Experimental West Nile Virus (WNV) Infection in Vaccinated and Unvaccinated Alpacas

Principal investigator:

Dr. Michelle Kutzler DVM, PhD, DACT
Oregon State University

The objectives of this research were to: 1) demonstrate the efficacy of a killed-virus vaccine against the clinical sequelae associated with WNV infection; 2) carefully monitor the time course of clinical WNV disease following an experimental challenge to better enable alpaca practitioners and owners in identifying and treating this disease; and 3) document end-organ manifestations of West Nile virus infection in both clinically normal and affected alpacas.

Two challenge experiments were conducted. In the first experiment, four vaccinated and eight unvaccinated alpacas were challenged with WNV infected mosquitoes . The vaccinated animals had received an initial series of three intramuscular vaccinations with West Nile Innovator (Fort Dodge Animal Health) given at three-week intervals more than one year prior to the mosquito challenge. A blood sample was taken to determine the serum neutralizing antibody titers prior to administering an annual vaccination booster three weeks before the experimental challenge. All twelve animals were challenged with infected mosquitoes and not one developed any clinical signs associated with WNV. Initially, both vaccinated and unvaccinated animals were negative for serum WNV RNA using reverse transcriptase polymerase chain reaction (RTPCR). Within 1 and 3 days following the mosquito challenge, two of the unvaccinated animals had detectable virus RNA present in the serum. WNV RNA was never identified in any of vaccinated animals. A second challenge experiment was then conducted in which five unvaccinated alpacas received an injection of WNV. Within 3 days following the challenge, two animals had detectable virus RNA present in the serum. This experiment allowed us to test the sensitivity of our new camelid-specific IgM capture ELISA that was developed with Alpaca Research Foundation funding from another grant. Within 9 days following the challenge, the presence of IgM antibodies can be detected to confirm acute WNV infection.