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A composite population of Haemonchus contortus was established with larvae from seven diverse sources, then maintained in Merino sheep bred to have either increased or decreased resistance to Haemonchus. After five, seven and 14 parasite generations, the two resulting lines of parasites were used to infect sheep from the increased resistance line, an unselected control line and the decreased resistance line. Line of sheep had a highly significant effect on average faecal egg counts 4 and 5 weeks after infection (geometric means 329, 735 and 1490 epg, respectively after the 14th generation), but the two lines of parasites yielded similar egg counts. There was no significant interaction between line of sheep and line of parasite, indicating that the parasite populations had not diverged significantly in their reproductive fitness, as measured by faecal egg count.

Lines of Merino sheep selected for increased (IRH) and decreased (DRH) resistance to Haemonchus contortus were compared with an unselected (CH) line, after approximately four generations of selection. Measurements were recorded on 69 IRH, 47 DRH and 84 CH animals. Following artificial challenge with H. contortus, the IRH line had significantly (P less than 0.001) lower faecal egg counts than the CH and DRH lines (2730, 12,720 and 17,400 epg, respectively). Significant differences (P less than 0.05) were found between all lines in the minimum packed cell volumes during artificial infection (25.7, 22.0 and 20.3%) and in faecal egg counts after natural infection (140, 3590 and 8750 epg). Differences were also recorded (P less than 0.05) following artificial challenge with Trichostrongylus colubriformis (490, 840 and 1340 epg). On a percentage basis, faecal egg counts in the IRH line deviated less from the CH line following artificial infection with T. colubriformis (42%) than with H. contortus (79%). The reverse was true for the DRH line (60 and 37%, respectively). Differences in egg output of this magnitude should have a marked effect on requirements for anthelmintic treatment, rate of development of drug resistance and level of pasture contamination when the lines are grazed separately.

Three paddocks were contaminated with Haemonchus contortus eggs from early spring to mid summer by yearling sheep. In Paddock 1, all contamination was derived from worms that had been previously inhibited in their development. The contamination in Paddock 2 was produced only by infections recently acquired from pasture, and in Paddock 3 contamination was attributable to both sources. When the yearling sheep were replaced in mid-summer by newly weaned lambs, severe outbreaks of haemonchosis occurred on all paddocks. It was concluded that inhibited H. contortus larvae are capable of producing sufficient eggs, when they resume development, to initiate an outbreak of haemonchosis in susceptible sheep.

A chronic infection was established in a group of twenty 18-month-old non-reproductive Merino ewes by oral administration of 3000 infective H. contortus larvae twice weekly for 12 weeks. Their live-weights and wool production were compared with those of 20 uninfected ewes grazing the same pasture. In the infected sheep, faecal egg counts increased over a period of 9 weeks to reach a mean of 5000 eggs per gram, accompanied by small, but significant effects on packed-cell volume and live-weight. The effects of infection on wool growth were also small, and not statistically significant, although there was evidence of a faster seasonal decline in wool growth in infected sheep. It was concluded that mortality induced by acute infection is the most important economic effect of this parasite.