West Nile Virus has been around at least since 1937, when it was first identified in the blood of a woman in the West Nile district of Uganda. Since that time, the range of the mosquito-borne virus, which often kills mammals and birds, has expanded to include Africa, Europe, the Middle East, west and central Asia, and, recently, North America.
In the early years, outbreaks of West Nile were short-lived and not overly virulent, and the expansion of the range of the virus was relatively slow. But upon reaching this country, the disease has taken off alarmingly. A large die-off of crows in the New York City area in August 1999 was the first time the disease was identified in the U.S. Just four years later, the Centers for Disease Control and Prevention (CDC) recorded 9,858 human cases in 47 states resulting in 262 deaths – all in 2003. West Nile Virus has now spread to Canada and Central America.
Just why the disease has been so fast-spreading and more lethal here than in Europe, Africa, and Asia has, up until now, been a mystery, but Dina Fonseca and her colleagues at the National Museum of Natural History in Washington, D.C., seem to have found at least part of the answer: a new, hybrid mosquito.
Fonseca collected mosquitoes in the Culex pipiens complex from 33 places around the world and examined their genetic fingerprints. That group of mosquitoes is considered to be the most important carrier of West Nile Virus, but it makes life difficult for entomologists because members of the C. pipiens group look very much alike yet differ markedly in their behavior and physiology.
In northern Europe, for example, there are two distinct populations of C. pipiens: one bites birds and hibernates in the winter; the other bites mammals and spends winters in sewers and subways. Only rarely do the mammal-biting mosquitoes go after birds or the bird-biting mosquitoes go for mammals.
In the U.S., however, more than 40 percent of the C. pipiens that Fonseca and her co-workers checked are a genetic hybrid of the two European strains. They feed on both birds and mammals and efficiently ferry the disease between the two.
West Nile virus lives in mosquito saliva and infects birds when a mosquito slurps up a meal of blood. The virus then multiplies in the bird for one to four days, long enough for another biting mosquito to pick up the disease. Most birds survive, but still the disease often takes its toll, especially on crows and jays.
If, instead of moving on to another bird, the infected mosquito bites a horse or a human, there might well be big trouble. Approximately 40 percent of infected horses die, and an unknown percentage of humans become sick. Despite becoming ill from the disease, humans and other mammals usually do not develop enough of the virus in the blood to re-infect mosquitoes.
In humans, West Nile Virus infections usually cause fever, headache, and body aches and are not life threatening, but if the virus crosses the blood-brain barrier, it causes encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord) and is often deadly.
Together, concentrated human populations in cities and suburbs and susceptible migrating birds have set the table for West Nile Virus. North American birds have had no previous exposure to the virus, and, consequently, with no immunity, they are no match for it. West Nile Virus has been identified in the dead bodies of at least 138 different species of birds.
Other mosquito species in parts of the U.S. besides C. pipiens also bite both birds and humans, and they, too, may be spreading the disease and perhaps are another reason for the virus’s virulence here. In addition, Harry Savage, of the CDC in Colorado, believes the viral strain here in North America may be unusually harsh.
How did these hybrid mosquitoes evolve? The mammal-biting mosquitoes in northern Europe appear to be recent immigrants from southern Europe and Africa that have been able to move north because subways and sewers provide suitable habitat for them during the winter. It’s likely that some similar mixing of previously separated species is responsible for the hybrid C. pipiens mosquitoes now plaguing North America. West Nile Virus seems to be on the rise in Europe, too, and it’s possible that the mixing of mosquito genes is underway there as well.
All in all, it appears that West Nile Virus is here to stay – a minor villain suddenly cast in a major role thanks to a hybrid mosquito well adapted to the sewers and subways of modern life. If the virus’s rate of increase this summer is anything like it has been since 1999, it’s going to be rough going for all of us: people, birds, and horses.