Fig. 1.

Generalized life cycle representing key gastrointestinal strongylid nematodes of small ruminants (adapted from Demeler, 2005). First-, second- and third-stage larvae (L1, L2 and L3, respectively) are ‘free-living’ in the environment. The fourth larval (L4) and adult stages (dioecious) are ‘parasitic’ in the gastrointestinal tract of the host. Disease in the host animal is caused by the adult and/or L4 stages, and depends on the species of nematode; intensity of infection; species, age and immunological/health status of the host; host response against the parasite; stress and other environmental and management factors (Kassai, 1999, Taylor et al., 2007).