There is a wide range of nematode species that inhabit, or migrate through, the lungs of deer in Europe (Table 24.1).

Dictyocaulus spp. are large whitish nematodes that, as adults, occupy the major bronchi and trachea of their hosts. The different species are not totally host-specific, with D. viviparus, predominantly a cattle parasite and D. filaria, a parasite of sheep and goats, both being reported in deer (Höglund et al. 1999; Movsesyan et al. 2021). There are, however, Dictyocaulus spp. that appear to be more deer-specific, namely D. eckerti, D. capreolus, D. cervi and D. skrjabini. These have been reported in various deer as detailed in Table 24.1.

Dictyocaulus spp. have a direct life-cycle, with deer infected by ingesting third-stage larvae (L₃) from the pasture. These penetrate the intestinal mucosa, migrate via the mesenteric lymph nodes and travel via lymph and blood to break out of capillaries into alveoli, primarily in the caudal lung lobes. They then travel to the major bronchi and trachea as they mature. The adults mate and the females lay thin-shelled eggs, which hatch almost immediately. The first-stage larvae (L₁) are transported up the ciliated tracheal epithelium and are swallowed, to be excreted in the faeces where they develop into L₃. This development, of L₁ to L₃, may take as short a time as three days in optimum temperatures (approximately 20 °C). However, it usually takes longer, up to 28 days, depending on environmental conditions. The L₃ then migrate from the faeces onto pasture, either by their own motility, mechanical means or via spores of the fungus like Pilobolus spp. (Figure 24.1) (Taylor et al. 2016).

The prepatent period, that is the time taken between ingestion of L₃ to L₁ being detected in the faeces, is usually two to three weeks and larvae can be produced for three weeks to three months, depending on host and species of Dictyocaulus (Deplazes et al. 2016).