Experimental Infection of Horses with West Nile virus

Abstract

A total of 12 horses of different breeds and ages were infected with West Nile virus (WNV) via the bites of infected Aedes albopictus mosquitoes. Half the horses were infected with a viral isolate from the brain of a horse (BC787), and half were infected with an isolate from crow brain (NY99-6625); both were NY99 isolates. Postinfection, uninfected female Ae. albopictus fed on eight of the infected horses. In the first trial, Nt antibody titers reached >1:320, 1:20, 1:160, and 1:80 for horses 1 to 4, respectively. In the second trial, the seven horses with subclinical infections developed Nt antibody titers >1:10 between days 7 and 11 post infection. The highest viremia level in horses fed upon by the recipient mosquitoes was approximately 460 Vero cell PFU/mL. All mosquitoes that fed upon viremic horses were negative for the virus. Horses infected with the NY99 strain of WNV develop low viremia levels of short duration; therefore, infected horses are unlikely to serve as important amplifying hosts for WNV in nature.

Incidence and effects of West Nile virus infection in vaccinated and unvaccinated horses in California

“…Our results are consistent with prior reports of retrospective studies that indicate that approximately 10% of WNV-infected horses develop clinical disease [5,7,16], which is markedly greater than the occurrence (< 1%) of severe disease in WNV-infected humans [4,7]. In contrast to the reported attenuation of virulence of WNV in the tropical ecosystems in Latin America and the Caribbean [14], there was no evidence that the virus circulating in California had attenuated following transcontinental spread from the original incursion to New York in 1999 because the frequency of clinical WND in the horses in our study population was similar to that of prior reports.…”

Antibody Responses Induced by Experimental West Nile Virus Infection with or without Previous Immunization with Inactivated Japanese Encephalitis Vaccine in Horses

“…These results suggest that viremia and illness caused by WNV infection might be milder in horses previously immunized with inactivated JE vaccine than in horses not immunized with the vaccine; however, our data is not adequate to indicate cross-protection because of the small group size. It should be noted that WNV infection in horses often results in a subclinical infection [3] and that only one horse showed clinical signs in experimental WNV infection of twelve horses [2]. Also, viremia levels in WNV-infected horses are often low and sometimes undetectable, as seen previously in an experimentally infected horse [2] that was similar to Horse 3 in the Non-Immune Group in the present study.…”

“…It should be noted that WNV infection in horses often results in a subclinical infection [3] and that only one horse showed clinical signs in experimental WNV infection of twelve horses [2]. Also, viremia levels in WNV-infected horses are often low and sometimes undetectable, as seen previously in an experimentally infected horse [2] that was similar to Horse 3 in the Non-Immune Group in the present study. Further experiments with larger group sizes will be required to determine whether cross-protection occurs in horses.…”

Equine Risk Assessment for Insecticides Used in Adult Mosquito Management

“…Horses experimentally infected with the virus did not show sufficient viremia for transmission back to a mosquito host (Bunning et al 2002). Therefore, horses, like other mammals, are most likely dead-end hosts for the virus (Dauphin et al 2004).…”