Climate Change & Malaria

Impacts of temperature. Malaria is caused by a parasite transmitted to humans via mosquitoes (the vector). Development of the

mosquito and, even more, of the malaria parasite is temperature-sensitive. Temperature can influence the timing and intensity
of malaria transmission by modifying the development speed of the parasite, the frequency of blood feeding by adult female
mosquitoes, and the time it takes malaria parasites to mature inside them (Githeko and Ndegwa, 2001). The lower temperature
threshold for the Anophelesmosquito vector  for biological activity is 8o C to 10o C (Patz and Olson, 2006).
 
The minimum temperature threshold for transmission of the Plasmodium falciparum
parasite  is 16o C to 19o C, and the maximum temperature threshold
is 33o C to 39o C (Patz and Olson, 2006).
 
Outside these thresholds, the parasite’s development may slow or stop (Githeko, 2009).
Temperature has a strong effect on the time it takes for the parasite to develop, mature and become infectious in the mosquito (i.e.,
the incubation period). For example, it takes malaria parasites 56 days to mature in the mosquito at 18° C (which is longer than the
23-day average life span of Anopheles gambiae mosquitoes), 19 days at 22° C and only 8 days at 30° C (Githeko, 2010).
 
However, given the high complexity associated with emergence of malaria epidemics, drawing a direct correlation between
temperature and malaria risk is unwise. Uncertainties remain regarding parasite-mosquito-temperature interactions. Moreover,
general epidemiological models use mean monthly temperatures as their variable, rather than diurnal temperature fluctuations,
which appear to be much more important than monthly averages in influencing the parasite’s life cycle (Paaijmans et al., 2009). Many
studies also use air temperature alone (versus water temperature) when assessing the influence of temperature on the development
rate of mosquito vectors, while mosquito larvae live in aquatic habitats (Paaijmas et al., 2010; Githeko et al, 2000).
 
Impacts of precipitation and humidity. Rapuro (n.d.: 3) points out that “warm conditions allow both the Anopheles [gambiae]
mosquito and the malarial parasite it carries to develop more quickly” and “wet conditions increase mosquito life expectancy
and provide breeding habitats.” Rainfall generally increases vector populations by creating new breeding sites for mosquitoes in
stagnant water (CCD Commission, 2009: 7). But heavy rains and floods can also wash away breeding sites, killing mosquito larvae
and potentially decreasing malaria transmission (CCD Commission, 2009; Imbahale et al., 2011). Githeko (2010:12) finds that in the
Africa “rainfall will normally increase the vector population 2–3 fold” and unusual heavy rains extend mosquito
breeding season, and both of these can lead to bigger epidemics.
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