mosquito-borne diseases

Global Warming and Malaria
Natalie Van Hoose

Introduction

            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.

    malarial molecules

Malaria
  http://www.buddycom.com/cells/malarial

            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 [2]. Death can occur within 24 hours, and the case fatality among untreated and/or nonimmune victims is 10% [3].

Transmission

            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 [2]. 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 [3].
                                    Anopheline mosquito

                                                            http://www.cdc.gov/ncidod/dvbid/arbor/anophel.htm

            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 [1]. 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. [3]


This process is illustrated in the figure below:

           Plasmodium
http://www.buddycom.com/cells/malarial

                                                                       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 [1].


Distribution of Malaria

            While 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 [2]. Scientists also cite rising global temperatures as a major contributing cause, as warmer temperatures give rise to more habitats for Anopheline mosquitoes [4]. 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 [3]. Its absence from these areas is largely due to effective control of vector mosquitoes and successful treatment of infected humans.

                  The map below shows the current distribution of malaria:

                            http://www.esri.com/news/arcnews/fall03articles/fall03gifs/p36p1-lg.jpg

            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 [3]. Changes in the survival of the vector for malaria will translate into changes in its distribution.

                                                                                   http://www.ciesin.org/docs/001-007/001-007.html

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 [5]. 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 [3]. 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 conditions [3].

                   Higher temperatures

                                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 [6]. 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 [3]. 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 [7]. While most areas may not experience such drastic warming, these experiments establish the connection between warmer temperatures and more rapid maturation of disease agents.

http://news.bbc.co.uk/1/hi/health/934536.stm

Increased Precipitation 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 [3].
Coastal Flooding 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 [3]. 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 [3].	http://news.bbc.co.uk/1/hi/world/africa/3630301.stm
Drought and Desertification 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 [3].

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 [7]. 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 [4]. 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 [3].

                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 distribution [2].

Taking a blood smear in an African hospital
http://www.gsk.com/malaria/photographs.htm

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 [8]. 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" [8]. MALSIM used Anopheles quadrimaculatus as the malaria vector, a species which already exists in the eastern U.S. The table below displays selected results:

                                                              http://www.ciesin.org/docs/001-365/001-365.html

               While 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" [8]. 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.


Conclusion

               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 [3]. Malaria is the only disease classified as "highly likely" to change distribution.

http://www.ciesin.org/docs/001-007/001-007.html

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" [7].

Sources

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, from http://www.ciesin.org/docs/001-007/001-007.html.

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/ docs/ 001-364/001-364.html.

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.