How Equids Cope with Macroparasites

In this chapter, we review exposure risk dynamics and coping mechanisms that equids employ against macroparasites, with a focus on gastrointestinal nematodes. As bulk-feeding grazers, equids are particularly exposed to nematodes—faecal-orally transmitted parasites that quest for hosts on low-lying vegetation. In contrast, ticks and biting flies seem to parasitize herbivores of all feeding guilds relatively equally. Mounting evidence suggests that equids have developed relatively tolerant immune systems to nematodes, investing in tissue repair and maintaining body condition in the face of chronic infection. In fact, several equid populations show signs of being hypertolerant to—or even commensal with—their nematode parasites. Grevy’s zebras (Equus grevyi) and plains zebras (Equus burchellii) in Kenya, for example, seasonally exhibit a positive relationship between nematode egg counts in faeces and body condition, rather than the negative relationship expected for a costly parasite. Faecal egg counts in equids (grazing perissodactyls) are more similar to those in black rhinoceroses (Diceros bicornis, browsing perissodactyls) than to those in African buffalo (Syncerus caffer, grazing ruminants), suggesting that gut morphology may be more important than feeding guild in establishing the patterns of chronic nematode infection, and potentially of immune tolerance. Exposure to ticks is unavoidable for equids, as it is for many other herbivores, and equids have developed both deterrence and resistance mechanisms that seem to be especially strong in donkeys. In contrast, equids deal with biting flies largely through avoiding exposure, including seeking out habitat refuges, gathering in groups to dilute fly exposure and to enable mutual tail switching, and potentially even by evolving fly-evasive coat patterns.