Anthelmintic Resistance in Sheep: Management

One of the most important factors determining the rate of development of anthelmintic resistance is the proportion of the parasite population which is exposed to drug. Maintenance of a worm population unexposed to drug (in refugia) is now accepted as the most appropriate means of slowing the rate of selection of resistance.  Optimising drug treatments either by targeting treatments times to ensure that some worms remain in refugia, or by a targeted selective treatment  (TST) approach (e.g. where only those animals that will most benefit from treatment are given anthelmintic) could provide the optimal conservation of drug efficacy. 

Recent work at Moredun has developed refugia-based management strategies to control roundworm parasites and have investigated the effects of using a TST regime on animal performance and drug efficacy.  Moredun scientists developed a decision support system to identify animals that required treatment on the basis of their performance.  The system also predicts the expected liveweight gain for each animal whilst allowing for environmental differences that could affect growth, such as herbage availability and environmental stress. Treatments were only given to those animals that had not performed as expected, whereas those animals that had met or exceeded their target weight gain were left untreated.  The number of drug treatments was halved in the TST approach, compared to a whole-group monthly treatment regime, with no negative effects on growth rate.  The whole-flock monthly treatment regime selected heavily for the development of anthelmintic resistance, whereas drug efficacy was maintained in the TST groups.  It is hoped that these approaches may help to provide effective, sustainable control for roundworm parasites in small ruminants.  

Current work

The current research in this area is directed towards:

• Developing methods to manage anthelmintic worms on farm, which help to slow the development of resistant worms and maintain efficacy of current and new drug families. 
• Determining if the TST approach will be applicable on commercial farms.