Spread of West Nile virus
West Nile virus, a virus that hitchhiked from Africa to America, probably via airplanes, swept across the continent in a few short
years. Birds are the virus’s primary host, but when viral loads in the blood are high enough, a mosquito can transfer the infection from bird to other animals, including humans.
“The virus is like a rat or a house mouse,” said Kilpatrick when I interviewed him for an article in the Santa Cruz Sentinel, “an animal that is helped out by human habitation.”
Kilpatrick’s paper in the October 21st issue of Science pulled together research and findings from the past decade.
“It looks like we have kind of set ourselves up,” he said. ” We have this amazing world of globalization, that increases the movement of the disease but also their vectors.”
This trend becomes clear when Kilpatrick pointed out that the disease is more common in urbanized and agricultural areas. These disrupted landscapes provide the habitat for mosquitos, standing water, and the main hosts. Birds most commonly infected with West Nile include the American robin, pigeons and the Common Crow.
American robin via Wikimedia CommonsRobins are what epidemiologists call ‘super-spreaders’. The robins only may be 1 in 10 of the birds, but they may be repsondible for half of all the infected mosquitos.
One example of a super-spreader was Mary Mallon, the infamous “Typoid Mary”, a cook in the New York City area in the early part of the 1900s. She had typhoid but never showed any symptoms. The presentation of a disease without symptoms was fairly unknown then, so health officials had a hard time convincing Mary that she needed to be quarantined. She infected 53 people with typhoid fever.
Another example Kilpatrick gave me was an individual in Bangladesh who got infected with the Nipah virus. The man happened to be a spiritual leader, well respected by the community. Everyone came and paid thier respects, so he became an accidental super-spreader.
Why are robins super-spreaders? Kilpatrick said that scientists don’t know if they are preferentially fed on by mosquitos, but that they suspect that robins don’t defend themselves well (perhaps they don’t avoid mosquitos, or they don’t shake a mosquito off when it bites).
Another trend that Kilpatrick pointed out was that the West Nile virus changed shortly after arriving in North America. By 2005, the virus had evolved into a local variant that completely replaced the original New York strain from Uganda. That local variant is more efficiently transmitted by both C. pipiens and C. tarsali.
It is possible that 2002 and 2003, big years for the West Nile virus nationally, had the perfect weather to encourage the spread of the virus. Or, it could be that once the virus enters an area it can cause an epidemic that declines afterward, perhaps as birds and humans build up immunity to the virus.
“We don’t really know,” said Kilpatrick. “There is kind of a decreasing trend but you don’t want to say that it is all over.”
CDC fact sheet on West Nile virus
CITATION: Kilpatrick, A.M. Globalization, Land Use, and the Invasion of West Nile Virus. Science 21 October 2011: Vol. 334 no. 6054 pp. 323-327.DOI: 10.1126/science.1201010