Field evaluation of an inactivated vaccine to control raccoon rabies in Ontario, Canada.

Since raccoon rabies first appeared in Ontario in 1999, >90,000 raccoons (Procyon lotor) have received IMRAB3 inactivated rabies vaccine via intramuscular (IM) injection and were released at the point of capture as part of a multiyear rabies control program, trap-vaccinate-release (TVR). Of the 132 confirmed cases infected with raccoon-variant rabies virus in Ontario between 1999 and 2005, two were from that vaccinated group, as indicated by the presence of identifying ear tags. During ongoing rabies control programs in 2003, sera were collected from 172 wild raccoons that had received IMRAB3 and tested for rabies-virus antibodies. Raccoons had one of three histories: 1) vaccinated in the current year only (to examine the response to primary vaccination), 2) vaccinated in the previous year only (to determine the duration of the primary antibody response), and 3) vaccinated in the previous year and current year (to examine antibody response to booster vaccination). Seroconversion in primary vaccinates could be detected as early as 1 wk postvaccination when sera were measured with the use of a competitive enzyme-linked immunosorbent assay (C-ELISA) with a cutoff value established to correspond to a neutralizing titer of 0.5 IU/ml. During weeks three and four postvaccination, 94% of sampled raccoons had detectable antibodies to rabies virus and 31% were still antibody positive the following year. Differences in the kinetics of the immune response were found in raccoons sampled from the two different TVR areas of the province. A strong anamnestic response was detected after booster vaccinations. IMRAB3 by IM injection was found to be an efficacious vaccine for rabies control in raccoons.