The pathogenesis of West Nile virus in dogs, cats, and house sparrows

Abstract

In the 1990s, a more aggressive and neurovirulent form of West Nile virus (WNV, family Flaviviridae, genus Flavivirus, species West Nile virus) emerged that has caused morbidity and mortality in human beings, horses, and most severely, in a large number of avian species. The unexpected enhancement of virulence, rapid global spread, and ready establishment of the virus in a diversity of ecosystems underscore the need to modify and enhance the body of basic knowledge about this zoonotic agent. This work describes what is known about WNV and explores the replication of this virus in three animal species: companion animals, dogs and cats, of potential import due to their close association with human beings; and house sparrows, important avian hosts of WNV. Domestic dogs and cats were infected by mosquito bite and evaluated as hosts for WNV. Each of four dogs developed a viremia of low magnitude and short duration, but did not display signs of disease. Twenty-one of 23 cats became viremic, with peak titers ranging from 101.0-104.2 plaque forming units/ml. Several of the cats showed mild, non-neurologic signs of disease. During the course of infection, WNV was not isolated from saliva of any of the dogs nor cats tested. An additional group of four cats were exposed to WNV orally, via ingestion of infected mice. Two cats consumed an infected mouse on three consecutive days and two cats consumed a single infected mouse. All of these cats developed viremia with magnitude and duration similar to that seen in cats infected by mosquito bite, but none of the four showed clinical signs. These results suggest that dogs and cats are readily infected by WNV. The efficiency of oral transmission observed with cats suggests that infected prey animals may serve as an important source of infection to carnivores. The magnitude of viremia found in dogs and cats indicates that neither species is likely to function as an epidemiologically- significant amplifying host, although the peak viremia observed in some cats may be high enough to infect mosquitoes at low efficiency. Passer domesticus, the house sparrow, has been identified as being likely to have key importance in the amplification and maintenance of WNV in the United States. In order to gain understanding of the pathogenesis of WNV in this significant host, virus isolation was performed at regular intervals on a battery of tissues obtained from experimentally infected birds. WNV rapidly established disseminated infection with wide tissue tropism. All tissues studied were infected in at least some of the birds. Skin was the tissue from which virus was most consistently isolated, and was also the first in which virus was detected - as early as 12 hours postinfection. Other tissues frequently containing virus included blood, heart, lung, liver, kidney, and testis.