Oral rabies vaccination of raccoons and striped skunks with ONRAB® baits: multiple factors influence field immunogenicity.

Multiple control methods have been used in North America to manage the spread of rabies caused by the raccoon (Procyon lotor) rabies virus variant (RRVV). Recently, oral vaccination with ONRAB(®) vaccine baits, which contain an adenovirus rabies glycoprotein recombinant, has been made available as an additional tool for rabies control. Our objectives were to estimate rabies antibody prevalence in wild-caught raccoons and striped skunks (Mephitis mephitis), and identify factors influencing the probability of being antibody positive at the individual level in these species, following oral rabies vaccination (ORV) campaigns in which ONRAB was distributed aerially in 2007-2009 in southern Québec, Canada. Following the aerial distribution of 43-155 ONRAB baits/km(2), the annual percentages of antibody-positive raccoons and skunks varied between 35% and 56% and 11% and 17%, respectively. In raccoons, the probability of being antibody positive was positively associated with age and density of ONRAB distributed, and influenced by the number of previous ORV campaigns conducted. Conversely, this probability was negatively associated with estimated abundance of raccoons in the trapping cell and proportion of residential areas near the raccoon capture location. None of the variables examined explained variation in the probability of being antibody positive in skunks. Our results indicate that the ONRAB density applied during ORV campaigns should be adjusted to account for variations in raccoon population density and presence of residential areas to increase the likelihood of creating an effective immunological barrier against RRVV. The high percentage of juvenile raccoons (annual mean =45 ± 3 [SE]%) and skunks (66 ± 2%) captured during post-ORV monitoring suggests that ORV campaigns should be conducted at least annually to account for the recruitment of naïve individuals into the populations. In Québec, the increasing use of ONRAB coincided with the elimination of rabies caused by RRVV. Nonetheless, our results indicate that improvements to this vaccine bait and/or the distribution techniques are required to increase its efficacy, especially in striped skunks.

Oral vaccination against raccoon rabies: landscape heterogeneity and timing of distribution influence wildlife contact rates with the ONRAB vaccine bait.

Aerial distribution of oral vaccine baits is one of the available strategies for controlling the spread of infectious wildlife diseases. This technique has commonly been used to control rabies in wild carnivores and, together with other techniques, was used to immunize wild populations of raccoons (Procyon lotor) and striped skunks (Mephitis mephitis) after the detection of the first rabid raccoon in the province of Quebec, Canada, in 2006. Vaccine bait distribution was conducted over large areas where agricultural land is dominant but interspersed with residual forest patches. Our objective was to evaluate the effect of habitat (forest vs. agricultural crops) in space and time on the contact rate between wildlife and the ONRAB(®) vaccine bait, a recent alternative to the V-RG(®). Four transects of eight vaccine baits each were installed parallel to, and at different distances from, the forest's edge (under forest cover, at field-forest edge, and at 50 and 200 m from forest edge in agricultural crops) at three sites composed of various crop types interspersed with forest patches. This experiment was conducted during three periods (late spring, 1-7 June; summer, 27 July-2 August; and fall, 24-30 October) in 2009. Contact rates with vaccine baits were monitored for 7 days in each period to evaluate the potential temporal variations generated within the habitat types. Contact rates with ONRAB vaccine baits were highest under forest cover and in the fall. Of 13 species observed in proximity to the vaccine baits, raccoons were the most frequent (49.5%, n=55 visits). Our study underlines the importance of taking into account landscape heterogeneity and timing of distribution when planning the distribution of vaccine baits to control rabies in raccoons.