EPIDEMIOLOGY OF GASTROINTESTINAL NEMATODE PARASITISM IN SUFFOLK AND GULF-COAST NATIVE SHEEP WITH SPECIAL EMPHASIS ON RELATIVE SUSCEPTIBILITY TO HAEMONCHUS-CONTORTUS INFECTION

An eight-year study was conducted to define the epidemiology of gastro intestinal nematode infection in Suffolk and Gulf Coast Native (Native ) breeds of sheep, and to determine if the Native sheep is more resist ant to infection. For the initial three years, each breed grazed separ ate pastures where anthelmintic treatments were administered to indivi dual animals on a salvage basis. For the last five years, both breeds grazed concurrently; anthelmintic treatments were administered to indi vidual animals on a salvage basis for the first three years, and to al l animals, when treatment criteria were met, for the last two years. T he fecal egg count (FEC) and blood packed cell volume (PCV) were monit ored, and tracer lamb nematode burdens were determined. Overall, FEC f or both breeds increased in the spring (periparturient rise) for most years and in the summer for all years. Under separate grazing conditio ns, Native ewes and lambs had consistently lower infection levels than Suffolk ewes and lambs. During the haemonchosis season (June-Septembe r) each year, Suffolk ewe and lamb PCV decreased, and Native ewe and l amb PCV remained relatively stable. The salvage treatment protocol res ulted in 27 treatments for Suffolk and one for Native ewes; similarly for lambs, 13 for Suffolk and zero for Native. Tracer lambs grazed wit h their respective breed, and the FEC and mean total nematode burden c orresponded with the pattern of infection for their respective breed. The predominant nematodes found in Suffolk and Native tracer lambs wer e Haemonchus contortus and Trichostrongylus spp., respectively. Under concurrent grazing conditions, the same seasonal repeatable pattern of infection was present and was exhibited by both breeds, with the Nati ve ewes and lambs being consistently and significantly (p less than or equal to 0.05) lower for FEC and higher for PCV. The salvage treatmen t protocol resulted in 57 and zero treatments for Suffolk and Native e wes, respectively: for lambs, 46 and 11. Tracer lamb nematode burdens again corresponded to their respective breed pattern of infection, wit h H. contortus and Trichostrongylus spp. being predominant in Suffolk and Native lambs, respectively. Data from all tracer lambs showed a re latively low level of hypobiosis (H. contortus only), and, although th ere was no consistent hypobiosis season, the tendency was for a higher level to occur in the fall. These results showed that the classic rep eatable seasonal pattern of gastrointestinal nematode infection occurr ed in both breeds of sheep, and that Native sheep were more resistant to infection (specifically H. contortus) than Suffolk sheep. (C) 1998 Elsevier Science B.V.