Gastrointestinal Parasitic Infections in Macaca fascicularis in Northeast Thailand: A One Health Perspective on Zoonotic Risks

Abstract

Gastrointestinal (GI) parasitic infections in non-human primates are of growing concern due to their implications for both veterinary and public health. Long-tailed macaques (Macaca fascicularis), commonly found in peri-urban and temple environments in Southeast Asia, may act as reservoirs for zoonotic parasites, posing risks to humans and domestic animals. This study investigated the prevalence and species diversity of GI parasites in free-ranging macaques from four provinces in Northeast Thailand (Loei, Khon Kaen, Bueng Kan, and Sisaket). A cross-sectional study was conducted between April and May 2025. A total of 445 fecal samples were examined using two parasitological techniques: agar plate culture (APC) and the formalin-ethyl acetate concentration technique (FECT). The overall prevalence of parasitic infection was 86.5%, with Strongyloides sp. (65.2%) as the most prevalent helminth and Balantioides coli-like (29.5%) and Entamoeba histolytica-like (28.8%) as the predominant protozoa. Other parasites identified included helminths (Trichuris sp., Ascaris sp.) and protozoa (Blastocystis sp., Iodamoeba bütschlii, Entamoeba coli, and Chilomastix mesnili). Mixed infections were frequently observed, with both helminths and protozoa co-occurring in 37.3% of cases. The high infection rates and parasite diversity reflect substantial environmental contamination and sustained transmission cycles. These findings underscore the importance of integrated surveillance in wildlife populations and the need for One Health-based approaches to minimize zoonotic transmission risks at the human-animal-environment interface.

Keywords: One Health; gastrointestinal parasites; long-tailed macaque; prevalence.

Figure 1

Map showing the locations of long-tailed macaque (Macaca fascicularis) study sites in Northeast Thailand. Red color indicates the provinces surveyed in the present study (Loei, Khon Kaen, Bueng Kan, and Sisaket). Sampling locations included Wat Tham Pha Mak Ho, Loei Province (n = 121); Wat Tham Pha Chor, Khon Kaen Province (n = 125); Ban Sang Community Forest, Bueng Kan Province (n = 105); and Ban Mueang Kan Monkey Park, Sisaket Province (n = 94). Gray color represents locations previously studied for gastrointestinal parasites in macaques, as previously reported [17,18,25,26]. The map was created by the authors using QGIS version 3.40. All other layers were produced by the authors and are copyright-free.

Figure 2

Satellite maps show the distance between long-tailed macaque habitats (red dots) and nearby village areas (white dots) in four provinces: (A) Sisaket; (B) Bueng Kan; (C) Khon Kaen; and (D) Loei. Dashed lines show the shortest path between macaques and people. Macaques in Sisaket and Bueng Kan live less than 500 m from villages. These short distances show high contact between humans and macaques. The map was created by the authors using Google Earth. All other layers were also produced by the authors and are copyright-free.

Figure 3

Microscopic images of protozoan parasites detected in fecal samples from long-tailed macaques. (A) Balantioides coli-like trophozoite; (B) Balantioides coli-like cyst; (C) Entamoeba coli cyst; (D) Entamoeba histolytica-like cyst; (E) Iodamoeba bütschlii cyst; (F) Chilomastix mesnili cyst. All images were captured under light microscopy at 400× magnification using iodine staining.

Figure 4

Microscopic images of helminth stages detected in fecal samples from long-tailed macaques. (A) Trichuris sp. egg; (B) Ascaris sp. fertilized egg; (C) Strongyloides sp. egg; (D) Strongyloides sp. rhabditiform larva. All images were captured under light microscopy at 400× magnification using iodine staining.