West Nile Virus

In 1999, West Nile virus made headlines when the disease killed 7 people and sent more than 60 to the hospital in New York. By December 2002, the virus has appeared all over the country, with more than 3,700 human cases confirmed in more than 30 states.

More than 200 deaths have been linked to the virus, and new cases are appearing almost daily. The Center for Disease Control and Prevention considers the West Nile virus to be established in North America permanently.

All mammals and many birds are at risk for the disease. The CDC notes that the West Nile virus poses potentially serious risks to humans and horses, as well as to domestic and wild birds. The CDC's report on the virus stated that the appearance of the virus in North America "may be an important milestone in the evolving history of this virus." Before 1999, the West Nile virus had never been seen in North America. Its appearance is a reminder that any disease is just a plane ride away.

But if you're concerned about yourself and your indoor pets relax. In people, less than 1 percent of those infected develop serious illness. Dogs and cats also can be infected, but as of the end of 2002, there have been few reports of illness in these pets. In the summer of 2002, there was a confirmed case of a dog in Illinois dying from West Nile virus. So far, this has been the only confirmed canine fatality. Despite this, dogs are not considered at significant risk of developing illness from WN virus.

While free-ranging birds have the highest risk of infection and severe illness, indoor birds are at low risk of getting infected. Unfortunatley, a number of horses have died as a result of contracting West Nile virus. However, research has shown that, as with people, most horses infected with West Nile virus recover.

The West Nile virus is transmitted when mosquitoes feed on infected host birds, which may circulate the virus in their blood for a few days. After an incubation period of 10 days to two weeks, infected mosquitoes can then transmit West Nile virus to humans, horses and other animals while biting to take blood. During blood feeding, the virus can be injected into the animal or human, where it may multiply, possibly causing illness.

The virus was discovered in 1937 in Uganda, in the blood of a woman suffering from fever. The virus is a member of the family Flaviviridae, which includes yellow fever and the St. Louis encephalitis virus. (The St. Louis virus is endemic in the eastern United States, causing sporadic outbreaks in humans and horses.)

Signs of Infection

Most birds are considered susceptible to infection, including the ringed-neck parakeet and the vasa parrot. Free-ranging birds are at the greatest risk while indoor companion birds are at the lowest risk. Birds showing signs of damage to the nervous system (such as abnormal head-positioning, circling, wing droop, stumbling, tremors and convulsions) should be evaluated by an avian veterinarian immediately.

The majority of humans and horses infected with West Nile virus remain clinically unaffected or develop only mild and temporary symptoms, followed by complete recovery. The people most at risk include the elderly and those with compromised immune systems. Dogs and cats can contract the virus but, as noted above, few have shown signs of serious illness (though pets with a compromised immune system would be at risk as well). The one confirmed canine fatality occurred in a dog with a compromised immune system. There is no reason to euthanize a dog, cat or horse diagnosed with West Nile virus. Most will recover with treatment or may even spontaneously recover.

The West Nile virus cannot be transmitted from person-to-person or pet-to-person contact. There is also no evidence of a horse-to-person or horse-to-horse transmission of West Nile virus. This means you cannot get infected with West Nile virus by caring for an infected horse, nor can a horse infected with West Nile virus infect other horses in neighboring stalls.

Less than 15 percent of infected humans develop more severe forms of the disease, characterized by aseptic meningitis or encephalitis, hepatitis, pancreatitis or myocarditis. In the few humans that are more severely infected, the virus causes progressive damage to the nervous system that can lead to death. People over the age of 50 are at greater risk.


Like most viral infections, there is no specific treatment for West Nile virus. Birds showing symptoms associated with West Nile virus could also be suffering bacterial, fungal, toxic, cancerous or other types of viral diseases. Following the appropriate diagnostic testing, an avian veterinarian may use antibiotics, immunostimulants, supportive nutrition and anticonvulsants.


There is no proven vaccination against West Nile virus. (A vaccine has been developed for horses, but it is still under study.) The only way to prevent infection is to keep mosquitoes from feeding on vulnerable humans and animals. Mosquitoes develop in standing water, so removing areas where water can stagnate around a home, barn or aviary can help decrease the threat. Spraying pesticides can be used in local areas. However, the advantage of spraying must be weighed against environmental and health risks associated with pesticides.

Bats, some birds and many species of wasp eat large numbers of flying insects, including mosquitoes, and neighborhood programs to increase these mosquito predators should be encouraged.

To learn more about West Nile virus, please click on West Nile virus In-depth.