Climate Change and Malaria

Projected Impact of Climate Change on the Malaria Disease System

Thomas W. Elston

Baylor University

Abstract

Malaria, the world’s greatest killer, is thought of as a tropical disease, being depending on the high temperature and moisture hallmarking that climate. In the face of global climate change the distribution of temperature, moisture, and the organisms sustained by specific climates, including the mosquito Anopheles gambiae, malaria’s vector (“transmitter”), are likely to shift. This paper seeks 1) to provide a brief introduction to malaria, its causes, effects and factors determining its distribution,  2) to examine the impact of the global warming climate change on the malaria disease system vector, 3) to investigate the impact of climate change on the malaria causing Plasmodium falciparum parasite itself, and 4) to comment on the cumulative impact on the human host.

 

I. Malaria: cause, effect, and current distribution

Malaria, an easily treatable but fatal if not disease impacting 3.3 billion people, near half the world’s population, extinguishes a human life approximately every 30 seconds (“Impact of malaria,” 2012 & “Malaria,” 2012). The disease, characterized by periodic high fevers and shaking chills, is thought to be caused by the parasitic protozoan Plasmodium falciparum. The malaria causing P. falciparfum infect humans through its vector species, the mosquito Anopheles gambiae. The female A. gambiae “takes [human] blood meals to carry out egg production,” mixing the mosquito’s blood with that of humans (“Anopheles Mosquitoes,” 2012). Thus, where A. gambiae is found, one may reasonably expect to find malaria. According to the US Center for Disease Control and Prevention, malaria is a primarily tropical disease distributed between 30 degrees north and south of the equator, especially the central region of the African continent. A 2007 investigation determined the temperature range for A. gambiae to be between 22 and 33°C, with 22-27°C as the optimal temperature (Impoinvil, Cardenas, et al, 2007). The same study determined that the malaria-carrying A. gambiae was not viable below 12°C nor above 42°C. The malaria distribution map below, developed by the CDC, displays the 2012 global malaria distribution Taken together, this information suggest that malaria distribution is dependent upon its vector’s, A. gambiae, distribution and, in terms of transmission, is limited by its vector’s limits.

Source: Center for Disease Control and Prevention malaria web page.

Source: Center for Disease Control and Prevention malaria web page.

 

II. Climate change and A. gambiae-dependent malaria distribution

In light of the now-known factors determining A. gambiae distribution it is possible to project the future distribution of malaria in the face of global climate change. The gradual warming of the planet, approximately 3°C since 1950 (Doyle, 2013), predicts that regions previously climatically shielded from the malaria vector will now become vulnerable. A 1999 study by Dutch and English biologists predicts a general encroachment of the disease further into the northern hemisphere (Martens, Kovats, Nijhof, et al., 1999) (see graph below). These authors agree that the “absolute latitudinal limits of the population at risk are set by the current distribution of the vector.” Thus, the management of the malaria disease is really the management of its vector. The most widely deployed solution to preventing the malaria-bestowing mosquito bites are insecticide treated nets (ITNs) placed above, surrounding, the bed and around the home. The distribution of these ITNs and other insecticides is a primary mandate of both the World Health Organization (WHO) and UNICEF.

“Potential malaria transmission changes (P. falciparum) for the 2020s, 2050s and 2080s due to climate change, compared to the baseline (1961–1990), due to the HadCM3 climate change scenario (based on monthly mean temperature and precipitation). (b) Potential malaria transmission changes (P. vivax) for the 2020s, 2050s and 2080s due to climate change, compared to the baseline (1961–1990), due to the HadCM3 climate change scenario (based on monthly mean temperature and precipitation)” Source: Martens et al, 1999.

“Potential malaria transmission changes (P. falciparum) for the 2020s, 2050s and 2080s due to climate change, compared to the baseline (1961–1990), due to the HadCM3 climate change scenario (based on monthly mean temperature and precipitation). (b) Potential malaria transmission changes (P. vivax) for the 2020s, 2050s and 2080s due to climate change, compared to the baseline (1961–1990), due to the HadCM3 climate change scenario (based on monthly mean temperature and precipitation)”
Source: Martens et al, 1999.

 

III. Climate change and Plasmodium falciparum

Regarding climate and P. falciparum the CDC states that “[generally], in warmer regions closer to the equator [1)] transmission will be more intense [and 2)] Malaria [will be] transmitted year-round” (“Where malaria occurs,” 2010). This suggests that warmer, wetter regions are better suited for P. falciparum development. The rise in global temperature also, via the water cycle, predicts greater humidity for coastal regions. An elevation of these two factors, temperature and moisture, key for the speed of biological life cycles, predicts that the rate of P. falciparum reproduction will increase, dovetailing with the second CDC statement above. Overall, we can expect P. falciparum to reproduce thus infect more intensely in regions currently effected and to be viable, that is, able to survive, and even thrive, in regions previously climatically limited.

It is difficult to predict the impact of climate change on the protozoan itself because the protozoan’s survival and proliferation is dependent upon its vector. Therefore, any statements made can only be generalizations. Interestingly, no work has been done to directly relate temperature to reproductive rate in P. falciparum. In light of the impending shift in vector dynamics, this line of inquiry seems both relevant and valuable.

IV. Cumulative impact on human hosts

The research above suggest that the malaria disease system will grow both in distribution and intensity in the years to come. Practically, for humans this insinuates that both the population of mosquitoes and the frequency at which humans are bit by mosquitoes will also increase. Research published in the Malaria Journal finds that malaria tends to lower the density of erythrocytes, red blood cells, in humans (Emami, Ranford-Cartwright, Ferguson, 2012). This would seem to account for the high rate of sickle-cell anemia endemic to Africans and citizens of African descent.

When viewed as an evolutionary force with magnitudinous adaptive pressure, it would not be unreasonable to expect the human population to gradually develop an immunity to the pervasive parasite, trending towards the genetic makeup manifested by African individuals. Recalling the introduction of slavery to the United States, this immunity to malaria is precisely the reason why African slaves were preferred to European indentured servants.

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