Report on a streptococcal outbreak in Balinese long-tailed macaques

Streptococcus equi ssp. zooepidemicus is a zoonotic bacterium associated with severe respiratory tract infections in primates. Several mortality episodes caused by this agent have been reported in primates, mostly in captivity. Here we report a mass mortality event affecting a population of long-tailed macaques (Macaca fascicularis) in central Bali (Indonesia) during spring 2022. Among the 11 social groups of this population, we recorded mortality in four groups with overlapping home ranges, with a progressive spread over time. The observed clinical signs included lethargy, respiratory distress and locomotion disorders. A total of 164 carcasses were found within two months and demographic counts revealed 20 to 60% of loss in infected groups. Adult females were the most affected age-sex class as they represented 44% of the carcasses found. Results from the necropsy of 4 dead animals followed by bacteriological culture, histopathological examination, and qPCR converged to the identification of S. equi ssp. zooepidemicus as the causative agent. Similar outbreaks due to this bacterium have been reported from the same population in 1994 and 2012, raising questions about the potential reservoirs of the bacterium and how it has been re-introduced in the macaque population. Given the zoonotic risk at the human-macaque interface, and the implications for conservation of this species recently uplisted to endangered by IUCN, it is paramount to carry out further studies on reservoirs and potential sources of infection with S. equi ssp. zooepidemicus, and other zoonotic pathogens.