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Fluctuating temperatures affect malaria transmission
Climate change is already bringing
malaria to altitudes that were previously too cold for the mosquitoes that
transmit the disease to survive long enough to pass it on. Crucially, in such
areas people haven’t built up natural immunity. Now it seems that fluctuations
in temperature during the day, not just the average temperature, are also
important.
Malaria transmission is a fairly complex
process and many of the parameters are temperature sensitive, explained Matthew
Thomas of Pennsylvania State University at the AAAS Meeting. A female Anopheles
mosquito must bite an infected human to pick up the malaria parasite, which
takes about ten to 14 days to incubate inside the mosquito, depending on temperature.
It turns out that the incubation period is also affected by temperature
fluctuations, perhaps by as much as 50-100%. This is crucial because if the
mosquito dies before the parasites have incubated, it won’t be able to pass on
the disease even though it is infected.
“At the moment we predict using
daily averages, but for malaria daily fluctuations matter,” said Thomas.
“Daily temperature fluctuations around a warmer average lengthen the
incubation time; temperature fluctuations around a cooler average shorten it.
We may be overestimating the risk in warm areas and underestimating it in
cooler areas.”
Temperature fluctuations also affect the
speed of development of mosquito larvae, which need about 10-20 days to mature
in water. To increase the chances of disease transmission it takes a certain
number of mosquitoes - and this number must build up rapidly. As for incubation
of the parasite, temperature fluctuations around higher temperatures slow the
process and variations around lower temperatures speed it up, which could mean
that scientists are underestimating the risk of disease transmission in cooler
locations.
Climate change could well affect both
daily average temperatures and the extent of the fluctuations. To complicate
the picture further, malaria parasites are highly sensitive to temperature -
particularly in the first 12 hours after they have infected the mosquito - and
will die if they get too hot. Although most mosquitoes feed at night some do
feed earlier in the day, which could remove malaria from some areas as climate
change progresses.
Thomas says it’s possible that, while
the areas where malaria occurs shift as a result of climate change, there may
be no net change overall. But the introduction of malaria into new areas will
have serious consequences as the local population will not have built up
natural immunity.
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