Oral vaccination and protection of red foxes (Vulpes vulpes) against rabies using ONRAB®, an adenovirus-rabies recombinant vaccine

Abstract

Twenty-seven red foxes (Vulpes vulpes) were each offered a bait containing ONRAB®, a recombinant oral rabies vaccine that uses a human adenovirus vector to express the immunogenic rabies virus glycoprotein; 10 controls received no vaccine baits. Serum samples collected from all foxes before treatment, and each week post-treatment for 16 weeks, were tested for the presence of rabies virus neutralizing antibody (RVNA). In the bait group, a fox was considered a responder to vaccination if serum samples from 3 or more consecutive weeks had RVNA ≥0.5IU/ml. Using this criterion, 79% of adult foxes (11/14) and 46% of juveniles (6/13) responded to vaccination with ONRAB®. Serum RVNA of adults first tested positive (≥0.5IU/ml) between weeks 1 and 3, about 4 weeks earlier than in juveniles. Adults also responded with higher levels of RVNA and these levels were maintained longer. Serum samples from juveniles tested positive for 1–4 consecutive weeks; in adults the range was 2–15 weeks, with almost half of adults maintaining titres above 0.5IU/ml for 9 or more consecutive weeks. Based on the kinetics of the antibody response to ONRAB®, the best time to sample sera of wild adult foxes for evidence of vaccination is 7–11 weeks following bait distribution. Thirty-four foxes (25 ONRAB®, 9 controls) were challenged with vulpine street virus 547 days post-vaccination. All controls developed rabies whereas eight of 13 adult vaccinates (62%) and four of 12 juvenile vaccinates (33%) survived. All foxes classed as non-responders to vaccination developed rabies. Of foxes considered responders to vaccination, 80% of adults (8/10) and 67% of juveniles (4/6) survived challenge. The duration of immunity conferred to foxes would appear adequate for bi-annual and annual bait distribution schedules as vaccinates were challenged 1.5 years post-vaccination.