A model to account for the consequences of host nutrition on the outcome of gastrointestinal parasitism in sheep: logic and concepts.

A deterministic, dynamic simulation model is developed to account for the interactions between gastrointestinal parasitism and host nutrition, and predict their consequences on performance and level of parasitism of sheep. Larval intake and established adult worms are assumed to result in nutrient loss for the host. In order to reduce this loss the host will mount an immune response, which will affect the establishment rate of incoming larvae, mortality rate of adult worms, and fecundity of female worms, as well as nutrient loss caused by larval intake per se. Host anorexia is modelled as a function of worm mass. Parasitism is also assumed to affect the allocation of ingested nutrients to the host's bodily functions, with maintenance getting absolute priority, and protein allocated to immunity and production proportionally to their requirements. Inputs to the model include the expected growth attributes of the animal, feed quality, various parasitological parameters and daily larval intake. Outputs include feed intake, growth rate and body composition, as well as worm burden and faecal egg counts. The model allows exploration of the consequences of gastrointestinal parasitism on sheep of different growth characteristics, kept under environments that vary in the provision of nutrients and exposure to parasites.