A targeted selective treatment approach for effective and sustainable sheep worm management: investigations in Western Australia

The effectiveness of a targeted selective treatment (TST) approach to the management of sheep worms in adult Merino ewes was compared with a normal worm control program on three properties in south-western Western Australia. For a TST strategy, a proportion of a flock is left undrenched when flock treatments are administered. This establishes a population of worms in refugia, isolated from the effects of anthelmintics, which dilutes the population of worms that survive drenching and, hence, minimises the development of anthelmintic resistance. The main worm genera present in this trial were Teladorsagia and Trichostrongylus. For the TST approach, an initial flock worm egg count was used to determine the proportion of the flock to be drenched at any time, and treatments were then given to that proportion of the flock, with treated individuals identified as those in the poorest body condition. Over a period of 10–12 months, the TST groups on the three properties received 60%, 53% and 73% of the number of drenches administered to the normal treatment groups, in which all sheep were drenched. No clinical signs of parasitism or adverse effects on reproductive indices occurred in the TST groups. Although bodyweight gain and wool weight were ~2 kg and 0.3 kg, respectively, lower in two of the TST groups than in the normal treatment groups, the differences were not significant, and body condition scores of the TST groups were consistently greater than optimal production recommendations. The TST concept has potential as an easily implemented approach to sustainable drench management and would reduce the cost and labour associated with worm control. However, further studies are required to develop decision indices for various environments to ensure a balance between worm control efficiency and sheep production.