Parasite insight: assessing fitness costs, infection risks and foraging benefits relating to gastrointestinal nematodes in wild mammalian herbivores

Abstract

Mammalian herbivores are typically infected by parasitic nematodes, which are acquired through direct, faecal-oral transmission. These parasites can cause significant production losses in domestic livestock, but much less is known about impacts on wild mammalian hosts. We review three elements of parasitism from the host's perspective: fitness costs of infection, risks of infection during foraging and benefits of nutritious pasture. The majority of wildlife studies have been observational, but experimental manipulation is increasing. Treatment with anthelmintics to manipulate parasite load has revealed varied impacts of parasites on fitness variables across host species, but has not produced consistent evidence for parasite-induced anorexia or impaired body condition. Some experimental studies of infection risk have manipulated faecal contamination and detected faecal avoidance by hosts. Only two field studies have explored the trade-off between infection risk and nutritional benefit generated by avoidance of contaminated patches. Overall, field studies of costs, risks and benefits of the host-parasite relationship are limited and few have examined more than one of these elements. Parasitism has much in common with predation, and future insights into anti-parasite responses by wild hosts could be gained from the conceptual and technical developments in research on anti-predator behaviour.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Keywords: anthelmintic treatment; anti-predator behaviour; faecal avoidance; gastrointestinal nematodes; mammalian herbivores; parasite co-infection.

Figure 1.

Nematode parasites in the stomach of a healthy adult male eastern grey kangaroo (Macropus giganteus). The larger parasites (Labiosimplex spp.) are about 100 mm in length and occur in numbers up to 500. The smaller parasites, predominantly Pharyngostrongylus kappa, Rugopharynx spp. and Cloacina spp., typically occur in tens of thousands. Photo by Graeme Coulson. (Online version in colour.)

Figure 2.

Free-ranging mammalian herbivores foraging on pasture heavily contaminated with conspecific faeces. (a) Adult female eastern grey kangaroo (Macropus giganteus) at Anglesea, Victoria, Australia. (b) Adult female Soay sheep (Ovis aries) at Hirta, St Kilda, Scotland, UK. Photos by Graeme Coulson. (Online version in colour.)