Humoral response to an equine encephalitis vaccine in healthy alpacas.
To determine humoral responses to an equine encephalitis vaccine in healthy alpacas. Clinical trial. 39 healthy alpacas on 1 farm and 86 healthy alpacas on a second farm. All alpacas were given 3 doses IM of a bivalent, killed-virus equine encephalitis vaccine, with 4 weeks between doses. Eastern equine encephalitis (EEE) virus neutralizing antibody responses were determined with a plaque reduction neutralization assay every 14 days in alpacas on the first farm and 70 days after the first dose of vaccine on the second farm. For alpacas on the first farm, geometric mean virus neutralizing antibody titer peaked 2 weeks after the third vaccine dose was given (ie, day 70). At this time, 29 of 38 (76%) animals were seropositive for antibodies against EEE virus, and percentage of animals <or= 2 years old that were seropositive (16/17) was significantly higher than percentage of animals > 6 years old that were seropositive (1/5). For alpacas on the second farm, 76 (88%) were seropositive on day 70, and percentage of animals <or= 2 years old that were seropositive (24/24) was significantly higher than percentage of animals > 6 years old that were seropositive (27/33). For both farms, geometric mean titer on day 70 was significantly higher in animals < 2 years old than in animals > 6 years old. Results suggested that inoculation of alpacas with 3 doses of a bivalent, killed-virus equine encephalitis vaccine induced a humoral antibody response against EEE virus.