Molecular evidence of host specificity in Buxtonella-like ciliates in mandrills (Mandrillus sphinx) and related primates.

Balantioides coli (formerly Balantidium coli) is a ciliate protozoan causing balantidiasis in humans and non-human primates (NHPs), yet it remains underrecognized in zoological settings. Morphologically similar ciliates described as Buxtonella-like organisms have been reported in NHPs and are generally considered asymptomatic. In this study, we report symptomatic infection in mandrills (Mandrillus sphinx) and investigate the prevalence and genetic diversity of Balantioides spp. and Buxtonella-like ciliates across multiple NHP species housed in Polish zoological gardens. Faecal samples were collected from 70 individually identified NHPs representing 25 species across five facilities between October 2023 and September 2025 and examined using coproscopic methods and molecular analyses. While standard coproscopy detected ciliate cysts in 5.7% of samples, PCR identified infections in 18.6% of individuals, demonstrating substantially higher sensitivity. Balantioides coli was detected in several NHP species, whereas a Buxtonella-like organism was found exclusively in seven mandrills, five of which exhibited gastrointestinal symptoms. Phylogenetic analyses revealed a distinct lineage associated with mandrills, suggesting host specificity and previously unrecognised pathogenic potential. These findings challenge the assumption of asymptomatic carriage of Buxtonella-like ciliates and underscore the importance of molecular diagnostics for accurate detection and assessment of clinically relevant protozoan infections in captive NHP populations.