Do parasites like it hot?

Today, I’m writing about a 2013 Ecology Letters paper entitled “Optimal temperature for malaria transmission is dramatically lower than previously predicted.”

Ro is the number of new disease cases that arise when just one infected individual is added to a susceptible population.  This is an important parameter for disease transmission models.  (Check out the wikipedia page for more information.)

Many factors affect Ro.  For malaria (Plasmodium falciparum), most of these factors are related to mosquito and parasite biology.  These are life history traits like the rate at which mosquitos bite people, the competence of the mosquito vector, and the mortality rate of the adult mosquitoes.  Mordecai et al. (2013) knew that these life history traits often don’t have linear relationships with temperature.  Instead, these life history trait rates tend to increase with temperature until some optimum, and then decline.


As it turns out, many biological responses have these nonlinear relationships with temperature because above some optimal temperature, enzymes/proteins become denatured. Photo Credit.


Using nonlinear estimates of the relationships between temperature and life history traits from the literature, Mordecai et al. (2013) mathematically modeled Ro.   They found that Ro peaked at 25°C, which is 6°C less than the peak Ro predicted if temperature is assumed to linearly affect life history traits and Ro.   Neat!


If you assume that temperature has a linear effect on life history traits, peak Ro occurs at 31°C (dotted line). If you allow for nonlinear relationships, peak Ro occurs at 25°C (solid line). Stylized results from Mordecai et al. (2013). 

You might be wondering why this 6°C difference is important.  First of all, the 6°C change equates to going from 77°F to 87.8°F, which is a big change!  Imagine which of those temperatures you’d prefer to work in on a summer day.

Second, temperature is one component of the global climate that is expected to change with climate change.  When people try to model how the range of malaria might shift with climate change, they need to know how mosquitos and the malaria parasite (and thus Ro) respond to various temperatures.  And the answer is:  that response is nonlinear, and the peak Ro is around 25°C! 


Predicted temperature increases around the world for 2070-2100. Photo Credit.

What do you think?  Modeling predicted range shifts is complicated business, and temperature is just one consideration.  Do you think malaria might end up in your continent/country/state/city?


Mordecai, E.A., K.P. Paaijmans, L.R. Johnson, C. Balzer, T. Ben-Horin, E. de Moor, A. McNally, S. Pawar, S.J. Ryan, T.C. Smith, K.D. Lafferty.  2013. Optimal temperature for malaria transmission is dramatically lower than previously predicted.  Ecology Letters 16(1): 22-30.