A. Axelsen

Effects of ivermectin residues in sheep dung on the development and survival of the bushfly, Musca vetustissima Walker and a scarabaeine dung beetle, Euoniticellus fulvus Goeze

During produced by sheep for up to 1 week post-treatment (drenching) with ivermectin caused significant mortality to newly emerged larvae of the bushfly Musca vetustissima. Dung from sheep drenched with a mixture of levamisole and oxfendazole also resulted in larval mortality, but here the effect was limited to the first 48 h after treatment. Flies reared through to the adult stage on dung collected up to 32 days post-drenching were tested for evidence of fluctuating asymmetry, which is an indicator of developmental stress. None was found for either drench treatment, although there was evidence that ivermectin residues directly affected wing size. The introduced dung beetle, Euoniticellus fulvus was also found to be sensitive to ivermectin residues in sheep dung. Dung produced during the first day after drenching caused mortality among newly emerged beetles and delayed the reproductive development of survivors. However, beetles in which ovarian development was impaired regained their reproductive capacity following transfer to nontoxic dung. Day 1 dung caused no mortality among sexually mature beetles, although there was a significant reduction in their fecundity. Dung collected from 2 to 10 days post-drenching had no detectable effects on either the survival or reproductive development of adult beetles, regardless of age. Residues in dung collected 1-2 days post-drenching caused 100% mortality in beetle larvae, but by Day 5 there was no evidence of acute toxicity. These findings indicate that insects feeding on the dung of ivermectin-treated sheep display adverse effects similar in range to those reported for cattle dung. However, their duration is much more transient, owing probably to differences in drug formulation and route of administration.

The effect of selection with levamisole on benzimidazole resistance in Ostertagia spp. of sheep.

A field population of Ostertagia spp. (predominantly O. circumcincta) in sheep, found to be resistant to oxfendazole, was exposed to selection with1 levamisole in the laboratory and the field. Progeny of the survivors of a single dose of levamisole in penned sheep showed a significantly lower level of resistance to oxfendazole in an anthelmintic assay, and a similarly lower level of resistance to the ovicidal activity of thiabendazole in vitro compared with the progeny of the survivors of a single dose of oxfendazole, although they remained more resistant than a known susceptible strain of 0. circumcincta. The progeny of worms present in grazing sheep after three doses of levamisole at 4-weekly intervals, and subjected to the same tests, behaved in a similar fashion. They also showed a substantial fall in benzimidazole resistance compared with the progeny of worms not exposed to any anthelmintic for 6 months. The results suggested that levamisole selected positively against benzimidazole resistance. In these circumstances some form of alternation in the use of different anthelmintics could delay the development of a high level of resistance and maintain the practical usefulness of existing anthelmintics.

Effects of reproduction, genotype and anthelmintic treatment of ewes on Ostertagia spp. populations

Merino and Border Leicester × Merino (BL × M) ewes, nearly all of the same age and reared at the same site, were either unmated or mated to Border Leicester rams. Ewes of each genotype and reproductive status were untreated or were given a single pre-lambing drench with thiabendazole at 50 or 100 mg/kg a week before the start of lambing in spring on pastures at Canberra which had been contaminated during autumn and winter by adult sheep. The two genotypes grazed together within each combination of reproductive status and anthelmintic treatment which grazed separately. Thiabendazole was highly effective in removing both fourth stage larvae and adults of Ostertagia spp., the most abundant genus. Eight weeks after the pre-lambing drench lactating ewes carried larger Ostertagia spp. populations than did unmated ewes of both genotypes, but as a result of reinfection after treatment, differences between drenched and undrenched ewes were not significant. At this time lactating as well as unmated ewes harboured large populations of arrested early fourth stage larvae of Ostertagia spp. acquired during the last 8 weeks, showing that arrest of development is not prevented by lactation. There was strong evidence that some ingested larvae which became arrested in lactating ewes were rejected by unmated ewes. At all stages of the reproductive cycle studied, BL × M ewes were substantially more resistant to Ostertagia spp. infection than Merinos. No persistent benefits in parasite control or in animal production were detected from the pre-lambing drench.

Growth of cattle on phalaris and lucerne pastures II. Helminth parasite populations and effects of anthelmintic treatment

In young beef cattle, the effects on helminth parasite populations of anthelmintic treatment, pasture type and stocking rate are reported from a four-year experiment at Canberra. The animal production results are given by Morley et al. (1978). Anthelmintic treatment was at three levels — none, at weaning in autumn, and frequent treatment intended to be suppressive. In the last two years of the experiment when helminth populations were moderately high, treatment in early autumn recouped roughly half the production loss from uncontrolled helminth infection. Most of this effect occurred in spring and not soon after treatment. Monthly anthelmintic treatment failed to suppress Ostertagia ostertagi infection and delayed the natural development of host resistance. Treatment at intervals of two weeks or less largely prevented O. ostertagi egg output and eventually reduced pasture infectivity and worm burdens to low levels. In three of the four years, worm burdens in untreated cattle were lower on lucerne than on phalaris pastures.