Targeted selective treatment (TST) has been proposed as a sustainable method of gastrointestinal nematode control that reduces the number of anthelmintic treatments administered, thereby preserving a susceptible nematode population in refugia.
On three farms in Ireland, weaned lambs were weighed and separated into three weight classes (light, medium, and heavy) to see if heavy lambs were more resilient than light lambs, if left untreated. Within the heavy (n = 225) and light (n = 218) weight classes, lambs were randomly allocated to two treatments; anthelmintic treatment or no anthelmintic treatment. All lambs in the medium weight class were treated. Animal performance and parasitological parameters were assessed over a 28-day period.
Anthelmintic treatment had a significant effect on fecal egg count, average daily weight gain, body condition score, and dag score. Withholding anthelmintic treatment from lambs had a negative effect on worm egg count and animal performance with no evidence that heavy lambs were more resilient than light lambs when left untreated. The proposition that live weight is a suitable criterion for identifying lambs that would benefit most from anthelmintic treatment was not supported
The helminth fauna observed in sheep in Ireland constitutes primarily of Teladorsagia, Nematodirus, Trichostrongylus, and Cooperia. Haemonchus is rare. There was no history of Haemonchus on any of the farms.
Source: Veterinary Parasitology. Volume 258. July 2018. Read abstract