Epidemiology of Strongylate Nematode and Coccidia (Eimeria) Infection in Sheep With Special Reference to Breed Resistance.

In the United States, the annual production losses in small ruminants due to internal parasites has been estimated at $45 million. In 1992, 1993 and 1994, breed specific responses to naturally acquired strongylate nematode and coccidia ({\it Eimeria\/}) infections were compared in three classes (suckling lambs, weaned lambs, and mature ewes) of pure-bred Suffolk and sheep native to Louisiana (Native) which were grazed together on infective pasture. Parasitological (fecal egg counts and nematode counts) and hematological (packed cell volumes, total immunoglobulin levels, total and class-specific anti-{\it Haemonchus contortus\/} antibody levels and eosinophil counts) data were collected. In suckling Native lambs, resistance to H. contortus infection developed by 7-10 weeks of age, manifested by reduced fecal egg counts, high packed cell volumes and reduction in adult nematode burdens and a slower decline in maternal antibodies. In contrast, suckling Suffolk lambs remained unresponsive to strongylate nematode infection and suffered mortality. Anthelmintic treatments administered to weaned Native lambs were not detrimental to the protective responses to H. contortus. Weaned Suffolk lambs and mature ewes continued to remain susceptible to strongylate nematode infection and susceptibility was complicated by anthelmintic resistance. Total H. contortus specific immunoglobulin levels indicated that both breeds became immune competent by three to four months of age. Total immunoglobulin levels and anti-H. contortus IgM, IgA, IgG1 and IgG2 levels were lower in weaned Suffolk lambs compared to Native lambs. No differences were seen in the blood eosinophil counts between the weaned lambs and mature ewes of the two breeds. The variability demonstrated in the parasitological and packed cell volume data indicates that breed substitution or possibly cross-breeding could alleviate production losses and the problems associated with anthelmintic resistance. In 1992 and 1993, no differences were detected between the overall coccidial oocyst output of both lambs and mature ewes of the two breeds. Eimeria crandallis was the predominant species. In suckling lambs, maximum numbers of E. crandallis oocysts excreted in feces occurred between six to nine weeks after turnout and no clinical coccidiosis occurred