Infection with nematode parasites detrimentally affects production efficiency in grazing animals, mainly through a reduction in food intake (anorexia).This thesis describes a series of six in vivo experiments designed to investigate the interactive effects of genotype, immune status and protein nutrition on the occurrence of parasite-induced anorexia of sheep. The experiments also investigated the role of the hormore leptin in immune response and anorexia following infection with gastrointestinal nematodes. In general, each experiment involved two breeds of lambs (Suffolk x Greyface, S, and Scottish Blackface, B) or ewes (Greyface cross, G, and Scottish Blackface, B) that are known to differ in their production potential. In each experiment, animals were either infected with the abomasal nematode Teladorsagia circumcincta and fed ad libitum or non-infected and fed either ad libitum or restrictedly..The first two experiments investigated the effect of a primary and a secondary infection on anorexia and plasma leptin concentrations (PLC) in growing lambs (Chapter 3). The secondary infection started two weeks after the discontinuation of the primary infection. The results showed that lambs of the S breed were more susceptible to nematode infection than B lambs, as judged from the differences in faecal egg counts. Primary infection resulted in anorexia in S lambs but not in B lambs and re-infection tended to affect the food intake of S lambs only. Infection did not result in an acute increase in PLC, but its effect was significant when variation in food intake between treatments was accounted for. These results suggest that anorexia can occur in previously infected lambs, thus the effect of re-infection on anorexia was further investigated (Chapter 4).Chapter 4 describes a series of 3 experiments with lambs. In these experiments, previously naïve lambs of approximately 3 (experiment I) or 7 (experiment II) months of age were infected with T. circumcincta for either 10 or 7 weeks, respectively. Lambs of experiment I were re-infected either 4 or 8 weeks after the end of the primary infection (experiment III). The results showed that the breed differences in resistance to infection were not associated with breed differenced in the degree of anorexia (experiment I) and infection of 7-month old lambs did not result in anorexia. Re-infection of previously infected lambs did also not result in anorexia when lambs were re-infected 4 or 8 weeks after the end of the primary infection. In addition, the results of these experiments showed that nematode (re)infection did not result in an increase of PLC. These results suggest that leptin may be involved in the response of Iambs to infection, but it is unlikely that leptin alone is responsible for the parasite induced anorexia in lambsThe last experiment (Chapter 5) investigated the consequences of protein supplementation on anorexia, and PLC in infected periparturient ewes. Infection resulted in a breakdown of immunity to parasites (PPRI) and a reduction in food intake in both breeds. The breeds differed in the extent of PPRI (G ewes having higher FEC than B ewes), but not in the magnitude of anorexia. Protein supplementation resulted in a reduction in FEC, but had no effect on the magnitude of anorexia. Plasma leptin concentrations changed significantly over time, but were not affected by protein supplementation or infection. It was concluded that infection with T. circumcincta in periparturient ewes results in anorexia that is not alleviated by protein supplementation. Leptin is unlikely to be responsible for the anorexia of nematode infection in periparturient ewes.The outcomes of the above experiments are brought together with the literature in the General Discussion (Chapter 6) and directions of future work, to elucidate the mechanisms as well as the functional significance of anorexia, are put forward
