MODELING RESPONSES TO SELECTION FOR RESISTANCE TO GASTROINTESTINAL PARASITES IN SHEEP

This paper describes a general framework which enables responses to se lection for resistance to gastro-intestinal parasites in sheep to be s tochastically modelled. The model incorporates between-animal variatio n for pasture intake, the proportion of larvae ingested from the pastu re which survive to become adults, the fecundity of the mature worm, a long with density-dependent control of this trait and the mortality ra te of the worms. The between-animal variation for each component is pa rtitioned into genetic, permanent and temporary environmental componen ts which vary with age. Infection rates are estimated from existing pa sture larval contamination and new contamination from infected animals . Using this framework, selection for reduced mean faecal egg count wa s practised, in silico, for a period of 10 years. Several general patt erns emerged. First, a curvilinear response to selection was observed, with responses initially being large then declining over time. Mean f aecal egg count declined from approximately 500 to 140 eggs per g in 1 0 years and worm burdens and pasture larval contamination showed simil ar patterns of response. The initial responses to selection were appro ximately 1.7 times that predicted by quantitative genetic theory becau se the epidemiology of the disease changed as the animals' genetic res istance improved. A method of partitioning selection responses into co mponents dire to the altered genotypes of the animals and components d ue to altered disease epidemiology is outlined. Secondly, the faecal e gg count distribution because more aggregated, or skewed, as selection progressed. Thirdly, correlating pasture contamination ion levels acr oss years (carry-over effects) resulted in even greater apparent respo nses to selection. Finally, regular anthelmintic treatment reduced mea n faecal egg counts but did not alter the patterns of response to sele ction, indicating that selective breeding should be feasible under a v ariety of anthelmintic regimes.