Global Warming and Malaria
Natalie Van Hoose
While global warming is
anticipated to have significant impacts on the Earth in the next century,
one indirect impact it could have is on the distribution of arthropod-borne
diseases, or arboviruses. It is difficult to predict exactly how these
diseases will be affected with changes in climate. However, arboviruses
are extremely sensitive to climate change, particularly changes in temperature
and precipitation, and as these factors will be greatly altered with global
warming, it is expected that arboviruses will be impacted as well. One
disease that may be highly affected is malaria, an arbovirus transmitted
by mosquitoes. Malaria is a serious disease which exists at epidemic
levels in many areas of the world, rendering it necessary to examine how
the distribution and severity of malaria may by altered by global warming.
Malaria is considered
to be the most important infectious disease and is a leading cause of morbidity
and mortality in the developing world [1, 2]. Forty-percent of the Earth's population
is classified "at risk," and approximately 3 million malaria victims die
annually, the majority of which are children. In humans, the symptoms of malaria
can include fever, chills, anemia, and blockage of blood vessels, and the
disease may progress to liver failure, coagulation, shock, encephalitis,
and coma . Death can occur within 24 hours, and the case fatality
among untreated and/or nonimmune victims is 10% .
Malaria is caused by four species of parasitic protozoa
in the genus Plasmodium, the two most prevalent malarial species
being Plasmodium falciparum and Plasmodium vivax . The
only vectors--organisms that transport
and transmit pathogens to other organisms--for Plasmodium are mosquitoes
in the genus Anopheles, which contains 400 different species, one-tenth
of which are potential vectors for Plasmodium .
When a female Anopheline mosquito takes a blood-meal from an infected host,
she picks up the parasite. Within the mosquito, the parasite undergoes
a cyclopropogative transmission process, meaning
that it changes morphologically and developmentally . This
maturation process can take 8-25 days,
the speed depending directly on temperature; generally, the higher
the temperature, the quicker this process will be.
Optimum temperature for Plasmodium maturation is 26 C, and
development cannot occur in temperatures below 16 C. After the parasite
is mature, it enters the salivary glands of the mosquito and is transmitted
to a human host when the mosquito takes a blood-meal. 
This process is illustrated in the figure below:
Figure from http://www-micro.msb.le.ac.uk/224/Bradley/Biology.html
The type and rate of
transmission of malaria are controlled by environmental factors, such as
minimum and average temperatures, the survival rate of the vector and the
parasite, the victims' level of immunity, the mosquitoes' biting rate, and
rainfall and water accumulation .
Distribution of Malaria
is mostly confined to the tropics, it has been spreading. Causes
for this spread include deforestation, increased travel, increase in population,
and increased resistance of mosquitoes and Plasmodium to pesticides
and anti-malarial drugs . Scientists also cite rising global
temperatures as a major contributing cause, as warmer temperatures give
rise to more habitats for Anopheline mosquitoes . Whether or not
global warming is to blame for the increased distribution of malaria is
a matter of hot debate.
It is important to note that malaria existed in the endemic form ("endemic" meaning that the
disease was present but not at epidemic levels) throughout much of Europe
and North America up until recent times . Its absence from these
areas is largely due to effective control of vector mosquitoes and successful
treatment of infected humans.
below shows the current distribution of malaria:
As with disease
transmission, disease distribution is also governed by environmental factors.
The table below, which was designed by the World Health Organization,
outlines the prime factors that control the survival of Anopheline mosquitoes
and the Plasmodium parasites within them . Changes in the
survival of the vector for malaria will translate into changes in its distribution.
Global Warming and the Spread of Malaria
Global warming could significantly
impact the distribution and severity of malaria on a global level, especially
as mosquito-borne diseases are highly sensitive to changes in climate .
Due to the increase
in atmospheric greenhouse gases, scientists predict that the next century
will hold significant changes for the Earth's environment. The
changes that will have the greatest impact on arthopod-borne diseases include
higher temperatures, increased precipitation, coastal flooding, drought
and desertification . It is important to consider the effects global
warming will have on mosquito-borne diseases since they are known to rapidly
change from an endemic status to an epidemic under favorable environmental
Warmer temperatures will be favorable to both the mosquito vector and the
Plasmodium parasite. As these
organisms survive best in the tropics, a rise in temperature could
produce more potential habitats for the Anopheline
mosquito and Plasmodium . In addition to increasing
the vector's geographic range, heat increases the reproduction rate of mosquitoes;
thus, the vector population would increase, providing more hosts to the
parasite. Also, the parasites' incubation period within the mosquito
would shorten . If the parasites' necessary development time is
decreased, they would be able to infect humans at a more rapid rate. Experiments
show that a 10 degree increase in temperature halves the development time
for most parasites . While most areas may not experience such drastic
warming, these experiments establish the connection between warmer temperatures
and more rapid maturation of disease agents.
An increase in precipitation
would also expand the geographic range of mosquitoes. Immature mosquitoes
are aquatic, and with more rainfall, there would be more potential breeding
and development grounds for mosquitoes .
Global warming is expected to produce a significant
rise in sea level, resulting in coastal flooding. Coastal flooding would
also provide Anopheline mosquitoes with more breeding grounds if it increased
the brackish water lagunae . However, if flooding results in an
increase in the salinity of coastal freshwater areas, then the breeding grounds
for mosquitoes would greatly decrease, which could possibly limit the spread
of malaria .
Some areas may experience an increase
in drought and desertification as a result of global warming. This would
not prove favorable to the spread of malaria, as mosquitoes require an aquatic
habitat to breed. Also, mosquito longevity is linked with relative
humidity, so the survival rate of mosquitoes in drier areas would decrease
Areas at Risk
Though it is difficult to predict exactly how climate
will change on a regional basis as a result of global warming, a few areas
have been outlined as possibly at risk for an increase in malarial cases.
Temperate zones may be at the greatest risk since the warming is expected
to affect them to the greatest degree . Immunity to malaria will
be lower in these areas, and the populations living in temperate zones may
be caught off guard in a malaria outbreak. These zones include the
African highlands in countries such as Ethiopia, Indonesia,
and Kenya, the United Kingdom, Europe, and the southeast United States
[3, 6]. Malaria has previously been endemic in many of these areas before,
and in some cases, Anopheline mosquitoes are already present . However,
due to effective control measures, the disease is presently rare. Were
the parasite to be introduced, though, it may be difficult to control with
its shortened maturation period and an increase in the mosquito population.
Areas that are already at the endemic level and areas bordering on
malarious countries are also at risk. In countries such as China and
South Africa, in which malaria is already present, the changes in climate
could raise malaria to an epidemic level, or at least increase the frequency
and/or severity of the disease .
While malaria is by no means an incurable disease, it is important
to remember that the drug resistance of the Plasmodium parasite is
increasing rapidly, as a result of uncontrolled and unregulated drug
blood smear in an African hospital
The MALSIM Model
MALSIM, a computer model predicting the effect of climate
change on the possible distribution of malaria in the United States, is
still being developed and refined . However, it has been used in
predicting possible outcomes under the following environmental relationships:
"1) the effect of temperature on the developmental rates of each vector
stage and the parasite in the mosquito; 2) the effect of temperature on
survival of immature stages; 3) the effect of temperature and saturation
deficit on adult survival; 4) the relationship between rainfall, available
area of aquatic habitat, immature density, and immature survival; and 5)
temeprature-induced hibernation of mated, nonblooded females" . MALSIM
used Anopheles quadrimaculatus as the malaria vector, a species which
already exists in the eastern U.S. The table below displays selected
it appears that some areas, such as Miami, Key West, and Orlando, are at
extremely high risk of malarial cases, this is no higher than their present
risk-level, according to the designer of the MALSIM model, D.G. Haile. In
fact, he states, "Overall, these simulations suggest that the proposed climatic
changes will have little if any impact on the transmission potential of
malaria in the United States" . However, Haile acknowledges that
this is only one mosquito-borne disease, and others may become more problematic.
He also notes that while usually accurate, computer models predicting
environmental responses are not always correct. The MALSIM model is
one example of how scientists are divided in their predictions for the spread
of malaria as a result of global warming.
The full report
on the MALSIM model can be found at http://www.ciesin.org/docs/001-365/001-365.html.
It is difficult to predict how malaria will respond to the climate
change that global warming will bring. It can only be said that a serious
disease is likely to become more serious, as the geographic range of the
disease vector could spread, the vector could significantly increase in population,
and the maturation period of the parasite could shorten. Malaria is
not an incurable disease; remedies are available and are being used in many
areas of the world today. However, it is wise to be prepared for possible
outbreaks. The following table from the World Health Organization
shows the most important arthropod-borne diseases and the likelihood that
their distribution will be affected by climate change . Malaria
is the only disease classified as "highly likely" to change distribution.
Though scientists are divided on how global warming will affect diseases,
the following quote from Global Warming and Biodiversity may be the
voice of reality: "Parasites and diseases will do well on a warming
earth. They are, by definition, organisms that colonize and exploit"
1. Daly, Howell V., Doyen, John T., and Alexander H. Purcell
III. 1998. Introduction to Insect Biology and Diversity, 2nd ed. New
York, NY: Oxford University Press.
2. Malaria Foundation International. (n.d.) Retrieved April
12, 2004, from http://www.malaria.org.
3. World Health Organization Geneva. 1990. Potential health
effects of climate change. Retrieved April 12, 2004,
4. BBC News. 2000. "Warming 'not spreading malaria.'" Retrieved
April 11, 2004, from http://news.bbc.co.uk/1/hi/health/934536.stm.
5. Reiter, Paul. 2000. "Malaria and global warming in perspective?"
Retrieved April 13, 2004, from http://www.cdc.gov/ncidod/eid/vol6no4/reiter_letter.htm.
6. BBC News. 1999. "Global warming disease warning." Retrieved April
11, 2004, from http://news.bbc.co.uk/1/hi/sci/tech/372219.stm.
7. Dobson A. and R. Carper. 1992. "Global warming and potential changes
in host-parasite and disease-vector relationships." In Global Warming
and Biodiversity, ed. R. L. Peters and T. E. Lovejoy. New Haven, CT: Yale
University Press. Retrieved April 20, 2004, from http://www.ciesin.org/
8. Haile, D.G. 1989. "Computer simulation of the effects of changes
in weather patterns on vector-borne disease transmission. In The Potential
Effects of Climate Change in the United States, ed. J. B. Smith and D. A.
Tirpak. Document no. 230-05-89-057, Appendix G. Washington, D.C.: U.S. Environmental
Protection Agency. Retrieved April 21, 2004, from http://www.ciesin.org/docs/001-365/001-365.html.
* Pictures are linked to their sites of origin.