Naphthalophos combinations with benzimidazoles or levamisole as effective anthelmintics for sheep

Objective To investigate the relative efficacy and safety of the anthelmintic naphthalophos in sheep, either given alone or in combination with benzimidazole (fenbendazole and albendazole) or levamisole anthelmintics. Design A parasitological study using faecal egg count reduction tests, a validating slaughter trial and field safety trials. Procedure Faecal egg count reduction tests were carried out on 13 farms. Naphthalophos and combinations of naphthalophos with levamisole and fenbendazole were included in the drench tests. On one property a controlled efficacy study was carried out to validate faecal egg count reduction test findings, In this trial, sheep were slaughtered 10 days after treatment and the remaining parasites recovered from the gastro-intestinal tract. Safety trials were carried out on eight farms where approximately 50 000 sheep were treated with naphthalophos and albendazole that were tank mixed in the backpack. Results The efficacy of naphthalophos alone in faecal egg count reduction tests ranged from 59 to 98% with one test showing greater than or equal to 95% reduction. The efficacy of naphthalophos and levamisole ranged from 74 to 100%, with 5 farms showing greater than or equal to 95% reduction. The efficacy of naphthalophos and fenbendazole ranged between 88 and 100% with greater than or equal to 95% reduction achieved on 10 farms. The controlled efficacy study showed a good correlation between the faecal egg count reduction tests and numbers of parasites recovered, except for Nematodirus where the faecal egg count reduction tests overestimated efficacy. The mortality rate in the safety trials was 0.05%, with most fatalities occurring on one farm. Conclusion The combination of naphthalophos and fenbendazole was more effective than a combination of naphthalophos and levamisole, and will provide a sufficiently safe drench rotation option.