Prevalence, risk factors, and molecular epidemiology of simian foamy virus in free-living Macaca fascicularis at human-macaque interfaces in eastern Thailand.

BACKGROUND AND AIM: Long-tailed macaques (Macaca fascicularis) are widely distributed in Thailand and often inhabit areas where they come into contact with humans, creating opportunities for zoonotic pathogen transmission. Simian foamy virus (SFV), a retrovirus of the subfamily Spumaretrovirinae, is of particular One Health relevance; however, site-specific epidemiological and molecular information from areas with frequent human-macaque interactions in Thailand remains limited. This study investigated the prevalence, associated risk factors, and molecular epidemiology of SFV infection in free-living long-tailed macaques at human-macaque interfaces in Chonburi Province, Thailand.

MATERIALS AND METHODS: A total of 244 macaques from Si Racha, Sattahip, and Mueang districts of Chonburi Province were sampled. Paired blood and oral mucosal swabs were collected from each animal and examined for SFV proviral DNA using nested polymerase chain reaction. Apparent prevalence was estimated, and potential risk factors were assessed using univariate and multivariable logistic regression. Agreement between sample types was evaluated using McNemar's test and the Kappa coefficient (κ). Partial pol gene sequences from three SFV-positive samples were subjected to phylogenetic analysis against reference SFV sequences from different geographic regions.

RESULTS: Overall, 137/244 macaques were SFV-positive, corresponding to a prevalence of 56.15%. SFV prevalence was 32.79% in oral swabs and 36.89% in blood samples, with no significant difference between sample types. In univariate analysis, adult macaques had higher odds of SFV infection than juveniles (OR = 2.69; 95% confidence interval [CI] = 1.54-4.70; p < 0.001), while macaques weighing >5.0 kg had higher odds than those weighing <5.0 kg (OR = 2.84; 95% CI = 1.54-5.24; p < 0.001). In multivariable analysis, only body weight remained significantly associated with SFV infection (adjusted OR = 1.40; 95% CI = 1.15-1.71; p < 0.001), whereas age, sex, and geographic location were not significant predictors. Phylogenetic analysis showed that all three Thai SFV isolates clustered within a broader lineage containing SFV sequences from humans and non-human primates in Asia.

CONCLUSION: SFV was highly prevalent among free-living long-tailed macaques in Chonburi Province, with body weight emerging as the principal independent factor associated with infection. The molecular findings further demonstrate the genetic relationship of Thai SFV isolates with viruses circulating among human and non-human primate hosts in Asia. These findings provide epidemiological and molecular baseline data for SFV surveillance and support continued One Health monitoring at human-macaque interfaces in Thailand.