Gut Parasites in Captive Macaques Reveal Close Genetic Ties to Human Infections

A large molecular survey of captive crab-eating macaques in China has revealed that these primates carry a strikingly diverse array of Entamoeba parasites, including species capable of infecting humans, and that the strains they harbor are genetically close to isolates recovered from people. The study, published in Acta Parasitologica, analyzed more than five hundred fecal samples from commercial breeding facilities in Beijing and Suzhou and offers one of the most detailed pictures yet of how these single-celled parasites circulate within non-human primate populations under managed care. For researchers tracking zoonotic disease, the findings carry a clear message: macaques living in close proximity to human caretakers may serve as reservoirs for parasites that are only a genetic step away from causing human disease.

Entamoeba species are intestinal protozoa with a worldwide distribution, infecting humans and a broad range of animals. While some members of the genus, most famously Entamoeba histolytica, are responsible for amoebic dysentery and liver abscesses in people, others colonize the gut without causing obvious harm. Because these organisms are transmitted through fecally contaminated food and water, environments where humans and animals share space create ideal conditions for cross-species transmission. Non-human primates are of particular concern in this regard, since their physiological and genetic similarity to humans means that many of their parasites face relatively few barriers when jumping between hosts.

The research team, led by scientists at Anhui Science and Technology University, collected a total of 504 fecal samples from crab-eating macaques, also known as long-tailed macaques (Macaca fascicularis), housed at commercial breeding farms in two geographically distinct Chinese cities. Rather than relying on microscopy, which can confuse morphologically identical Entamoeba species, the researchers used polymerase chain reaction amplification and sequencing of the small subunit ribosomal RNA gene, a genetic marker that allows precise differentiation among six distinct Entamoeba species. This molecular approach is essential because species such as E. histolytica, E. dispar, and E. moshkovskii look identical under the microscope yet differ dramatically in their clinical significance.

The results revealed a layered picture of infection. Entamoeba coli was by far the most prevalent species, detected in 49.01 percent of samples, or 247 of the 504 specimens tested, with a 95 percent confidence interval spanning 44.6 to 53.4 percent. E. dispar followed at 36.51 percent, while E. histolytica, the most medically consequential species, was found in 8.13 percent of samples. The remaining species appeared at lower frequencies: E. chattoni at 5.95 percent, E. moshkovskii at 0.79 percent, and E. nuttalli at just 0.6 percent. The detection of E. histolytica in more than one in twelve animals is noteworthy, as this species remains a leading cause of parasitic death in humans globally.

Statistical analysis of the infection data uncovered a significant relationship between host age and the risk of carrying E. coli, with the association reaching a p-value below 0.0001. This strong age dependence suggests that older macaques accumulate infections over time, possibly through prolonged exposure to contaminated enclosures, changes in immune competence, or altered grooming and foraging behaviors. Understanding such risk factors is critical for breeding facilities, where managing parasite burden can affect animal welfare, the validity of biomedical research using these animals, and the safety of staff who work in close contact with the colonies.

The most striking findings emerged from the phylogenetic analysis. When the researchers reconstructed evolutionary relationships among the 247 E. coli isolates recovered from the macaques, they identified two distinct genotypes, designated SZ E. coli CEMs1 and SZ E. coli CEMs2. Both genotypes clustered firmly within the major E. coli clade and showed close genetic affinity with strains previously recovered from humans and other primate species. This pattern of shared lineages across host species is precisely what would be expected if the parasites are moving between humans and macaques, rather than evolving in isolated host-specific populations.

The presence of E. nuttalli, although rare in this cohort, adds another dimension to the zoonotic picture. This species, once confused with E. histolytica, is known to infect rhesus macaques and can cause liver abscesses in experimental animals, and previous studies have documented its genetic differentiation across macaque populations in Nepal, Myanmar, and China. Similarly, the detection of E. moshkovskii, an organism long considered a free-living amoeba but increasingly reported in human patients with diarrhea, underscores how fluid the boundaries between environmental, animal, and human Entamoeba lineages can be. The low prevalence of these species in the sampled colonies does not diminish their significance; rather, it highlights the value of sensitive molecular surveillance in detecting rare but potentially important infections.

From a practical standpoint, the findings carry implications for multiple stakeholders. For breeding facilities that supply macaques for biomedical research, high parasite prevalence complicates efforts to produce animals of defined health status, since subclinical infections can confound immunological and gastrointestinal studies. For public health authorities, the phylogenetic proximity of macaque-derived E. coli strains to human isolates argues for treating captive macaque colonies as potential zoonotic reservoirs warranting routine monitoring. And for the growing field of One Health, which examines the interconnected health of humans, animals, and environments, the study provides a concrete example of how intensively managed animal populations can harbor parasites with demonstrated potential for cross-host transmission.

The authors emphasize that enhanced surveillance is needed, both within breeding facilities and in contexts where macaques and humans interact more loosely, such as zoological gardens and areas where free-ranging macaques contact tourists. Prior studies of captive primates and their handlers in European zoos have already documented shared intestinal protists, suggesting that occupational exposure is a realistic concern. The Chinese breeding farms surveyed here represent a controlled setting, yet even under such conditions, nearly half the animals carried E. coli and a meaningful fraction harbored potentially pathogenic species, indicating that standard husbandry practices may not be sufficient to interrupt transmission cycles.

Looking forward, the genetic characterization of the two macaque E. coli genotypes opens avenues for finer-scale epidemiological work. Comparing these lineages with sequence data from human patients, livestock, and wild primates across Asia could clarify the directionality of transmission events and identify which host populations act as sources and which as sinks. Such work would build on a growing body of molecular epidemiology that has transformed Entamoeba from a morphologically defined genus into a genetically structured assemblage of species with distinct host associations and virulence potentials. As the new study demonstrates, even a parasite long regarded as a harmless commensal can reveal important truths about the porous boundaries between animal and human health, and about the responsibility that comes with keeping our closest relatives in close quarters.

Subject of Research: Zoonotic Entamoeba parasite prevalence and genetic diversity in captive crab-eating macaques in China

Article Title: Distribution of Zoonotic Entamoeba spp. in Crab-Eating Macaques Across China

Article References: Guo, Q., Gong, R., Ge, X., Li, M., Ma, J., Liu, X., & Li, W. (2026). Distribution of Zoonotic Entamoeba spp. in Crab-Eating Macaques Across China. Acta Parasitologica, 71(5), Article 216. https://doi.org/10.1007/s11686-026-01399-7

Image Credits: AI Generated

DOI: 10.1007/s11686-026-01399-7

Keywords: Entamoeba, crab-eating macaques, zoonotic parasites, E. histolytica, E. coli, E. dispar, SSU rRNA, phylogenetics, China, non-human primates, One Health, molecular epidemiology