Identifying areas of malarial infection risk depends more on
daily temperature variation than on the average monthly temperatures, according
to a team of researchers, who believe that their results may also apply to
environmentally temperature-dependent organisms other than the malaria
"Temperature is a key driver of several of the
essential mosquito and parasite life history traits that combine to determine
transmission intensity, including mosquito development rate, biting rate,
development rate and survival of the parasite within the mosquito," said
Justine I. Blanford, research associate in geography, Penn State's the Geovista
While other variables, such as the necessary rainfall to
support mosquito development, also influence malaria transmission, temperature
controls a variety of lifestages of both the mosquito and the malaria parasite.
How the research was
The Penn State researchers include Blanford; Simon Blanford,
senior research associate in biology; Robert G. Crane, professor of geography
and director of Penn State's Alliance for Education, Science, Engineering and
Development in Africa; Michael Mann, Distinguished Professor of Meteorology;
Krijn P. Paaijmans, post doctoral researcher; and Matthew Thomas, professor in
ecological entomology and Center for Infectious Disease Dynamics; and Kathleen
V. Schreiber, professor of geography, Millersville University of Pennsylvania.
The researchers first looked at four locations in Kenya that
represented four different climates, including warm arid conditions and cool
upland conditions. They looked specifically at the Extrinsic Incubation Period,
the length of time it takes for a parasite to complete development inside a
mosquito from initial acquisition through an infected blood meal to
transmission to a host via another blood meal. The researchers published their
results in the current issue of Scientific Reports.
They found that EIP measurements based on mean monthly
temperatures and mean daily temperatures were similar to each other in all
cases. For the measurements based on hourly temperatures, the researchers found
that for warmer locations, the EIPs were significantly longer. For cooler
locations, the EIPs were substantially shorter than those calculated using mean
monthly or mean daily temperatures.
"To estimate 'true' EIP we need to capture the
influence of daily temperature fluctuations using hourly temperature
data," said Justine Blanford. "But hourly temperature data does not
exist for all locations in Kenya or across Africa."
The researchers estimated hourly temperatures using data on
minimum monthly and maximum monthly temperatures. This does not equal the
hourly temperatures but can be compared to the mean monthly temperatures
usually used to determine malaria risk. This estimation method compared well
with the data from the four Kenyan locations.
What the study found
Applying this method to other locations in Kenya, they found that
"mean temperatures overestimate parasite development rate under warm
conditions, provide a good approximation of growth under intermediate
conditions and underestimate development under cool conditions."
When looking at all of Africa, the area in and around the
equator has similar EIP measurements regardless of the method used. However,
there are areas in Africa where current methods over or under estimate EIP by
100% or more.
"The vast majority of ecological studies examining
temperature-dependent effects consider mean temperature alone," said
According to the researchers, daily temperature dynamics
could have marked effects on many species, affecting understanding of both
current ecology and the expected responses to future climate change.