Search

In merino sheep bred for either increased or decreased resistance to Haemonchus contortus, faecal worm egg counts (FEC) were lower in the resistant line (6,831 vs 17,645 epg, P < 0.01), and circulating eosinophils (EOS) were higher, but not significantly so (3.40 x 10(4) ml(-1) vs 1.40 x 10(4) ml(-1), P = 0.1 1). Another flock was artificially infected with Trichostrongylus colubriformis and significant genetic variation was found in both FEC (heritability 0.40 +/- 0.11) and EOS (0.19 +/- 0.08). In a third flock comprising tropical sheep breeds, a natural challenge with T. colubriformis resulted in significant sire effects on FEC (heritability 0.20 +/- 0.10) but not EOS (heritability inestimable). We conclude that EOS offers no advantage over FEC as it selection criterion for resistance.

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.

From autumn (April) to spring (November), groups of pregnant and dry Merino ewes grazed pasture contaminated with infective larvae of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis. A periparturient rise in faecal egg counts occurred in the pregnant ewes, following the maturation in early spring of arrested fourth-stage larvae of H. contortus and failure of pregnant ewes to expel the resulting adult worms. Peak egg counts were seen in September, just before lambing. In dry ewes, egg counts were slightly elevated at the time of the rise in the pregnant ewes, but fell to very low levels as adult worms developing from previously arrested larvae were expelled. Lactating ewes acquired greater burdens of O. circumcincta and T. colubriformis than did dry ewes, but were equally refractory to new infections with H. contortus. It is therefore suggested that the impairment of immunity to helminth infection seen in reproductive ewes may be more specific than was previously envisaged.