High prevalence of antibodies against canine adenovirus (CAV) type 2in domestic dog populations in South Africa precludes the use of CAV-based recombinant rabies vaccines

Abstract

Rabies in dogs can be controlled through mass vaccination. Oral vaccination of domestic dogs would beuseful in the developing world, where greater vaccination coverage is needed especially in inaccessibleareas or places with large numbers of free-roaming dogs. From this perspective, recent research hasfocused on development of new recombinant vaccines that can be administered orally in a bait to beused as adjunct for parenteral vaccination. One such candidate, a recombinant canine adenovirus type 2vaccine expressing the rabies virus glycoprotein (CAV2-RG), is considered a promising option for dogs,given host specificity and safety. To assess the potential use of this vaccine in domestic dog populations,we investigated the prevalence of antibodies against canine adenovirus type 2 in South African dogs.Blood was collected from 241 dogs from the Gauteng and KwaZulu-Natal provinces. Sampled dogs hadnot previously been vaccinated against canine adenovirus type 1 (CAV1) or canine adenovirus type 2(CAV2). Animals from both provinces had a high percentage of seropositivity (45% and 62%), suggestingthat CAV2 circulates extensively among domestic dog populations in South Africa. Given this finding,we evaluated the effect of pre-existing CAV-specific antibodies on the efficacy of the CAV2-RG vaccinedelivered via the oral route in dogs. Purpose-bred Beagle dogs, which received prior vaccination againstcanine parvovirus, canine distemper virus and CAV, were immunized by oral administration of CAV2-RG.After rabies virus (RABV) infection all animals, except one vaccinated dog, developed rabies. This studydemonstrated that pre-existing antibodies against CAV, such as naturally occurs in South African dogs,inhibits the development of neutralizing antibodies against RABV when immunized with a CAV-basedrabies recombinant vaccine.