SELECTION FOR A STRAIN OF HAEMONCHUS-CONTORTUS THAT EXHIBITS PERIPARTURIENT EGG RISE IN SHEEP

Periparturient egg rise (PPR) is a principal means for some nematode p arasites to survive over winter and to provide transmission of infecti ve larvae from ewes to lambs during the spring. Routine laboratory pro pagation techniques probably have selected for those characteristics t hat would not promote PPR in conventional laboratory strains of Haemon chus contortus. An established isolate of H. contortus (BPL) was the s ource of the initial inoculum to select for a strain that readily exhi bits characteristics of PPR. The selection process included inoculatio n of helminth-free pregnant ewes during mid-gestation, collection of n ematode eggs during lactation, and storage of infective larvae at 4 C for 4 mo. After 10 generations, the 2 strains, BPL and PPR from lambs and pregnant ewes, were compared for reproductive, morphological, and population differences in lambs and pregnant ewes. After lambing, ewes inoculated with the PPR strain had significantly higher fecal egg con centrations. Lambs inoculated with the PPR strain had higher egg conce ntrations, higher total daily egg production, fewer adult worms, large r female worms, and higher fecundity. Repeated selection in the approp riate host, after prolonged storage of the inoculum, produced a PPR st rain of H. contortus morphologically and reproductively distinct from the initial source isolate.