Seasonal variation in bait uptake and seropositivity during a multi-year biannual oral rabies fox vaccination programme in Kosovo (2010-2015).

The European Union supported programmes for rabies control in Kosovo between 2010 and 2015, including spring and autumn biannual oral vaccination campaigns targeting foxes. Throughout the programmes foxes were obtained to provide samples for monitoring the campaigns. This paper explores the seasonal pattern of bait uptake and seropositivity in the fox population. Bait uptake varied by season and by the phase of the project supporting the programme (the main differences between phases being the number of baits distributed and flight line separation). Seropositivity varied by season and by titre of the vaccine used in the preceding campaign. The analyses also suggested a negative effect of higher daytime temperature on bait uptake, and possible association between geographic location of sampling and concordance between bait uptake and seropositivity, but the dataset was too unbalanced to support robust conclusions on these detailed aspects. Descriptive summaries of the data and the multilevel analyses showed that the proportion of sampled foxes that were positive for bait uptake and the proportion seropositive were both high through winter, following the autumn campaigns, and declined through spring and summer, with a low point in samples collected during the time when juvenile foxes are typically dispersing from their birth dens. The percentage of foxes positive for bait uptake was below 30 % (first project phase) and 40 % (second project phase) from mid-July to mid-October following a spring campaign, compared with around 70 % (first project phase) and 80 % (second project phase) in the periods between autumn and the following spring campaigns. As could be expected, the percent of samples that were seropositive followed a similar seasonal pattern, with some additional variation associated with the titre of vaccine used. This seasonal pattern is likely because the population sampled in the late summer months includes increasing numbers of young foxes that could not have been effectively exposed to the spring vaccination campaign, and would have lost any possible maternal immunity by late summer. The main finding of high levels of bait uptake and seroprevalence through winter, following the autumn campaigns, declining through summer despite the implementation of spring campaigns, supports advice that countries lacking financial resources to support biannual campaigns should focus resources on once per year vaccination in late autumn or early winter. This pattern also indicates that a fox population may rapidly become naïve after cessation of vaccination programmes, therefore strongly coordinated regional programmes and good surveillance will be needed.