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Helminth infections were more common in Volcanoes golden monkeys than in Gishwati

Golden monkey eating bamboo in Volcanoes National Park, Rwanda
Research · Disease, parasites & microbiology

A 2026 study of Rwanda’s two remaining golden monkey populations found a higher prevalence of helminth infection in Volcanoes National Park than in Gishwati Forest. Helminths are parasitic worms that can infect the digestive tract and other tissues of their hosts. Golden monkeys are the distinctive subspecies Cercopithecus mitis kandti of the blue monkey, and the two populations now live in very different forest landscapes.

The larger population, numbering more than 4,000 animals, lives on the Virunga Massif, including Volcanoes National Park. The much smaller Gishwati population contains roughly 200 animals. The forests were once connected, but the researchers note that decades of deforestation have separated them and reduced forest cover substantially, particularly around Gishwati.

The study used records of helminths identified morphologically from faecal samples. The researchers compared the two populations and also examined indicators of contact with human-modified areas, including crop foraging and distance from the forest edge.

Helminth richness was similar, but infection prevalence differed

Both populations had the same helminth richness, meaning that the number of helminth types detected was similar. Their infection patterns nevertheless differed. Golden monkeys in Volcanoes National Park had a higher overall prevalence of helminth infection and were more often infected with several helminth taxa at the same time than animals in Gishwati Forest.

Forest-edge effects differed between parasite patterns

Forest edges were also associated with some infection patterns. In Gishwati, samples collected closer to the forest edge were more likely to contain at least one helminth taxon; the same relationship was not detected in Volcanoes National Park. For Capillaria, however, distance from the forest edge was important in both populations: infection became less likely farther from the edge. Groups that foraged on crops or lived near forest edges also had a higher prevalence of Capillaria infection.

The authors interpret the results as suggesting that population density, how permeable forest edges are to animal movement, and human presence outside the forest may influence parasite-infection risk. The contrast is not simply one of greater parasite diversity in one forest, since helminth richness was the same in both populations; rather, the prevalence and frequency of multiple infections differed.

The two populations provide an initial comparison of parasite-related health risks under very different ecological conditions after long-term forest loss and fragmentation.

Sources and image credit

  1. Johnston, A., Uwamahoro, C., Gategeko, A., Červená, B., & Cords, M. (2026). Helminth infections in wild golden monkeys (Cercopithecus mitis kandti) in Rwanda. American Journal of Primatology, 88(5), e70158. https://doi.org/10.1002/ajp.70158

Image: Charles J. Sharp — golden monkey (Cercopithecus mitis kandti) eating bamboo in Volcanoes National Park, Rwanda; exact subspecies and one of the two study populations. Source: Wikimedia Commons. · CC BY-SA 4.0 · Source

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