Spotlight on Zoo Science
May 19,
2003
West Nile Virus on the Move
A National Zoo scientist and his Czech colleague investigate the spread of West Nile virus.
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| American crows are highly susceptible to West Nile virus. (Photo by Greg Gough) |
In August, 1999, a resident of New York City became ill with what would be diagnosed as the first documented case of West Nile virus (WNV) in the New World. The disease, more typically found in Africa, can cause encephalitis—an inflammation of the brain due to a viral or bacterial infection. The principal vectors or transmitters of WNV are mosquitoes, especially Culex pipiens.
The disease was first described in a human patient in Uganda in 1937. By the 1950s, outbreaks occurred in Egypt and Israel. By the next decade, West Nile virus spread to horses in France and Egypt. Then, in 1999, horses and humans in North America came down with the disease. Clearly WNV had “jumped” from the Old World and was spreading. But how?
John Rappole, a National Zoo scientist based at the Conservation and Research Center in Front Royal, Virginia, and colleague Zdenek Hubalek, from the Institute of Vertebrate Biology in the Czech Republic, investigated the introduction and movement of WNV from its New York City origin point as it spread to the north, south, and west.
The most likely candidates for transmitting the disease to North America were birds (either imports, usual migrants, or storm-driven birds blown off course). Once WNV was established in New York, its spread fairly rapidly—roughly 27 miles per month. By the end of 2002, the virus was reported in every state of union except Oregon, Nevada, Utah, and Arizona.
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| American crows may be responsible for spreading West Nile virus to people. (Photo by Greg Gough) |
Rappole and Hubalek investigated migratory birds as well as non-migratory birds as possible hosts that could spread WNV. The two scientists first believed that susceptible New World migratory birds might be the principal transmitters that brought the virus to sites hundreds of miles from New York.
However, if New World migrant birds were the culprits, one would expect the virus to move hundreds of miles in a matter of days. Rappole and Hubalek did not find this pattern of movement. “Instead,” wrote the two researchers, “the virus moved a maximum of 300 km [about 116 miles] during three months of known activity in 1999, and another 400 km [154 miles] during seven months of known activity in 2000. This rate of travel is very slow if migratory birds are involved.” Also, migrants move on a north-south axis and the virus moved nearly as far west as it did north and south.
These facts led Rappole and Hubalek to consider especially susceptible non-migratory birds, such as house sparrows (pictured above left) and American crows, as even more likely candidates for spreading WNV.
House sparrows, for example, travel significant distances during dispersal periods—a young adult can travel six or more miles from its point of origin. House sparrows were introduced into North America in New York City in 1851. By 1886, 35 years later, they were established in south Texas. Today, house sparrows populate a vast area throughout North America, from central Canada south to Nicaragua.
The jury is still out on which candidate is the principal vector (transmitter) of WNV. Rappole and Hubalek believe there may be more than one type of carrier. They are seeking more evidence regarding migratory birds because the virus has reached the Caribbean island of Cayman Brac, 215 miles south of Key West. While susceptible non-migratory birds very likely play a major role in the spread of the disease, migratory birds and wind-blown mosquitoes (and possibly even ticks that hitch-hike on avian migrants) may transmit WNV to distant locations.
Meanwhile, other scientists are also collecting more data on the spread of West Nile virus. In the near future, researchers will no doubt understand more about which species are transmitting the disease.
At the moment, concern is growing over what will happen when West Nile virus reaches the New World Tropics—as it eventually will. Rappole and Hubalek predict that WNV will spread rapidly through the region, given the year-round abundance of both mosquitoes and susceptible birds. The two scientists point out that the “likely results of such an epidemic are unknown.”
Yet, Rappole and Hubalek remain hopeful that, for animals and humans in the New World tropics, previous exposure to similar viruses, such as yellow fever, may confer some level of immunity and moderate the damage that WNV may cause.
Reference: J. H. Rappole and Z. Hubalek. Migratory birds and West Nile virus. Journal of Applied Microbiology, 94, 47s-58s.
For more information, visit the Centers
for Disease Control website.
