Risk Factors Associated with Gastrointestinal Parasite Infections in Urban Long-tailed Macaques (Macaca fascicularis): The Role of Network Centrality and Synanthropy

Evaluating risk factors of infections in primates is essential to understand infection
dynamics and predict epizootic threats at the human-primate interfaces. In Bali,
Indonesia, long-tailed macaques (Macaca fascicularis) frequently interact with
humans in touristic areas, living sometimes in high demographic density, which may
increase exposure to or alter transmission dynamics of gastrointestinal (GI) para
sites. This study investigates risk factors of infection in macaques, specifically how
their social network centrality, individual traits, and human-macaque interactions
influence GI parasite infections in synanthropic macaques. Over two years (2022
2023), we opportunistically collected 142 fecal samples from 53 macaques in the
Ubud Monkey Forest, and we analyzed GI parasites using direct smear and flotation
techniques. We recorded behavioral data, including macaque grooming interactions
and human-macaque contacts, through focal sampling and integrated the former
into social network analysis. Generalized linear mixed models assessed the effects
of social centrality, synanthropic nature, and host characteristics of the macaques
on their infection risk. We found GI parasites in 75% of the samples and we identi
f
ied six GI parasite taxa: Entamoeba spp., Iodamoeba spp., Balantioides-like cili
ate, Strongyloides spp., Trichuris spp., and Strongylida (fam. gen. indet). Individuals
central in the grooming network had fewer parasite species, suggesting a potential
social buffering effect. Human-macaque contacts positively influenced the presence
of Iodamoeba spp. and showed a positive trend in influencing GI parasite richness.
These findings highlight that infection dynamics in primates result from complex
interactions between social, anthropogenic, and biological factors.