NEWS RELEASE
New York, NY, June 2008: Scientists at Wildlife Trust’s Consortium for Conservation Medicine (CCM), New York, and the New York State Department of Health’s Wadsworth Center report in a paper published today in the journal Public Library of Science Pathogens that warmer temperatures helped a new strain of West Nile virus invade and spread across North America.
The introduction of West Nile virus to New York City in 1999 was accompanied by a die-off of crows and other birds, and 62 human cases, including 7 deaths. In the two years following the introduction, transmission of West Nile virus was relatively low, with only 21 and 66 reported human cases, in 2000 and 2001, respectively, as the virus spread along the Atlantic seaboard. However, in 2002, a new strain of the virus emerged and rapidly spread throughout North America, completely displacing the old strain by 2005. Coincident with the spread of this new strain were two of the largest epidemics of West Nile virus in North America recorded to date, with 11,356 cases in 2003, and 4,582 cases in 2002, including more than 270 deaths in both years. Since then, there have been 2,500 to nearly 6,000 cases and over 100 deaths each year, and the virus shows no sign of retreating.
Dr. A. Marm Kilpatrick, with Wildlife Trust’s Consortium for Conservation Medicine, and the University of California, Santa Cruz, and Dr. Laura Kramer’s laboratory at the New York State Department of Health’s Wadsworth Center conducted a study that examined the impact of different temperatures on the transmission of the two viral strains. They found that increases in temperature greatly accelerated the transmission of the virus by mosquitoes, and showed that the advantage of the new strain increased with warmer temperatures. Thus, warmer climates helped facilitate the invasion of the new strain and its spread across North America. As a result, even a few degrees increase in temperature, such as those currently occurring as a result of global warming, would sharply increase the efficiency of viral replication, and possibly increase the intensity of West Nile virus epidemics in some regions. Crucially, the study provides a model to predict the impact of increasing temperatures on West Nile virus transmission by mosquitoes.
This new research set out to determine how the new strain of West Nile virus had displaced the introduced strain, and what effect temperature had on transmission by mosquitoes. Dr. Kramer explained, “A previous study in our lab demonstrated that the new strains of the virus were more efficient at replicating in mosquitoes, which may have increased the intensity of epidemics seen in the field. We wanted to examine whether temperature might have played a role in the invasion of the new strain, and whether its success may have been related to increasing temperatures.” Dr. Kramer’s lab performed a series of laboratory studies that involved infecting one group of mosquitoes with the introduced 1999 strain of West Nile virus, and siblings with the recently evolved strain, holding the mosquitoes at different temperatures and for different lengths of time, and then determining what fraction could transmit the virus. They found that at 32°C/90°F both strains could occasionally be transmitted as early as 12 hours after feeding on infected blood but the new strain was more efficient than the introduced strain at nearly all temperatures and time points after infection. Dr. Kilpatrick, who analyzed the data and developed models to predict the impact of temperature on transmission, stated, “the results provide a striking example of how climate and evolution can interact to increase the transmission of this virus. They also suggest that relatively small increases in temperature can have large impacts, due to the nonlinear acceleration of transmission with temperature.” Dr. Peter Daszak, Executive Director of Wildlife Trust’s Consortium for Conservation Medicine stated, “This study shows how direct the impacts of climate change could be for us all — it isn’t just about a rise in sea-level or the melting of a glacier in Alaska — it’s also about our health and welfare.”
Source: Wildlife Trust
EVOLUTION AND CLIMATE INTENSIFY WEST NILE VIRUS TRANSMISSION
