West Nile virus infection: Essential facts for small-animal practitioners - Veterinary Medicine
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West Nile virus infection: Essential facts for small-animal practitioners
Your clients may be concerned about this virus in their pets. Are you prepared to answer their questions?


In one experiment, cats fed West Nile virus-infected mice developed systemic viral infections and later seroconverted.12 This study suggests that cats allowed outdoors are at additional risk for contracting West Nile virus, not only from greater exposure to mosquitoes but also through predation. Birds or other small animals weakened by West Nile virus infection may be particularly easy targets for hunting cats. Therefore, hunting and scavenging should be discouraged or prevented to avoid infection through oral exposure.

Enough virus was measured in the bloodstream of some cats12 to possibly infect the most susceptible species of mosquitoes,9 but the level of West Nile viremia in most of the cats was below the amount thought to be necessary to infect feeding mosquitoes.12-14 Thus, like horses,15,16 dogs and cats can generally be considered incidental hosts of West Nile virus, contributing little, if at all, to the environmental maintenance of the virus. Additionally, all animals in the studies cleared West Nile virus from their bloodstreams within days and mounted an antibody response—no evidence of prolonged or persistent infection was found in any of the dogs or cats studied.


Clinical disease from West Nile virus infection in cats is rare but has been documented.17 In fact, an apparently stray, seizing cat in New Jersey was among the first animals found to be infected with West Nile virus when the virus was first detected in 1999.18 Brief episodes of febrility, lethargy, or decreased appetite occurred in several of the cats in the laboratory studies after infection through mosquito bites, but most cats did not display any signs of disease.12,19

Clinical signs were not observed in any dogs infected in the laboratory,12-14,19 but a small number of clinical West Nile virus cases in dogs and other Canidae have been reported as the virus has spread (Bunning ML, United States Air Force; Vandeventer S, Iowa State University; Austgen LE, Colorado State University: Unpublished data, 2005).17,20,21 One large serologic study detected antibody against West Nile virus in more than 26% of 442 dogs and 9% of 138 cats from a combination of shelters and privately owned populations tested,22 indicating that exposure to West Nile virus is common.

The finding of high seroprevalence, combined with the rarity of clinical disease, suggests that the overwhelming number of exposed dogs and cats likely suffer no or mild illness as a result of West Nile virus infection. However, it is important to consider that West Nile virus-induced disease in companion animals may go largely unrecognized until more information about clinical presentation and diagnosis becomes available.

Although few cases have been analyzed, West Nile virus in dogs seems to present much as it does in horses, with signs attributable to any combination of meningitis, encephalitis, and myelitis. The most commonly affected regions of the central nervous system are the meninges, dorsal horn, brainstem, and cerebellar Purkinje cells, but the virus may be found in other areas, including the cortex. Clinical signs are nonspecific and vary greatly with the extent and location of central nervous system involvement; diagnosis cannot be made from clinical signs alone. The most frequent clinical findings in dogs with West Nile virus infection are

  • Ataxia, particularly caudal; may ascend
  • Weakness or paresis
  • An altered mental state, such as listlessness, hyperesthesia, or somnolence
  • Muscle tremors or focal twitching
  • Pain—may not be localizable; varieties include myalgic, meningeal, and abdominal
  • Fever

In horses and people, fever tends to peak at or before the onset of neurologic signs and has often resolved by the time of clinical evaluation. The same is likely true in dogs and cats and some other mammals kept as pets.


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