Genetic Resistance of Scottish Blackface Sheep to Gastric Nematodes: Association With the Eosinophil

The gastric nematodes, Haemonchus contortus and Ostertagia (Teladrosagia) circumcincta, are the most pathogenic and economically important parasites of sheep in tropical and temperate areas, respectively. There are no vaccines available and the control methods rely entirely on grazing management and anthelmintic treatment. However, there are increasing constraints on the continued use of these conventional methods and alternative approaches for controlling gastric nematodes are under investigation. One of these, namely, genetic resistance to helminth infection is considered in this thesis. Chapter 1 is a general introduction and review of gastric nematodes with respect to production losses, pathogenesis, limitations of the conventional methods of control, and evidence for genetic variation in resistance. The association between eosinophilia and parasite infection, including, the role of T-lymphocytes, chemotactic factors and possible effector mechanisms, are considered. Chapter 2 is a general description of the materials and methods and includes, the type of sheep, their management, laboratory techniques and the methods of statistical analysis used to test for differences between groups or individual animals, including other interactive factors such as sex, time, drug treatment. Chapter 3 reports the results of an investigation on the eosinophil response of lambs to natural mixed nematode infection, predominantly O.circumcincta. Faecal egg counts and peripheral blood eosinophil counts were found to be overdispersed, with wide variations among the lambs. The correlation coefficient between faecal egg count and peripheral blood eosinophilia was negative and significant suggesting that peripheral blood eosinophilia might be an indication of resistance to gastric nematode infection. The repeatability of faecal egg counts taken three times at monthly intervals was not significant from zero (repeatability was 0.06), while that of peripheral blood eosinophil counts was 0.25 and was significant. Chapter 4 describes the results of an investigation on the existence of diurnal variation of peripheral blood eosinophil counts in sheep infected with 50,000 L3 O.circumcincta. Diumal variation was observed during low to moderate levels of peripheral eosinophilia with the highest counts around 0900 hrs and the lowest around midnight. However, at time of peak peripheral blood eosinophilia following infection, diumal rhythm was not observed. Thus, the time of sampling peripheral blood eosinophilia is an important factor to consider during low to moderate levels of infection, but not at peak infection period. The factors that might influence diumal variation, including cortisol levels in blood are considered. Chapter 5 presents the results of an investigation on eosinophil responses of 20 Scottish Blackface lambs following three experimental infections with 10,000 L3 H.contortus. The greatest decrease in growth rate of infected lambs was observed during the first infection. Following the second infection, there was a slight increase in growth rates, while following the third infection, growth further increased. There were marked differences in the degree of resistance to infection with H.contortus. Four lambs died before the end of the experiment and were classified as susceptible. The 16 lambs that survived, classified as resistant, showed marked differences in weight gains, in faecal egg counts, in the ability to maintain stable packed red cell volumes, in peripheral blood and abomasal tissue eosinophilia, and in worm burdens. There was a significant negative correlation between faecal egg output and body weight gains during the second and third infections and, between PCV and body weight gains, there was a significant positive correlation during the third infection. In all three infections, the correlation between faecal egg output and PCV was significant and positive. With respect to peripheral blood eosinophilia, it was significantly and positively correlated with body weight gains during the first and second infections, significantly and negatively correlated with faecal egg counts during the second and third infections, and significantly and positively correlated with PCV values in the second and third infections. At the same time, peripheral blood eosinophilia and abomasal tissue eosinophilia were significantly and positively correlated; both were significantly and negatively correlated with worm burdens. Thus, peripheral blood eosinophilia was significantly correlated to parameters that reflect resilience (body weight) and parameters that reflect resistance (worm burden, faecal egg output and PCV) and might have a potential as a marker for selection. Chapter 6 is a general discussion with proposals for further investigations on eosinophils, including, heritability estimates of eosinophilia, association of eosinophilia with major histocompatibility complex, and eosinophil chemotaxis and adherence