Measurement of IgA activity against parasitic larvae, fecal egg count and growth rate in naturally infected sheep

Gastrointestinal parasitism is one of the greatest causes of disease and lost productivity in domestic animals. Anthelmintic treatment is the mainstay of current control methods. However, with the widespread use of anthelmintics, the problem of parasite resistance has emerged. Other modalities of prevention and treatment are urgently needed. One of the most promising developments in this respect is the use of genetically resistant sheep. Selective breeding of animals resistant to gastrointestinal parasitism is particularly attractive, but identifying the phenotypic and genetic markers of resistance on which selection will be based is a major problem. Hence, a study was undertaken to investigate some of the phenotypic determinants of resistance to nematode infection in Scottish Blackface sheep when naturally infected with the gastrointestinal nematodes, particularly Teladorsagia circumcincta, and how these determinants may facilitate the successful selection of resistant animals. The present study has not found any correlations between plasma IgA activity against thirdstage larvae of T. circumcincta and fecal egg. Moreover, there was no significant correlation between growth rates in 24 week old lambs. The results suggest that IgA activity against fourthstage larvae is probably a better marker of nematode resistance than IgA activity against thirdstage larvae. The work detailed here has further increased our understanding of the complex host/parasite relationship, and has confirmed that selective breeding, using the various phenotypic markers.