Vail Pet Talk: Facts about West Nile virus | VailDaily.com
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Vail Pet Talk: Facts about West Nile virus

Nadine Lober
Pet Talk
Vail, CO Colorado

VAIL, Colorado – The topic of West Nile has popped up a few times recently. I will reiterate some facts about the virus. This virus emerged in North America in 1999. Historically, this virus spanned through Africa, the Middle East and much of India and Australia. West Nile virus was traditionally associated with mild fever in people and was not a pathogen of major concern, but the West Nile virus strain introduced into the U.S. has caused sickness and death in people, horses and mostly in birds.

The virus is perpetuated in a bird-mosquito cycle, in which the infected mosquito transmits the virus to a bird. The virus then circulates in the bird’s blood. All mosquito species tested support West Nile virus replication. Studies show that infected mosquitoes transmit the virus both while feeding on hosts and when certain bird species consume them. The virus in mosquito saliva is thought to be the critical factor in viral transmission. Once infected, mosquitoes may harbor and transmit the West Nile virus for the remainder of their lifespan. Believe it or not, mosquitoes can hibernate over the winter, which likely contributes to the re-emergence of West Nile virus in the springtime. Case of West Nile virus infection are highest in late summer and early fall, the hottest time of the year.

Birds are affected the most when carrying the West Nile virus. Birds will usually die within hours of showing clinical signs: lethargy, inappetence, dehydration, emaciation, pale beak, open-mouth breathing, ataxia, circling, etc. Mortality among birds vary with the species, as well as the bird’s age and the viral strain.



Horses in the U.S. are probably second to birds in frequency of clinical disease from West Nile virus infection. Most horses develop a low-level virus in the blood after exposure. Importantly, illness in horses occurs after the virus has been in the blood, unlike in humans. It is unlikely for virus-infected horses to spread disease to other horses or handlers, because the virus is absent from feces and saliva of the affected horses. Maybe, 90 to 99 percent of horses exposed to West Nile virus experience subclinical infections. Those that do develop clinical disease may exhibit a mild course such as lethargy, or they may develop neurological signs. Signs initially may be of colic, low grade. The most common sign at presentation is muscle tremors. Other neurological manifestations include hindquarter or forequarter weakness, circling, head-pressing or hypersensitivity. Disease may resolve or progress usually rapidly, and lead to seizures. About 60 to 70 percent of horses treated for West Nile virus infection survive.

Now, here is the concern with dogs and cats. Happily, both the experimental data and the rarity of cases of disease in dogs and cats from West Nile virus does not make this an issue of major concern. However, both dogs and cats can develop severe or even fatal disease from West Nile virus infection. There was a cat euthanized in New York due to neurological signs, and West Nile virus was isolated from the cat’s brain. A modest number of clinical cases of West Nile virus infection in dogs were reported to the Centers for Disease Control and Prevention in 2002 and 2003. Studies of dogs’ blood indicate that many dogs and cats are exposed to West Nile virus. In recent studies, none of the dogs infected with the mosquito bites showed signs of illness. There were a few that developed the virus in the blood, but no clinical signs of disease were noted. So, our domestic pets, dogs and cats, are not at a great risk of severe disease.



The best treatment for horses is prevention with the vaccination. Mosquito repellants are important. Shelter should be provided for the animals at dusk and dawn, when mosquitoes feed the most. Remove standing water when possible, where mosquitoes lay eggs. Frequent water changes in tanks will prevent mosquito eggs from hatching.

West Nile virus is difficult to control, poorly understood and will be present for a while.

Veterinarian Dr. Nadine Lober can be reached at 970-949-7972.


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