The Virunga Massif mountain gorilla population has been periodically monitored since the early 1970s, with gradually increasing effort. The population declined drastically in the 1970s, but the numbers stabilized in the 1980s. Since then, the population has been steadily increasing within their limited habitat fragment that is surrounded by a dense human population. We examined fecal samples collected during the Virunga 2015-2016 surveys in monitored and unmonitored gorilla groups and quantified strongylid and tapeworm infections using egg counts per gram to determine environmental and host factors that shape these helminth infections. We showed that higher strongylid infections were present in gorilla groups with smaller size of the 500-m buffered minimum-convex polygon (MCP) of detected nest sites per gorilla group, but in higher gorilla densities and inhabiting vegetation types occurring at higher elevations with higher precipitation and lower temperatures. On the contrary, the impact of monitoring (habituation) was minor, detected in tapeworms and only when in the interaction with environmental variables and MCP area. Our results suggest that the Virunga mountain gorilla population may be partially regulated by strongylid nematodes at higher gorilla densities. New health challenges are probably emerging among mountain gorillas because of the success of conservation efforts, as manifested by significant increases in gorilla numbers in recent decades, but few possibilities for the population expansion due to limited amounts of habitat.

Biology Centre Institute of Parasitology Czech Academy of Sciences České Budějovice Czech Republic

Department of Botany and Zoology Faculty of Science Masaryk University Brno Czech Republic

Department of Pathology and Parasitology Faculty of Veterinary Medicine University of Veterinary Sciences Brno Czech Republic

Department of Physical Geography and Geoecology Faculty of Science Charles University Prague Czech Republic

Department of Primatology Max Planck Institute for Evolutionary Anthropology Leipzig Germany

Department of Veterinary Sciences CINeZ Faculty of Agrobiology Food and Natural Resources Czech University of Life Sciences Prague Prague Czech Republic

Dian Fossey Gorilla Fund Musanze Rwanda

Gorilla Doctors Davis CA USA

Here There Mapping Solutions Berlin Germany

Institut Congolais pour la Conservation de la Nature Kinshasa Congo

Institut Congolais pour la Conservation de la Nature Parc National de Kahuzi Biega Bukavu Congo

Institute of Vertebrate Biology Czech Academy of Sciences Brno Czech Republic

International Gorilla Conservation Programme Kigali Rwanda

Liberec Zoo Liberec Czech Republic

Rwanda Development Board Kigali Rwanda

School of Veterinary Medicine University of California Davis CA USA

Sydney School of Veterinary Science Faculty of Science The University of Sydney New South Wales 2006 Australia

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