Search

This thesis describes a series of three experiments designed to estimate the nutritional cost of the immune response to the gastrointestinal nematodes Trichostrongylus colubriformis and Teladorsagia circumcincta in sheep. For each experiment, animals were allocated hierarchically by liveweight into one of four groups that were either infected (group IF), similarly infected and concurrently immuno-suppressed with weekly intramuscular injections of 1.3mg kg liveweight (LW)⁻¹ of methylprednisolone acetate (group ISIF), immunosuppressed only (group IS) or remained as controls (group C). Body composition of all animals was estimated using x-ray computer tomography prior to infection and at the conclusion of each study with bodyweight and faecal nematode egg counts (FEC; eggs gram⁻¹ of fresh faeces (epg)) measured along with blood samples taken for the determination of levels of serum proteins, phosphate and antibodies. In the first trial (Chapter 3), the nutritional cost of both the acquisition and maintenance of immunity to gastro-intestinal nematodes was investigated using immunologically naive 5-month-old lambs and immunologically competent 17-month-old ewes during infection with 2,000 and 4,000 L3 infective T. colubriformis larvae d⁻¹, respectively (80 L3 T. colubriformis larvae kgLW⁻¹ d⁻¹). Profiles of FEC and comparative worm burdens at slaughter indicated an effective immune response was maintained in IF ewes and developed in IF lambs while successfully suppressed in both ISIF lambs and ISIF ewes and was confirmed by serum antibody titres. The typical reduction in voluntary feed intake as a consequence of infection was observed in IF lambs (0.30, p0.05). Infection proportionately reduced the gross efficiency of ME utilization in immunologically normal animals by 0.23 in HIF (p=0.09) and by 0.51 in LIF (p=0.01), but not in immuno-suppressed animals. Immuno-suppression did not suppress serum L3 IgA levels in seven of the eight HISIF and four of the eight LISIF animals. Furthermore, only four out of the eight immunologically normal animals from both the HIF and LIF groups displayed an L3 IgA response. Consequently, regardless of immunosuppression treatment, animals were termed as IgA responders (HR or LR) or non-responders (HN or LN). Feed intake was proportionately reduced from day 22 by 0.15 in HR (p=0.03) and by 0.32 in LR (p=0.01), but was not significantly reduced in HN or LN. Gross efficiency of ME utilization was significantly reduced for LN animals only, being proportionately 0.59 (p