This paper examines the known immunological and genetic factors associated with sheep resistance to infection by Haemonchus contortus. Such resistance is an inheritable genetic trait (h 2, 0.22–0.63) associated with certain sheep breeds. Resistant sheep do not completely reject the disease; they only harbor fewer parasites than susceptible sheep and therefore have a lower fecal egg count. Protective immune response to haemonchosis is an expression of genetic resistance. Genes associated with resistance and susceptibility are described. Genetically resistant sheep have nonspecific mechanisms that block the initial colonization by Haemonchus contortus larvae. These sheep also have an efficacious Th2 type response (e.g., increases in blood and tissue eosinophils, specific IgE class antibodies, mast cells, IL-5, IL-13, and TNFα) that protects them against the infection; in contrast, susceptible sheep do not efficiently establish this type of immune response. Finally, the main reported antigens of H. contortus were reviewed.

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Correlation of parasitological immunological parameters in sheep with high and low resistance to haemonchosis. Can. J. Anim. Sci. 86: 363-371. Parasitological and immunological parameters of Blackbelly (Bb), a haemonchosis high-resistance sheep breed, and Columbia (Cb) a haemonchosis low-resistance breed were compared. Two experiments were performed: Bb and Cb lambs were infected weekly with 1000 third-stage larvae (L3) of Haemonchus contortus for 6 wk. In the first experiment, faecal egg counts (FEC) and blood eosinophils (BE) were determined for 15 wk. In the second experiment, the following parameters were measured weekly: FEC, plasma proteins, serum IgG to H. contortus L3 levels by ELISA and L3 antigens recognized by Western blot; animals were euthanized after 15 weeks and autopsies were performed to evaluate the total adult phases (AP), eosinophils and mast cells in the fundic abomasal region (FAR), pyloric abomasal region (PAR) and abomasal lymph node (ALN).Fecal egg counts and AP were significantly higher (P < 0.05) in Cb lambs. BE showed a significant increase (P < 0.05) in both breeds at week 9 and, subsequently, Cb lambs presented a sudden decrease in these levels. The Bb lambs revealed a higher amount (P < 0.05) of antibodies between weeks 9 and 15 of the experiment, and Cb lambs showed a reduction in antibody levels at week 9. The Western blot revealed 11 bands of H. contortus L3 antigens; no differences were observed between breeds. The number of tissue eosinophils per cm 2 in PAR and ALN was higher (P < 0.05) in the Bb lambs. No differences were found (P > 0.05) in the number of mast cells per cm 2 in the sampled regions of both infected groups. The increase and maintenance of serum antibodies and circulating and local eosinophils in resistant animals agrees with the development and maintenance of a Th 2 type immune response. The initial increase and further reduction of serum antibodies and circulating and local eosinophils in the low-resistance lamb breed suggest an initial Th 2 type response and a subsequent change to a Th 1 type response. Les animaux ont été sacrifiés au bout de 15 semaines et on a pratiqué une autopsie pour évaluer le nombre total de phases adultes du parasite (PA), d'éosinophiles et de mastocytes dans la région gastrique (RG), la région pylorique (RPC) et les ganglions lymphatiques (GL) de la caillette. Le nombre d'oeufs et de PA dans les fèces était significativement plus élevé (P < 0,05) chez les agneaux Cb. Les auteurs ont relevé une hausse significative (P < 0,05) du NES chez les deux races la neuvième semaine; par la suite, cette concentration baissait subitement chez les agneaux Cb. Les agneaux Bb présentaient une concentration plus élevée (P < 0,05) d'anticorps entre la 9 e et la 15 e semaine de l'expérience alors que cette concentration était plus basse chez les agneaux Cb, la neuvième semaine. Le transfert de Western révèle 11 bandes pour les antigènes des L3 de H. contortus; aucune variation n'a été relevée à cet égard entre les deux races. Les agneaux Bb présentaie...

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