Is it possible to orally vaccinate juvenile red foxes against rabies in spring campaigns?

The rabies antibody status of juvenile foxes (Vulpes vulpes) was evaluated in large-scale, long-term oral vaccination campaigns. Between 9% (n = 659) and 21% (n = 42) of the juvenile foxes examined in 1993-94 and 1997, respectively, showed rabies virus neutralizing antibody (nAb)-titers > or = 0.5 IU/ml following bait distribution in spring. The presence of nAb may be due to either the passive transfer of maternal antibodies, or active immunization derived from spring vaccination campaigns. The latter alternative is supported by the finding of nAb throughout late spring and the summer months, and the finding of the tetracycline (TC) biomarker, used in the vaccine-baits, in 27% (n = 43) and 37% (n = 155) of juveniles in 1993-94 and 1997, respectively. It was not possible to distinguish nAb originating from passive immunity from that arising from active immunization. However, biological data on the whelping period of red foxes, on dynamics of maternal antibodies and the timing of oral vaccination, gave evidence that a superposition of these processes is likely. Evidence from these studies suggests that oral vaccination coinciding with the spring perinatal period may produce immunity in both parents and only in a certain percentage of the offspring simultaneously. This phenomenon should be useful in further enhancing the efficacy of oral vaccination in red foxes.