Recurring Streptococcal Outbreak Threats in Urban Long-Tailed Macaques (Macaca fascicularis) in Bali

Streptococcus equi ssp. zooepidemicus is a zoonotic bacterium known to cause severe respiratory tract infections in primates, and most documented mortality episodes occur in captivity. Here we report a mass mortality event in a wild population of long-tailed macaques (Macaca fascicularis) inhabiting an urban mosaic habitat in central Bali, Indonesia, and describe the demographic impact and clinical patterns of the outbreak. Mortality was observed in three of the nine social groups that shared overlapping ranges, and the infection spread progressively over time. Clinical signs included lethargy, respiratory distress, and locomotion disorders. Over a 2-mo period during March-May 2022, 170 carcasses were found. Demographic surveys revealed a mortality rate ranging from 16% to 53% among the three affected groups. Adult females and adult males were the most affected age-sex classes, representing 36% and 35% of the carcasses found, respectively. Necropsy findings from four individuals, combined with bacteriological culture, histopathology, qPCR, and 16S rRNA metabarcoding analysis, all suggested Streptococcus equi ssp. zooepidemicus as the most likely causative agent. Similar streptococcal outbreaks had occurred in this population in 1994 and 2012, raising concerns about the recurrent introduction of the bacterium or potential reservoirs either within or outside the macaque population. The recurrence of Streptococcus equi ssp. zooepidemicus outbreaks in this population highlights the impact of lethal bacterial epidemics in wild primates, which remain poorly documented. Strengthening long-term surveillance, including noninvasive serological monitoring, is essential for better understanding infection dynamics and improving conservation strategies.