Bovine rabies in Rio Grande do Sul: positivity rates, geographical distribution, and seasonal occurrence

Abstract

Rabies is a zoonotic disease characterized by acute viral encephalitis. It is almost 100% fatal for infected animals. While cases of human rabies have still been registered in Brazil, in the state of Rio Grande do Sul rabies is considered a controlled disease in urban areas. However, the transmission of bovine rabies – which has been largely associated with the Desmodus rotundus bat – is endemic. Additionally, there are estimates that only 10% of rabies cases are reported. Therefore, this study aimed to describe the positivity rates, geographical distribution, and seasonal occurrence of bovine rabies in RS, as well as the sampling practices in rabies surveillance. Using samples submitted for laboratory diagnosis from 2016 to 2019, we conducted a retrospective study of the sample results, positivity rates, and seasonal occurrence of rabies in cattle. Throughout these four years, less than a third of state municipalities sent samples for diagnosis, suggesting that the occurrence of rabies might still be underestimated in non-sampled areas. A higher number of rabies cases were reported in 2019, a year in which the most diagnosed samples and the highest positivity percentages were recorded. However, the case numbers registered between 2016 and 2019 were not significantly different. We found that the summer and autumn months presented statistically different positivity rates. Besides this, we did not find any positive D. rotundus samples, despite the positivity rates for bovine rabies. Active surveillance, especially of bat populations and under-sampled regions, must be strengthened to correctly estimate the impact of rabies.