Canine distemper is endemic in wild raccoons in Toronto and surrounding suburbs. Raccoons represent a constant threat to the Metro Toronto Zoo collection. We have tried to create a "barrier" of protected raccoons on the zoo site to minimize the pressure of infection on the collection, but it has been our experience that vaccinated animals that are subsequently released remain susceptible and may succumb to the disease. There have been no controlled studies of humoral response of raccoons to canine distemper vaccination.
A controlled vaccination trial using a modified-live virus vaccine in 48 raccoon pups of known immune status was conducted. Antibody titers were measured for 3 mo following various vaccination schedules. Raccoons with and without titers were challenged with a virulent strain of canine distemper. They were monitored for clinical signs and humoral response for a period of 42 days. All vaccinated pups seroconverted within 2 wk of inoculation. Pups vaccinated once at 8-wk-old and pups vaccinated three times at 8, 12 and 16-wk-old had similar titers at 20-wk-old. Eight-week-old raccoons mounted titers similar to those of raccoons first inoculated at 16-wk-old.
Maternal immunity had completely waned by 16-wk-old in all pups. Pups with maternal antibodies did not mount titers as high as immune-naive pups, but all vaccinated animals survived a challenge with a virulent, raccoon-origin canine distemper virus that killed five out of six controls. Results of this study suggest that modified-live virus vaccination in raccoon pups was efficacious and yielded protection from clinical disease.