Roundworms have a direct life-cycle and eggs from infected animals are passed out in faeces. Fertilised eggs will embryonate and develop into larvae. Generally, the larvae will hatch and undergo two moults to become infective larvae (L₃), which are ingested with herbage (except Nematodirus species, where development to infective larvae takes place in the egg before hatching). Once ingested, the larvae start the parasitic dioecious (sexual) phase of their life-cycle. Depending on the time of year, larvae will either undergo further development to become adult male or female worms, mate and females will produce eggs that are passed out with the faeces to complete the life-cycle or they will arrest their development (particularly over winter). Emergence and continued development of arrested larvae have been associated with a disease similar to type II Ostertagiasis in cattle (Mackintosh 2022).

Wild (Table 19.1) and farmed deer have been shown to be infected with a variety of generalist roundworm species that are commonly found in other grazing ruminants, particularly in species of deer, such as roe deer, which are known to commonly graze livestock pastures (Brown et al. 2022). The nematode species in deer occupy the same predilection sites as in livestock such as the abomasum and small and large intestines. Studies assessing the faecal egg count (FEC) and species composition of samples collected from farmed and/or culled deer have shown that red, fallow, sika and reindeer all carry a range of gastrointestinal nematode (GIN) species in the United Kingdom (McGregor 2024). The most prevalent species in both wild and farmed deer populations were Oesophagostomum venulosum (being found on average in 42% and 94%, respectively of the wild and farmed deer examined, respectively) and Ostertagia leptospicularis (27% and 4%, respectively); however, more variation was observed in wild deer populations, including the presence of Trichostrongylus spp., Teladorsagia circumcincta, Ostertagia ostertagi and Cooperia oncophora. Spiculopteragia, Skrjabinagia, Rinadia and Apteragia have also been reported in deer (Munro 1994a).

Farmed and wild deer tend to have low FECs (Figure 19.1) in comparison to sheep and cattle. These infected animals could act as vehicles for roundworm movement, including anthelmintic-resistant parasites, and/or could act as a reservoir for ‘non-treated’ roundworms that may act as a source of refugia for livestock if they co-graze pastures (van Dijk et al. 2010; Chintoan-Uta et al. 2014). Deer have been shown to carry benzimidazole-resistant Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus axei, albeit in small numbers (Chintoan-Uta et al. 2014; McGregor 2024).

In common with most grazing ruminants in temperate regions, deer are also susceptible to infection with trematode (fluke) parasites, most notably liver fluke (Fasciola hepatica) and rumen fluke (several paramphistome species). Infection is acquired by deer ingesting metacercarial cysts on pasture that have been shed by infected mud snails. The liver fluke is the acknowledged pathogen in deer. Historically, the prevalence of liver fluke in deer was thought to differ between species, with roe deer most commonly infected, followed by red deer and fallow deer, with sika deer rarely infected (Munro 1994b). More recent reports have revealed that between ~10 and 50% of wild red deer (Cervus elaphus) in Scotland are infected with liver fluke, with prevalence depending on location and environmental/climatic conditions (French et al. 2019). There is very little information on liver fluke prevalence in farmed deer, although reputable sources suggest prevalence could be high if enclosed grazing contains suitable mud snail habitat (Munro 1994b).

There is much less information about rumen fluke generally and there have been no recent studies of rumen fluke infections in UK deer; however, a study of roe, red and sika deer in Ireland indicated that Calicophoron daubneyi and Paramphistomum leydeni were the predominant species (O’Toole et al. 2014). The situation is likely to be the same in UK deer. These species are also found in grazing livestock and it has been suggested that deer may act as reservoirs of rumen fluke infection. Interestingly, the predominant rumen fluke species in UK livestock has been shown to be C. daubneyi (Gordon et al. 2013). Previously, it was thought to be Paramphistomum cervi, with deer as the presumed wildlife source (Taylor et al. 2016).

One other perceived trematode threat to UK deer is the giant liver fluke, Fascioloides magna (Juhász and Stothard 2023). This parasite is prevalent in Central and Eastern Europe, although it has yet to be reported in Great Britain or Ireland. One concern would be that it employs the same mud snail, Galba truncatula

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