West Nile Virus Transmission Linked to Land Use Patterns and 'Super-Spreaders'

ScienceDaily (Oct. 20, 2011) — After its initial appearance in New York in 1999, West Nile virus spread across the United States in just a few years and is now well established throughout North and South America. Both the mosquitoes that transmit it and the birds that are important hosts for the virus are abundant in areas that have been modified by human activities. As a result, transmission of West Nile virus is highest in urbanized and agricultural habitats.

"The virus has had an important impact on human health in the United States partly because it took advantage of species that do well around people," said Marm Kilpatrick, a biologist at the University of California, Santa Cruz, who studies the ecology of infectious diseases.

West Nile virus can infect a wide range of animals, including more than 300 species of birds and 60 species of mosquitoes. It also infects mammals, reptiles, and even amphibians. But researchers have found that in most places only a few key species of bird "hosts" and mosquito "vectors" are important in transmission of the virus.

"We now know that in any given location, only one or two species of mosquitoes play a big role, and only a handful of birds appear to be important in overall transmission rates," said Kilpatrick, who reviewed a decade of research on the ecology and evolution of West Nile virus in a paper published in the October 21 issue of Science.

According to Kilpatrick, the familiar American robin plays a key role in the transmission of West Nile virus across much of North America. It is such an important host species that Kilpatrick calls robins "super-spreaders" of West Nile virus. The reason is not so much the abundance of robins, but rather the feeding patterns of the mosquitoes that transmit the virus. The mosquito species important in transmission seem to prefer robins over other, more abundant species of birds such as house sparrows.

"Robins are more important in transmission than their abundance alone would suggest," Kilpatrick said. "The peculiar feeding habits of the vectors play a really important role in transmission, and this idea applies to many different diseases. It's one of the really interesting things we've learned from the past decade of research on West Nile virus."

Insights gained from research on West Nile virus could help public health officials deal with other introduced diseases in the future. "The spread of disease-causing organisms is likely to only increase in the coming years," said Sam Scheiner, director of the Evolution and Ecology of Infectious Diseases program at the National Science Foundation (NSF). "West Nile virus has provided a test of our ability to respond to such spread. This research shows that predicting disease incidence in humans and other animals is more complex than first imagined, but that greater understanding of such complexities is possible--knowledge that can be applied to the next threat."

The globalization of trade and travel has spread many invasive species, including infectious pathogens like West Nile virus. Although its exact route of entry to New York is unknown, West Nile virus may have arrived in an infected mosquito carried across the Atlantic by an airplane, Kilpatrick said. The virus then adapted quickly to its new environment, evolving a new strain that was transmitted more efficiently by local mosquitoes than the introduced strain. By 2005, the new strain had completely displaced the introduced one throughout North America.

Three species of mosquitoes are key vectors for transmitting West Nile virus in much of North America. Interestingly, these mosquitoes are not among the species that feed frequently on people. They are bird specialists that happen to bite people often enough to cause human infections. "The mosquitoes that bite humans most are actually not as important in transmission of West Nile virus to humans because they rarely bite birds and thus rarely get infected in the first place," Kilpatrick said. "Instead, it's the species that feed mostly on birds and frequently get infected, but occasionally feed on people, that are most important."

Millions of birds have died from West Nile virus infection, with dramatic effects on the populations of some species. Crows, for example, are much less abundant than they were before the virus arrived. The robin population, which had been growing rapidly, has leveled off.

"Robins were on a steady upward trajectory thought to be linked to human land use--they love lawns and agricultural fields," Kilpatrick said. "Crow populations were growing even faster. Now crow populations have crashed downward and robins have leveled off, and we suspect that's due to West Nile virus."

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New Threat Closes in On Iconic Galápagos Wildlife

ScienceDaily (Sep. 17, 2011) — Renewed vigilance over the biosecurity of the Galápagos Islands is needed, based on new research on the risk posed by West Nile virus. Scientists from the Zoological Society of London (ZSL), the University of Leeds and the New York State Department of Health, together with the Galápagos National Park Service and University of Guayaquil, have been studying the disease threat posed by Islands' mosquito populations. They have discovered that a species of these biting insects is capable of transmitting West Nile virus, a potentially dangerous disease for the archipelago's unique wildlife.
West Nile virus (WNV) most commonly affects birds, but can infect mammals, including humans, and reptiles. Previous studies of West Nile virus impact in the USA have linked the virus to declines in several bird populations, demonstrating the high risk it poses to the Galápagos' endemic species. The virus recently invaded South America, but has yet to reach the Galápagos.

Recent studies on tourist boats and planes have shown that the mosquito species Culex quinquefasciatus (also known as the Southern house mosquito) is hitching a ride onto the Galápagos on airliners. Culex species are well-known vectors of WNV elsewhere in the world, so their presence on the Islands has caused concern amongst the scientific community.

The ability of mosquitoes to transmit particular disease agents effectively often varies between species, or between populations within species. Therefore to understand the risk posed by C. quinquefasciatus in Galápagos, the research team measured the ability of Galápagos C. quinquefasciatus to pick up and transmit WNV in the lab, under conditions that simulated those in the wild. They found that Galápagos C. quinquefasciatus were indeed effective vectors for the virus.
Prof Andrew Cunningham from ZSL says: "We now know that mosquitoes capable of carrying West Nile virus have a route onto the Galápagos, and once there, the virus could also spread into the local mosquito population. This means there is potential for large impacts on endemic species. There is no doubt that West Nile virus poses a serious threat to the survival of the Galápagos' iconic wildlife."
In order to reduce the chances of West Nile virus reaching the islands, the authors suggest further research to determine the presence of WNV in the mainland Ecuador, plus strict enforcement insect control measures on aircraft and ships moving between the mainland and islands.

Dr Simon Goodman from the University of Leeds says: "Piece by piece we are building up a comprehensive picture of the disease ecology in Galápagos and what could happen if WNV were to reach the islands. Once WNV has been introduced onto the Galápagos, it would be much harder to contain. Therefore the best strategy is to have strict preventive measures to reduce the chance of the disease reaching the islands in the first place."

Lead author PhD student Gillian Eastwood says: "Whilst WNV does not yet exist in Galápagos, it is important to envisage what future disease scenarios could be by looking at how this particular virus would interact within this unique ecosystem. Evaluating the role that mosquitoes could play is therefore vital. This recent part of our work is however only one aspect to understanding potential WNV transmission on the Islands; it remains to see how severely Galapagos wildlife might be affected but all precautions should be taken."

The research is published in the current edition of the American Journal of Tropical Medicine Hygiene.

http://www.sciencedaily.com/releases/2011/09/110916102406.htm