels were higher in the children than in the
adults, and they were even higher than some of the sprayers in Chiapas. The high
levels in the children can be explained as a result of ingestion of human breast milk,
which represents
Table 8. Blood Serum DDT Levels from Two Areas in Mexico (Adapted from Yanez et ai, 2002).
Group
Children
Adults
Sprayers
Mean
67.8
27.1
165.5
CHlAPAS
Range
21.8-113.1
11.2-57.1
73.7-216.8
Mean
2004
13.2
OAXACA
Range
7.5-53.3
3.1-29.6
the children's main source of food for the first two years of its life. Although breast
milk was not investigated in this study, the situation is probably similar to the
Kwazulu study, where DDT was found to be transmitted to the nursing infant from
the mother via the breast milk. A second reason for higher levels among children in
Chiapas is that during DDT spraying household dust and adjacent surface soil is
highly contaminated with DDT, and children are exposed through dermal contact
during play.
17
In Sao Paulo, Brazil, serum DDT levels were compared between 26 workers
engaged in spraying DDT and hexachlorohexane (HCH) and 16 unexposed workers.
Total DDT levels for the exposed worker ranged from 7.5 to 473.5 ug/l with a mean
value of76.9 ugll. Total DDT levels for the unexposed group ranged from 5.1 to 32.9
ug/l with a mean value of 16.1 ug/l. The background levels are likely due to previous
widespread DDT use in Brazil for agricultural purposes (Minelli and Ribeiro, 1996).
In a study by Torres et al (2002) DDT residues were found in urban soils,
river sediments, and fish samples in the Madeira and Tapajos River watersheds, both
of which are important goldmining areas in the Brazilian Amazon. In the Rato River
area in the Tapajos Basin, soil samples yielded total DDT concentrations ranging
from 281 ug/kg at the riverbank to 1224 uglkg in the village. This negative gradient
as you move toward the river may be explained by evaporation of some DDT
metabolites and photooxidation that occurs on the deforested riverbank. River
sediment samples ranged from 3.2 to 61.5ug/kg. In the Madeira River area, DDT
concentrations were much lower, with soil values ranging from 4 to 123 ug/kg and
sediment values ranging from 0.0 1 to 1.1 ug/kg. In fish samples that were analyzed
from both basins, total DDT levels ranged from 7 to 536 ug/kg. In general, DDT
levels were lower than in major agIicultural areas where large amounts of DDT were
previously used (Torres et aI, 2002).
v.
DISCUSSION
While excess use of DDT for agricultural purposes has resulted in trace
amounts of the chemical being found throughout the biosphere and has been
associated with harmful effects to wildlife, it has never been proven to cause
widespread harm to humans. In many parts of the world, the use of DDT still works
for control of malaria vectors. Most of these areas are undeveloped countries that do
not have large public health budgets (Murdock, 2001). In addition, changes in
weather patterns due to global warming can potentially aid in the spread of malaria
vector habitat, and therefore, the spread of the parasite as well (Reiter, 2001). The
parasite has been able to develop resistance to introduced treatments and is likely to
18
in vaccine development, as well.
The data shown from malaria endemic countries (Brazil, Mexico, India, and
South Africa) imply that the DDT burden is distributed world-wide, and further, that
it has been there for about as long as DDT spraying has been in operation. Although
it is persistent in nature, overall levels have dropped since agricultural use of DDT
was discontinued, and this trend should likely continue.
Table 9. Summary of DDT Levels for Various Endemic Areas. (Values shown
are ppb unless otherwise stated, N/D= not determined
Country
Brazil
Blood
Serum
76.9
Mexico
11.2-113.1
N/D
NID
South
Africa
India
140.9
575
N/D
38.1
145
29
-
Breastmilk
Bovine
Milk
Soil
Water
Infant Load
(mg/kg/day)
N/D
NID
281
1224
3316
8229
N/D
N/D
NlD
NID
N/D
NID
0.07
0.1-0.375
NID
270.5
Humau Health Effects
DDT primarily affects the central nervous system, and sYmptoms of acute
poisoning would include vomiting, headache, fatigue, and convulsions. Death from
acute poisoning is extremely rare and no syndrome of chronic DDT exposure has
been recognized in humans (National Institute of Health (Nlli.), 1978).
Several studies have been done for occupational exposures of pesticide
applicators. In Kwazulu no relationship between blood serum levels and age or
worker or between worker and general population was established. (Bouwman et aI,
1991). In Costa Rica, occupational exposure to DDT has been associated with a
permanent decline in neurological functioning (van Wendel de Joode et all, 2001).
Ayotte et al (2001) found a correlation between DDE (a DDT metabolite) levels and
reductions in semen volume and sperm count in Mexican men.
Studies of factory workers with DDT serum levels ranging from 737 ug/I to
1373 ug/l showed no evidence for liver disease or abnormal liver function. In fact, no
relation between cancer or overall mortality and blood serum DDT levels has been
observed (Bouwman et aI, 1994).
19
mortality (Longenecker et ai, 2001). In the Kwazulu breast milk study, infants were
getting dosages of between 20 and 75 times the ADI. While these levels are not
expected to harm the mothers, these levels should be considered a possible health risk
to the infants. This risk should be balanced against the known health benefits of
breast-feeding, especially in developing countries. Contaminated water supplies
could also lead to higher rates of diarrhea (Bouwman et aI, 1990a).
High levels of DDT in fish caught for subsistence eating near Triana,
Alabama, in 1978 caused elevated levels of DDT metabolites in the persons eating
those fish. A correlation was established between these elevated levels and elevated
serum triglycerides. No association was found for any other health effects
(Committee on Environmental Epidemiology, NRC, 1991, p206). In South Africa,
where DDT is used for malaria control, low levels of DDT were found in fish samples
collected from the Pongolo River. No significant changes in levels were found before
or after DDT application The levels found in the fish were not found to pose health
hazards from human consumption, but harmful effects to species in higher trophic
levels such as alligators and eagles could not be ruled out (Bouwman et aI, 1990).
A study of breast cancer patients and DDTIDDE tissue levels was conducted
between 1994 and 1997. Adjusting for age, the DDT and DDT tissue levels were
similar between the cases and controls. DDT levels were 51.8 ppb in the cases and
55.6 ppb in the controls and DDE levels were 736.5 ppb in the cases and 784.1 ppb in
the controls. These results do not show association between DDT/DDE tissue levels
and incidence of breast cancer (Zheng et ai, 1999).
Two other studies collected and stored blood samples from healthy women.
\Vhen some of the women later developed cancer, the blood samples were analyzed
for high concentrations of DDT and PCBs. A New York University study found a
fourfold risk of cancer for women with high levels ofDDE, but a California study
found no link between cancer and DDE levels (Colborn et aI, 1997, p184).
20
The amount of DDT being used for malaria control is a small fraction of what
was used for agricultural purposes when DDT use was common. In 1993, the Pan
American Health Organization reported a total of 1, 172,077kg of DDT being applied
to house walls in all of South, Central, and North America. This represents about 6%
of the amount used in 1968 in the US alone, where it was commonly applied to crops
(Roberts et aI, 1997).
An outright ban on DDT for malaria control could have significant health
costs. After discontinuing the use of DDT in 1993, Guyana and ParaguaylPeru
reported a 78% and 92% increase, respectively, in malaria cases. Ecuador increased
its use of DDT in the same year and reported 62% decrease in new malaria cases
(Roberts et aI, 1997).
Throughout the Americas malaria rates have been increasing. At the same
time in-house spraying has decreased. Figure 2 shows a graph reflecting annual
statistics compiled by the Pan American Health organization for house spray rates and
incidence of parasite appearance. In 1959, the annual parasite index (# of positive
blood smears per 1000 persons) was 0.39. In 1996, the API was 2.46, and for persons
in malaria-specific areas it was 12.50. During this same time frame the house spray
rate (HSR) had decreased from 71.55 houses sprayed per 1000 persons to 4.12
(Butler, 2000).
Pan American Health Data 1959-1993
- =Q)
3
nJa.
;~
a. >< 2
Q)
~"C
c: c: 1
c:
'ii)
60
>.
eQ.::r:Ui
40
~ ;;
20
::l"C
0 c:
1/1
Q)
::r:
0:(
Year
-II- Annual Parasite Index
--+-- House Spray Index
FIGlJRE 2. Parasite Incidence and House Spray Data from Pan American Health Association
Data (Butler 2000)
21
Mpumalanga a switch was made from using DDT to synthetic pyrethroids for in
house spraying while Swaziland continued the use of DDT. Malaria cases increased
by 350% in Kwazulu-Natal and 185% in Mpumalanga while they remained steady in
Swaziland (Govere et aI, 2002). South Africa switched from using DDT to
pyrethroids in the mid 1990s. After five years they switched back to using DDT after
the AnopheLes mosquitoes had become resistant to them. They had previously reiied
heavily on pyrethroids against plant pests, which likely contributed to the mosquitoes
developing resistance (Raloff, 2000). Resistance to pyrethroids is appearing in field
vector populations throughout the world (Roberts and Andre, 1994).
Romi et aI, reported on a malaria control program using DDT in Madagascar,
where malaria is the second largest cause of mortality and morbidity. The coastal
regions are characterized by stable malaria while the highlands are characterized by
unstable malaria due to weak and irregular rainfalls. In the highlands, the main
malarial vector, Anophelesfunestus, was eradicated as a result of malaria control
campaigns carried out in the 1950's and 1960's that used DDT for indoor spraying
and treatment with chloroquine. A military takeover in 1975 resulted in a disastrous
economic decline, which created shortages of medicine and caused many of the
health clinics to close (Reiter, 2001). As a result, A. funestus slowly recolonized the
highlands, and a series of malaria outbreaks occurred, resulting in over 40,000 deaths
between 1985 and 1990.
In response, a national control program was initiated in 1988 to stop the
spread of the vector. Chloroquine was administered, and DDT was used for indoor
spraying where the heaviest cases of malaria occurred, specifically, villages located
between 1000 and 1500 meters in elevation. The program was funded by the World
Bank and provided for five one-year vector control campaigns between 1993 and
1998. The campaigns were known as the Operation de Pulverisation Intra
Domiciliare (OPID) Program. Based on parasite and vector surveys, the program was
a success.
Tables 10 and 11 show the parasite index and the occurrences of the vector A.
funestus, for the areas that were sprayed in the OPID vector control campaigns, and
22
mean total parasite index, or the percentage of blood smears that were positive for
plasmodium, dropped from 25.6 before OPID to 0.3 after the aPID campaigns.
Figure 3 shows a gradual decline in TPI, year by year, for the villages of Antanetibe,
Merimandroso, Analaroa, and Alasora. This figure clearly shows that after the DDT
spraying campaIgns started, the incidence of malaria decreased each year until it was
practically nonexistent.
Tablel0. Parasite Survey Results for schoolchildren jn Madagascar highlands before and
OPID program (Romi et ai, 2002).
Before OPID
After OPID
Total Parasite Index
Total Parasite Index
Altitude (m)
# of
# subjects
mean Min-max # of subjects
mean
viHaoes
examined
examined
2
474
57.6
43.6-65.0 235
0
900-1000
28
4272
23.8
0-72.2
2590
0.4
1000-1500
> 1500
2
309
0.6-2.3
206
0
1.3
3031
0.3
32
25.6
Total
5055
after
Min
max
-
0-2.0
-
Anophelines fimestus was common in the majority of the villages between 1000
and 1500 meters prior to the indoor residual spraying. Occurrence of the vector was
dramatically reduced during the spraying campaigns.
Table 11. Abundance of Anopheles funestus in villages of the Madagascar highlands before and
after OPID vector control campaif!J1s (Romi et ai, 2002).
Before OPID
After OPID
Altitude
(m)
# of
#
villages
positive
900-1000
1000-1500
>1500
Total
2
26
7
35
2
21
0
23
-
Mean #
of bites
per
person
per
night
47-112
1.7-19.5
Mean # of
mosquitoes
per room
# of
47.4-82.2
0.1-56
-
-
4
12
3
19
-
23
# positive
villages
3
3
0
6
Mean #
of bites
per
person
per
room
0- 1.5
.08-.45
Mean # of
mosquitoe
per room
0-3.6
0.1-0.1
-
-
-
-
Total Parasite Index for Schoolchildren from Four
Villages in Madagascar Highlands
~
70
-; 60
Q)
-g 50
...
'iii
40
;
20
Q)
l'CI
a..
30
10
~ 0
l'CI
"""'-~:7""'"""-,--,-------,--c---~~~
+--f-\;--;
---+--- Antanetibe
+----'-"':tt-"7
___ Merimandroso
+-..,-;-.-~~ ~,,:--::-,--..:...-..c~,--_-"------=-,--_-+-_~
Analaroa
--"'.-- Alasora
+--~...".-./--'-"-'----;-'---'--'"-i'---'----.,.---'-"'".
1993
1994
1995
1996
1997
1998
Year
Figure 3. Parasite Index for Schoolchildren from Four Villages in Madagascar Highlands (Romi
et ai, 2002).
Cost Comparisons with Alternative Insecticides
While developed nations can afford a variety of insecticides, health budgets in
third-world countries are sjgnificantly less. Annual health budgets in these countries
are often less than $5.00 (US) per person. In these cases, any jncreases jn cost
become significant (Murdock, 2001).
The costs of alterna6ve insecticides have been an issue in malaria control.
Several cost studies have been conducted comparing the cost of DDT with other
insecticides. Statistics from the Pan American Health Organizatjon show that use of
the insecticide malathion to be five times that of DDT (Roberts and Laughlin, 1997).
Other organochlorine insecticides such as dieldren and gamma-BHC/HCH were used
for malaria control until the 1960's but were discontinued due to toxicity in humans
and also resistance jn the target vector species (Walker, 2000).
Table 12, from Walker (2000), shows comparative prices for DDT and five
other pesticides in use. All were more expensive than DDT, with Propoxur being the
most costly at over twenty-three times the cost of DDT. While other insec6cides may
require less actual product to be used, their cost are greater. Because many of these
countries are so poor, no realistic alternatives to DDT exist (Butler, 2000).
24
2000).
Insecticide
Apllications/6 months
DDT
Malathion
DeItamethrin
Permethrin
Fcnitrothion
Propoxur
1
Cost per Kg
-
3.00
2.10
25-28.00
30.00
7.50
9.30
2
I
2
2
2
Cost per house per 6
months
1.60
3.36
5-5.60
6.00
12.00
37.20
Resistance to Pesticides and Drugs
Resistance to chemicals is almost universal among pest species. Resjstance
acquisition occurs through evolutionary means. Over time, random genetic changes
occur, which preadapt some individuals in populations to survive exposure to given
chemicals. When these chemicals are used, these preadapted individuals will survive
to reproduce and pass their genetic advantages on to their offspring (NRC, 2000,
p55). Acquisition of resistance to one agent can assist in the development of
resistance to other agents, such as the situation when South Africa switched from
using DDT to synthetic pyrethroids, only to switch back again 5 years later, when the
mosquitoes were showing resistance to them (Raloff, 2000).
Insecticide Resistance in the Vector
Reports of insecticide resistant in mosquitoes appeared as early as the 1950's.
In 1956, resistance by Anopheles gambiae from Nigeria was reported against dieldrin,
chlordane, aldrin and benzene hexachloride, but remained susceptible to DDT in
laboratory experiments (Coetzee and Horne, 1999).
As of 1983,4 billion pounds of DDT had been applied to outside areas,
mainly for agriculture, which greatly accelerated the pace of mosquito resistance. By
1959, seven species of Anophelines were resistant to DDT and another eight species
were resistant to dieldrin. By 1962, the numbers of resistant species had increased to
nine and twenty-six, and by 1969 had increased further to fifteen and thirty-seven.
Fifty species have demonstrated some resistance to DDT as of 1994 (Wargo, 1998, p
51 ).
25
generations became resistant to even the highest dosages. In some cases, cross
resistance occurred to chemically similar compounds (Reiter, 2001), which happened
with synthetic pyrethrum in South Africa (Raloff, 2000).
More recently, in India, Anopheles fluvialillis have become resistant to
hexachlorohexane (HCH), which was introduced as a replacement for DDT.
Studies
by the Malaria Research Centre have shown malaria rates to be largely unaffected
(Sarala et aI, 1988). Table 13 shows a comparison of responses of A. fluvialilis to a
one-hour exposure ofHCH and DDT in four different districts. Mortality for HCH
varied by location between 24.0 and 41.9 percent, while the DDT showed 100%
mortality (Sahu and Patra, 1995).
Table 13. Mortality of A. fluviatisis to HCH and DDT (Sahu and Patra, 1995).
Primary Health District
Malkan~iri
KGumma
Korkunda
Khairput
HCH application, %mortality
24.0
26.7
41.9
26.6
DDT application, %mortality
100
100
lOa
100
Even when not outright killed by the insecticide, vectors often exhibit a
characteristic known as excito-repellency, where the insecticide still adversely affects
the vector of interest. These effects can range from resting on a sprayed surface for
less time than an unsprayed surface, or avoiding it altogether, to completely avoiding
rooms that have been sprayed (Najera and Zaim, 2001). Although the mosquitoes
may show resistance to DDT, at least four anopheline species have demonstrated this
'excito-repellency' where the mosquito is not killed by the DDT, but avoids it
nonetheless. The ultimate result of this behavior is that the mosquitoes do not bite
humans or transmit malaria (Butler, 2000).
This exito-repellency, however, may well represent an evolutionary step
towards resistance. Greece began DDT spraying in 1946. By 1949, large numbers of
Anopheles sacharovi mosquitoes were observed in large numbers under road bridges,
and soon were also observed in caves, culverts, and outbuildings. Within a few
months they were able to remain in houses on walls treated with DDT (Carson, 1962,
p 269).
26
DDT occurred slowly, even factoring in previous usage of DDT for agricultural
purposes. The proportion of resistant mosquitoes in the population increased slowly,
and so DDT remained effective for a long time. In some areas of Central America,
DDT was still effective, even when tests showed vector survivability of up to 40%
(Najera and Zaim, 2001, p 45).
Drug Resistance in the Parasite
The same mechanisms by which mosquitoes become resistant to insecticides
allow the Plasmodium parasite to become resistant to animalarial drugs. A wide
range of drugs is used, and in recent years more emphasis has been placed on these
than on insecticidal methods (Reiter, 2001). It is recognized that some level of
resistance to the antimalarial drugs will be unavoidable in most countries (Sherman,
1998, p16). Appendix B shows the current occurrences of parasite resistance
throughout the world.
Quinine, which is isolated from bark from the cinchona tree, was the primary
treatment for malaria until synthetic antimalarials were developed between the two
World Wars. The first isolated reports of resistance to quinine occurred in 1910
(Wargo, 1998, p53). Chloroquine, which was introduced at the end ofWWII, was the
primary antimalarial drug for the next four years due to its effectiveness, safety, and
low cost. Parasitic resistance started to appear for chloroquine in the late 1950s to
1960's in Asia and South America, and now occurs in all endemic areas (Salako,
1991). Resistance to the main alternative to chloroquine, sulfadoxine/pyrimethamin,
occurs in Southeast Asia and across South America. Mefloquine resistance is
common in the border area between Cambodia and Thailand. Resistance to quinine
also occurs in the Amazon region and in Southeast Asia (WHO, 2000).
Combination drug therapy has been widely accepted as a method to counter
drug resistance in the parasite. Using drug combinations slows the development of
the parasite's resistance to either drug, and therefore, helps prevent transmission of
the disease. This technique has also been used with various levels of success in
treating some cancers and AIDS (Key et ai, 1999). Unfortunately, because malaria
27
for these drugs is often quite small (Tren and Bate, 2001).
The CDC currently recommends the following five prescription drugs for
malaria control and treatment: mefloquine, doxycycline, malarone, chloroquine, and
hydroxychloroquine sulfate. All have potential for mild side effects, which include
itching, dizziness, nausea, vomiting, blurred vision, and sleep disorders. Doxycycline
increases susceptibility to sunburn. Mefloquine can also have major side effects,
which include severe anxiety, hallucinations, and seizures (CDC, 2003).
Implications From Global Warming
Our planet's climate has always been in a state of flux. For the past 300 years
or so the planet has experienced a warming trend. This trend has been exacerbated,
according to some models, by human-induced activities. The ecology and survival of
mosquitoes and the dynamics of the diseases that they help to transmi t are strongly
influenced by these climatic factors. Temperature, rainfall, and humidity are
especially important (Reiter, 2001). As the global temperature rises, so can the area
plagued by mosquitoes and the associated diseases they carry. This can occur in the
vertical as well as the horizontal dimension as temperatures rise, the insects will find
hospitable temperatures at higher elevations. The increased temperature also raises
the rate of maturation of parasites. P. faciparum takes 26 days to fully develop at 68
degrees F, but matures in half the time at 77 degrees F (Epstein, 2000).
The global temperature will affect precipitation patterns, which will affect the
ability ofthe vector to multiply. This ability is dependent upon whether stagnant or
moving water is encountered and also in the type of vegetation present (Martens et aI,
1997).
While Anophelines mosquitoes are generally not found more than a few miles
from their development area, they are readily transported by wind currents (10M,
p28). This phenomenon
lS
likely to contribute to the mosquito's ability to spread as
the mean annual temperature increases.
Localized epidemics due to climatic variations are already thought to have
occurred. In Bangladesh, heavy rains in usually arid regions resulted in extensive
28
Madagascar, and Ethiopia experienced epidemics following prolonged rains, high
temperatures, and humid summers. In Sri Lanka in the 1980's, lack of normal
monsoons resulted in the normally humid valleys experiencing pooling water from
the rivers, which produced an environment conducive to the breeding of A.
culicifacies in these areas (Sherman, 1998, p14).
Higher precipitation levels should create an increase in vector breeding sites
and should result in an increased opportunity for the parasite to be transmitted
between individuals (Lindsay and Birley, 1996). A correlation was found between
fever deaths due to malaria and rainfall in the Punjab regIOn near the India-Pakistan
border. This correlation between precipitation and fever death is shown in figure 4.
,--------_._--_._----------------------------
Rainfall and Malaria Deaths in the Punjab Region, 1868 to
1908
900
3
800
2.5
700
-2
600
E
E 500
0
0
0
.....
iii
-
..r::::
-ra
1.5
r::: 400
ra
0:::
. 1
300
ra
-11- Rainfall, mm
0
-+- Fever deaths/1
<ll
ra
";:
ra
ra
::2:
200 - .
0.5
100 -
o -H-H-++t-'H-++t-'+++-++t++-+++++-+++++-++-H4++++-++
0
Year
----------------FIGURE 4. Relationship between fever deaths and precipitation (Lindsay and Birley, 1996)
29
world's population (Epstein, 2000). Appendix C shows the current infectious areas
and the potential increase in area with a two degree Fahrenheit increase.
A small number of malaria cases occur in the United States. The Centers for
Disease Control (CDC) estimates that it receives between 1,000 and 1,500 cases per
year. The majority of these cases are from travelers recently returning from malaria
endemic areas. A few cases, however, are believed to be mosquito-transmitted.
Several competent malarial vector species exist in the United States (CDC 2002).
Global climate change can only exacerbate this condition.
VI.
SUMMARY AND RECOMMENDATIONS
DDT has been shown to be very persistent in the environment. Mostly
through excessive use as an agricultural pesticide in the middle 1900's, it has become
ubiquitous in nature. It has bioaccumulated throughout the global food web and trace
tissue levels of DDT have been found in numerous animal and human studies. Trace
amounts have been found in soil and sediment samples around the world, as well as in
the oceans.
Malaria has been a major pathogen to humans throughout history, and has
caused much suffering and many millions of deaths. A major attempt was made at
worldwide eradication of the disease in the middle 1950's. While not successful, it
did significantly reduce the incidence of the disease. Since then, then parasite has
become resistant to different drugs in different regions, and the vector mosquitoes
have become resistant to certain insecticides.
Concerns about damage to nontarget organisms by DDT exposure resulted in
significant reductions of its use beginning in the 1970's. Malarial rates have
increased in several areas as a result. Reintroduction of DDT in South Africa and
Madagascar has resulted in downward trends of malaria incidence in those areas.
Without the excessive amounts of DDT used for agriculture in the 1950's and 1960's,
human levels of DDT and its metabolites have decreased. Worldwide DDT levels in
human breast milk are decreasing between 11 and 21 % per year, and have dropped
30
from more than 5000 ug/kg to less than 1000 ug/kg (Butler, 2000). It is likely that
DDT levels found in wildlife have decreased, as well.
New antimalarial drugs are expensive, costing as much eight dollars per pill
and well beyond the affordability for most Third World countries (Speilman and
D' Aantonio, 2001, p95). InitiaIly one of twelve POP's to be banned worldwide,
continued use of DDT for malaria control has been allowed (Maurice, 2001). The use
of DDT for indoor spraying does not introduce sufficient amounts of DDT into the
environment to bioaccumulate into the food chain (10M, 1991, P 133; Tren and Bate,
2001). Attention must be paid to prevent rinsing of spray equipment or disposal of
unused insecticide into natural bodies of water (Najera and Zaim, 2001).
DDT has not been shown to be a human health risk (Henderson, 2000;
Roberts et aI, 2000). There is little evidence of environmental harm when DDT is
used indoors for control of Anophelines mosquitoes (Curtis, 2002).
While the symptoms of malaria can generaIly be treated, prevention of
infection in the first place would be a more desirable condition. It is generally
accepted that limited use of DDT should be allowed, especially where no reasonable
alternatives exist, for public health purposes (Turusov, 2002). Although DDT's
persistence makes for a long-lasting treatment, which does not require frequently
repeated applications, it does accumulate in the environment, and exposure to the
chemical does not end with substitution of another insecticide (Yanez et ai, 2002).
House spraying with DDT has its limitations, but it remains an effective antimalarial
tool, which should continue to be used (Roberts et aI, 2000).
For malaria control to be effective, the overaIl strategy must be flexible and
continuously responsive to changes in effectiveness of various methods used. For
insecticide applications, applicators must be trained in proper and safe methods of
application and supervisors must insure that required protective equipment and
clothing is in place both for the protection of the applicators as weIl as the conununity
inhabitants (Najera and Zaim, 2001).
Increased monitoring of both malaria incidence and vector resistance is vital
to an effective malaria control strategy. Although no actual assessments have been
made since 1982, fewer students seem to be training in vector biology, and many of
31
the educators in the field have retired without being replaced (lOC, 1991, P 134).
Jobs and training for field ecology and entomology is severely lacking, and
fundamental changes in policy and funding for national and international public
health organizations are required to improve this situation (Roberts and Andre, 1994).
It is not likely that any single method can be used to control malaria.
Continued instances of parasite and vector resistance, as well as the implications of
global warming can only exacerbate the situation. As long as DDT remains an
effective insecticide agamst Anopheline mosquitoes, it should continue to be used
where appropriate. Current WHO guidelines, as outlined in their Ro11 Back Malaria
Campaign, calls for research into new medicines, vaccines, and insecticides. It also
allows for the use of DDT where effective. In my opinion, this represents a sound
public health policy.
32
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36
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37
APPENDIX A
Global Malaria Transmission Areas
o
o
o
•.
."
.",
o
o
0
Global Malaria Transmission Areas (World Health Organization, 2000)
APPENDIXB
Global Malaria Parasite Resistance
Q
.
I
Incidence of Parasite Resistance (World Health Organization, 2000)
APPENDIX C
Potential for Increase of Malaria Transmission Area Assuming a
Two-Degree Fahrenheit Increase in Temperature
a
.•
Potential for Increase in Malaria Transmission Area Assuming a Two-Degree Fahrenheit
Increase in Temperature (Epstein, 2000, World Health Organization, 2000)
