Assessment of the efficiency of oral vaccination against rabies in the fox population in Bulgaria

Since 1980, sylvatic rabies largely occurs in Bulgaria, specifically involving red foxes (Vulpes vulpes) as virus reservoirs and vectors. In this study, a total of 3 321 380 baits including vaccine doses (Lysvulpen modified live vaccine) and tetracycline were disseminated over 19 districts (55 995 km2 area) mainly in Northern and Western Bulgaria during 3 vaccination campaigns (in spring and autumn 2009 and in spring 2010). The efficiency of oral vaccination was assessed in shot foxes directly by measurement of the circulating anti-rabies antibodies with a specific ELISA test and indirectly by determining tetracycline accumulation in teeth. Concomitantly, the presence of the wild rabies virus was investigated in brain samples from various animals with evocated clinical signs or dead (including shot foxes) using fluorescent antibody test (FAT) and mouse inoculation test (MIT) during 2007-2010. Seroconversion rates in the fox population were 18.3% and 17.0% in 2009 and 2010, respectively and the frequencies of tetracycline accumulation in teeth were 11.1% and 12.0%. Only 7 foxes in 2009 and 4 in 2010 were positive for the 2 methods. In parallel, the overall rabies prevalences have dramatically and gradually declined since 2009. These results demonstrate the efficacy of oral vaccination campaigns in the fox population at a large scale although the seroconversion rate remained moderate and that improvements (in hiding vaccine doses in baits, extending to other wild species and regularly conducting such programs) in the vaccination procedure may be brought.