West Nile sickens animals in different ways | VailDaily.com
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West Nile sickens animals in different ways

Nadine Lober

West Nile virus emerged in North America in 1999. Historically, this virus spanned Africa, the Middle East and much of India and Australia.

West Nile virus was traditionally associated with mild fevers in people and was not a pathogen of major concern. But the West Nile virus strain now causing illness in the U.S. has killed birds, horses and people.



The virus is perpetuated in a bird-mosquito cycle, in which infected mosquitos transmit the virus to birds, which develop a blood virus capable of infecting feeding mosquitos.

Studies show that infected mosquitoes transmit the virus while feeding on people and animals as well as when they are eaten by certain birds.



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. In addition, mosquitos can hibernate over the winter, which likely contributes to the re-emergence of West Nile virus in the springtime.

Case rates of disease from West Nile virus infection are highest in late summer and early fall, the hottest time of the year.

Birds suffer the highest death rate among the important West Nile virus hosts. Birds will usually die within hours of showing clinical signs: lethargy, low appetite, dehydration, emaciation, pale beak, open-mouth breathing, ataxia and circling, among other symptoms. Mortality among birds vary with the species, age and the viral strain.



Horses in the U.S. are probably second to birds in frequency of West Nile virus infection. Most horses develop a low-level viremia – or, virus in the blood – after exposure.

Importantly, illness in horses has been found to occur after viremia has cleared, unlike in humans. But it is unlikely for virus-infected horses to spread disease to other horses or handlers because the virus is absent from feces and saliva.

The best treatment for horses is vaccination though mosquito repellents are also important. Shelter should be provided for horses at dusk and dawn when mosquitoes feed the most.

Another way to attempt the disease is to remove standing water where mosquitoes lay eggs. Frequent changes of water in tanks can also prevent mosquito eggs from hatching.

As for dog and cats, both the experimental data and the rarity of West Nile illness suggest that infections in these pets are sub-clinical in nature. Still, dogs and cats can develop severe or even die from West Nile virus.

There was a cat euthanized in New York after displaying neurological symptoms and West Nile virus was isolated from the cat’s brain. And 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.

Blood samples from dogs and cats indicate that many of these animals are exposed to West Nile virus. But in recent studies, none of the dogs infected with the mosquito bites showed signs of illness, though there were a few that developed viremia. Our domestic pets, therefore, are not at a great risk of severe disease.

Still, West Nile virus is difficult to control, poorly understood and here to stay for a while.

Dr. Nadine Lober 949-7972.


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