This paper reviews the previous mathematical and conceptual models for the parasitic phase of a range of trichostrongylid nematode parasites of cattle and sheep. It reassesses the results of single and trickle infection experiments and suggests as a working hypothesis that the common trichostrongylids are essentially identical with respect to the processes that determine their survivorship in the host. Parasite abundance in the parasitic phase is explained in terms of immune exclusion, which acts on recently ingested third stage larvae, and mortality of established (fifth stage) parasites. The functional forms used to describe immune exclusion and the mortality of fifth stage worms are defined, respectively, as a declining sigmoid and an asymptotic curve.