Higher temperatures accelerate the spread of a new strain of West Nile virus
Higher temperatures have been correlated with the spread of a new strain of West Nile virus across North America. The researcher speculate that global warming could sharply accelerate transmission of the virus and possibly lead to more severe epidemics of West Nile virus in some cooler regions.
The first occurrence of West Nile virus (WNV) in the western hemisphere was in New York City in 1999. It has subsequently spread throughout much of the Americas. It is primarily transmitted between birds and Culex mosquitoes. From the period 1999-2007 the WNV has caused a total of 32,135 reported cases, 11,243 cases of encephalitis and 1,125 deaths. In addition, the WNV has evolved over these years and, in 2002, a new strain of the virus emerged and rapidly spread throughout North America, displacing the old strain by 2005. Coincident with the spread of this new strain were two of the largest epidemics of West Nile virus recorded to date in North America, in 2002 and 2003. The authors of this paper set out to determine how the new strain of West Nile virus had displaced the first strain, and what effect temperature had on transmission by mosquitoes. Laboratory tests showed that for both strains, increases in temperature greatly accelerated transmission of the virus by increasing the efficiency of viral replication in the mosquitoes. However, the new strain is more efficient at infecting, disseminating and being transmitted than the older strain, and the advantage of the new strain increases with higher temperatures. As a result, regional temperature increases of just a few degrees due to global warming could sharply accelerate transmission of the virus and possibly lead to more severe epidemics of West Nile virus.
Source: Kilpatrick, A.M., M.A. Meola, R.M. Moudy and L.D. Kramer, 2008. Temperature, viral genetics, and the transmission of West Nile Virus by Culex pipiens Mosquitoes, PLoS Pathogens, 4(6): e1000092. doi:10.1371/journal.ppat.1000092.