The life cycle of Plasmodium parasites depends on temperature

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[CHICAGO] Climate change could have unexpected effects on the incidence of malaria — even reducing it in unforeseen places, says an insect expert.

In malaria-prone areas, the daily fluctuations in temperature that global warming might bring could threaten a critical phase of the malaria parasite's life cycle, according to Matthew Thomas, of Pennsylvania State University in the United States. Cases in these areas could drop as a result.

His findings illustrate the complexities faced by researchers trying to understand the effects of climate change on infectious diseases, the annual meeting of the American Association for the Advancement of Science (AAAS) in Chicago, United States, heard last week (14 February).

Thomas' insights also reveal, conversely, that some cooler areas might be more severely hit by rising temperatures than previously thought.

The life cycle of Plasmodium parasites is known to depend on temperature. They are sucked up from human blood by mosquitoes when they bite. They multiply and mature in the mosquito, eventually passing back to a human during a later bite.

Most mosquitoes live for two to three weeks so for the parasite to be passed on, it must develop within that time. Below about 16 degrees Celsius parasites do not develop sufficiently fast; and they are killed by temperatures above 40 degrees Celsius.

But, said Thomas, there is a critical window of the parasite's life when it actually needs to be cool. For the first 12 hours after entering the mosquito, it requires a temperature below 31–32 degrees Celsius or it will die.

Since mosquitoes bite at night, a feeding late at night puts the parasite at risk of not completing its 12-hour incubation before temperatures rise.

"If climate change increases the frequency of days when the temperature quickly exceeds the threshold temperature, then entire cohorts of mosquitoes could fail to develop the parasite," said Thomas.

Conversely, the spread of malaria up mountains as a result of rising temperatures might have been underestimated.

In these marginal areas projections of malaria incidence are based on a rise in mean temperature from, say, a malaria-free 15 degrees Celsius to a parasite-friendly 17 degrees Celsius.

But the mean temperature could disguise large fluctuations during the day. For a substantial part of the day temperatures might be much higher than the mean — and the parasite's development would accelerate.

Thomas said that some predictions about the effect of warming on malaria incidence may be wrong by as much as 50–100 per cent because of these findings.

His predictions were supported by new, unpublished data about the incidence of malaria in the western Kenya highlands, which associate the small observed temperature increase of 0.5 degrees Celsius with a four-to-five-fold increase in the incidence of malaria.

The scientist behind the research, Mercedes Pascual, of the University of Michigan, Ann Arbor in the United States, told the meeting: "The predictions we make [using current models] are below the observed number of cases."