Population biology of the parasitic phase of Ostertagia circumcincta.

Four current models for the parasitic phase of the Ostertagia circumcincta life-cycle were evaluated with respect to their ability to represent the outcome of experimental infection studies and the population biology of the parasite in the field. Neither of the two discrete-time models was able to mimic the rise and fall in parasite numbers that characterize trickle infection experiments. When the infection rate is constant, both models predict that parasite numbers will eventually reach an asymptotic equilibrium value. This happens because both models are formulated such that parasite mortality is constant when the infection rate is constant. The two continuous-time models are able to mimic trickle infection experiments because both of them represent parasite mortality as an increasing function of the infection rate and the duration of infection. However, the continuous-time models do not adequately represent the demography of the parasitic phase in the field because neither of them takes any account of the effect of variations in infection rate from host to host on the overall mean parasite death rate.