West Nile virus is closely linked to birds. When it first appeared in 1999 some rare species of birds died suddenly in a New York zoo. At the same time, several people came down with encephalitis (a disease which affects the brain). It was later found to be West Nile virus, a virus never before seen in the United States. It is called "West Nile" because it was first identified in the West Nile sub-region in the East African nation of Uganda in 1937. Over the next five years, the virus spread across the continental U.S., north into Canada, and southward into the Caribbean Islands and Latin America. WNV is now in Africa, Asia, Australia, the Middle East, Europe and in the U.S.
Currently in the U.S., we are experiencing one of the worst epidemics. The disease has been reported in people, birds or mosquitoes in 48 U.S. states.
How did is spread so far and wide? Migratory birds play a role in the spread of infectious diseases like West Nile. Their ability to travel over long distances and through a variety of habitats exposes them to a wide range of microorganisms. Migratory birds carry the virus and the mosquito picks up the virus by biting birds, and then spreading the virus by biting other birds, humans and other animals. Yes birds get mosquito bites too! Birds are the only reservoir of WNV. Although other animals, including several mammals and captive alligators, are susceptible to WNV infection, there is currently no evidence that animals other than birds naturally develop a high enough virus load to transmit the infection to an uninfected mosquito.
In some species of birds like crows and robins, the infection is fatal in 4-5 days. This bird fatality cycle been shown to appear 15-16 days before humans become ill. This may be due to the high mortality, and thus depletion of the preferred hosts, (the specific bird species). The mosquitoes become less selective and begin feeding more readily on other animal types, such as humans and horses which are considered incidental hosts.
During the original outbreak 13 years ago, large numbers of dead birds were found. Native bird populations in the U.S. were not previously exposed to the virus. It is not unusual for a new disease to cause high rates of infection or death because they do not have natural immunity to the infection. Currently, WNV has been detected in more than 225 wild and captive bird species.
Signs of infection in wildlife, like in humans, can range from no symptoms to severe symptoms of neurologic illness. Commonly reported signs in animals have included: weakness, stumbling, trembling, head tremors, inability to fly/walk and lack of awareness that allowed them to be easily approached and handled.
In one location, the virus was found in 16 dead birds recently, more than those found in the entire year for each of the past three years. The big question is: has the virus mutated. A number of studies are indicating that we are definitely seeing evolution of the virus.
The number of U.S. cases of West Nile virus rose 25 percent in the past several weeks, and the number of deaths jumped 32 percent. So far this year, 1,993 cases have been reported to federal health officials, up from 1,590 reported the week before, the U.S. Centers for Disease Control and Prevention said in its weekly update of outbreak data. A total of 87 people have now died from the disease compared with 66 reported one week ago.
Virologists blame this year's resurgence partly on a mild winter, which may have helped the virus to persist through the season, followed by a scorching summer, which could have sped up replication.
West Nile virus can also be transmitted directly from infected adult mosquitoes to their eggs, so that newly hatched aquatic larvae are born infected. There are at least 36 species of mosquitoes that can carry the WNV. This is unusual as most illnesses carried by mosquitoes are usually narrowed down to only a few species of mosquitoes. The virus can persevere through the winter, even in many northern states in mosquito eggs.
Joanne Stolen is retired from teaching microbiology at Rutgers University.