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 parasites to survive over winter and to provide transmission of infective larvae from ewes to lambs during the spring. Routine laboratory propagation techniques probably have selected for those characteristics that would not promote PPR in conventional laboratory strains of Haemonchus contortus. An established isolate of H. contortus (BPL) was the source of the initial inoculum to select for a strain that readily exhibits characteristics of PPR. The selection process included inoculation of helminth-free pregnant ewes during mid-gestation, collection of nematode 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 concentrations. Lambs inoculated with the PPR strain had higher egg concentrations, higher total daily egg production, fewer adult worms, larger female worms, and higher fecundity. Repeated selection in the appropriate host, after prolonged storage of the inoculum, produced a PPR strain of H. contortus morphologically and reproductively distinct from the initial source isolate.