Impacts of targeted selective treatment on performance, nematode faecal egg counts and anthelmintic use in sheep: A meta-analysis

Gastrointestinal nematode (GIN) infections of ruminant livestock have traditionally been controlled through regular, "suppressive", treatment with anthelmintic drugs, but this has resulted in the evolution of anthelmintic resistance and reduced efficacy. There is now a critical need to reduce anthelmintic use to maintain anthelmintic efficacy by creating refugia where susceptible GIN reside. Targeted selective treatment (TST) strategies only treat animals that need it, ensuring animals affected by GIN maintain their performance, while animals unaffected by GIN are not treated unnecessarily. TST is generally agreed to reduce anthelmintic use without drastically reducing animal performance or increasing GIN faecal egg count (FEC), but there is considerable variation in results reported by studies comparing TST with other strategies. To explore the effectiveness of TST and identify factors affecting previous results, we applied meta-analysis to data collected from 20 studies comparing TST to other strategies. Overall, TST was associated with a statistically significant, but small, reduction in animal performance, no effect on worm FEC, and 50 % lower anthelmintic use. Experimental design affected study outcome: basing treatment decisions on body condition scores resulted in less favourable outcomes compared to other methods. Meanwhile, TST was associated with negligible reductions in performance compared to "suppressive" treatment, and although FEC was increased slightly in the TST group, anthelmintic use was reduced by around two-thirds. Differences between TST and "strategic" treatments were smaller, but TST still reduced anthelmintic use by 25 %. Overall, the results support the use of TST to reduce anthelmintic use while maintaining efficacy and animal performance.