Tropical rainforest flies carrying pathogens form associations with a wild non-human primate social group

Living in groups provides benefits but also incurs costs such as attracting disease vectors. For example, synanthropic flies associate with human settlements, and higher fly densities increase pathogen transmission. We investigated whether such associations are unique to modern human populations, perhaps due to their sedentary lifestyles, or also exist in highly mobile non-human primate groups (NHP). We studied flies in a group of wild sooty mangabeys (Cercocebus atys atys) in Taï National Park, Côte d’Ivoire. We observed a 706% higher fly density within the mangabey group than outside the social group. A mark-recapture experiment showed that flies stayed with the group for up to 12 days and for up to 1.3 km. To gain insights into the disease risk posed by this fly association, we tested flies for pathogens affecting mangabeys in this ecosystem, causing rainforest anthrax (Bacillus cereus biovar anthracis: Bcbva) and yaws (Treponema pallidum pertenue). We found flies contained treponemal (6/103) and Bcbva (7/103) DNA and we detected high Bcbva copy numbers in flies. We were able to culture Bcbva from all seven of these positive flies, confirming flies contained viable Bcbva and suggesting they have the potential to be involved in Bcbva transmission and dissemination through the environment. Whole genome sequences of these Bcbva isolates revealed a large diversity of Bcbva, likely derived from a diversity of sources. We conclude that flies actively track mangabeys and can carry infectious bacterial pathogens. Such fly associations represent an understudied cost of sociality, potentially exposing animals to a large diversity of pathogens.