Immunosuppression of the periparturient ewe: a causative role for nutrient supply?

Reproductive ewes suffer a temporary relaxation of immunity to gastrointestinal nematode parasites that often begins a few weeks prior to parturition. The maintenance and therefore the duration of this immunosuppression is determined by physiological factors associated with lactation and the balance between nutrient supply and demand, particularly the demand for metabolisable protein. Changes to the systemic host immune response during late pregnancy can be detected prior to changes in worm egg count and these changes are often accompanied by mobilisation of protein from skeletal muscle and lipid from fat reserves. Mobilisation of tissue reserves augments nutrient status and is an important source from which periparturient ewes may draw resources to bolster immunity. Experimental contrasts involving Merino ewes differing in reproductive physiology and nutrient balance enabled elucidation of the effects of lactation 'per se' on immunity to 'Trichostrongylus colubriformis' that were independent of effects on body composition. Evidence that previous and current nutrient supply has a causative relationship with immunosuppression of the periparturient ewe is discussed.