Assessment of parasite control and weight gain after use of an ivermectin sustained-release bolus in calves.

OBJECTIVE: To assess parasite control and weight gain after administration of an ivermectin sustained-release bolus over 135 days to calves grazing in the midwestern United States. DESIGN: Replicated pasture study. ANIMALS: 56 Bos taurus calves. PROCEDURE: Calves were matched for body weight and randomly allocated to remain untreated or to receive an ivermectin sustained-release bolus before turnout on day 0. Calves were grazed by treatment group on B pastures (4 replicates). Body weights and fecal egg counts were recorded on days- 1 and 28, and then at 28-day intervals until day 168. RESULTS: Parasitism was not clinically evident prior to or during the study. In treated calves, mean fecal egg counts were at or near 0 at all posttreatment evaluations. Although the mean egg count exceeded 20 ova/g only once in control calves, the cumulative egg output was > 42 million/calf. For the treated group, it was < 0.1% of this number. Mean total weight gain was 33.9 kg (74.6 lb) greater for ivermectin-treated calves than for untreated control calves (P < 0.02): a 34% increase. CLINICAL IMPLICATIONS: Fecal trichostrongyle eggs from calves can accumulate over a grazing season to provide enormous potential for augmenting pasture infectivity. An ivermectin sustained-release bolus (administered to calves being placed on pasture) controls parasitism, limits pasture infectivity, and can substantially influence growth by limiting the impact of subclinical parasitism.

Production losses in Merino ewes and financial penalties caused by trichostrongylid infections during winter and spring.

The productivity of Merino ewes treated with ivermectin and a controlled-release capsule containing albendazole was compared with untreated ewes grazing the same pastures on each of 3 farms. Treated breeding ewes had significantly increased greasy fleece weights (GFW, 6.5% and 7.1%) compared with untreated breeding ewes, on 2 of 3 farms but treatment caused no significant increase in the GFW of non-breeding ewes. Other benefits of treatment on all farms were a significant increase in body weight gain (from 1.7 to 3.7 kg) and a significant decrease in the weight of dag removed at crutching (from 42 to 622 g). These benefits occurred despite the presence on each farm of worms resistant to benzimidazoles. One disadvantage of treatment was an increase in mean fibre diameter of wool from treated ewes of from 0.12 to 0.41 microns. This increase reduces the value of the wool. Partial budgets indicated a net loss of from 8 to 62 cents per ewe for treatment. However, potential benefits such as increased growth rate and wool production from lambs of treated ewes were not included in the analysis. On one farm ewes were classified as 'susceptible' to severe dag if they had a dag score > or = 4 at the time of capsule treatment. Ewes classified as 'susceptible' were about 7 times more likely to develop severe dag than were other ewes. There was no significant difference between the GFW of untreated 'susceptible' ewes and untreated ewes not classified as 'susceptible'.(ABSTRACT TRUNCATED AT 250 WORDS)

Mathematical models of the population biology of Ostertagia ostertagi and Teladorsagia circumcincta, and the economic evaluation of disease control strategies.

The construction and use of mathematical models of the population biology of Ostertagia ostertagi and Teladorsagia circumcincta is discussed. Simulated field trials implemented by deterministic mathematical models currently share with actual field trials the disadvantage that they convey no information concerning the risk associated with the net return demonstrated by the trial. This has important implications when it is necessary to rank disease control strategies in order of usefulness.

An economic appraisal of helminth parasites in sheep.

The economic effects of the common helminth parasites of sheep are described and the current methods of prophylaxis adopted by sheep farmers in the United Kingdom reviewed. More efficient prophylactic programmes are outlined and the economic benefit accruing from two programmes designed to control fascioliasis and parasitic gastroenteritis are presented in detail.