Refining premating parasite control for ewes: identifying those most likely to benefit from anthelmintic treatment

GASTROINTESTINAL nematode (GIN) infections are responsible for significant reductions in the production efficiency of pasture-based small ruminant industries worldwide.1 Control of GINs typically relies on the use of anthelmintics; however, their intensive application at the whole-flock level has resulted in widespread anthelmintic resistance, reducing their effectiveness.2, 3 To tackle the problem of emerging anthelmintic resistance, several studies have investigated strategies for sustainable GIN control, primarily through changes to whole-flock treatment protocols.4, 6 Often these strategies focus on timing anthelmintic applications to maximise epidemiological benefits and reduce the number of unnecessary treatments. However, an alternative approach is targeted selective treatment (TST), in which treatment decisions are made at the animal level rather than the flock level. TST relies on leaving a proportion of the flock untreated, based on the principle that most GINs are concentrated in a few individuals; that is, treating only the animals with a high burden would have a large effect on overall GIN transmission and flock health.7 Leaving some sheep untreated also promotes the maintenance of in refugia parasite populations (ie, populations not exposed to anthelmintics, and thus remaining susceptible), including eggs and immature worms already established on the pasture before anthelmintic treatment, as well as adult worms residing within untreated animals.8 These structured treatment regimens, alongside planned grazing, can help maintain in refugia populations and reduce the proportion of anthelmintic-resistant larvae on the pasture. Implementing TST to reduce the number of anthelmintic treatments during the premating period may offer an opportunity to slow the development of anthelmintic resistance with minimal impacts on productivity. However, there is currently a lack of research into the effects of TST on ewe reproductive performance. To this end, a recent study by Williams and colleagues,18 summarised on p 133 of this issue of Vet Record, aimed to assess the feasibility of TST in premating ewes to reduce the number of anthelmintic treatments applied without compromising ewe reproductive performance. In addition, the authors retrospectively analysed TST indicators to identify those most closely associated with reproductive performance. During the trial,18 ewes were randomly assigned to either receive anthelmintic treatment before mating or remain untreated. Data on TST indicators, including body condition score (BCS), liveweight and dag score, were collected at three time points: preflushing (three weeks before mating), premating (one day before ram introduction) and postmating (11 weeks after mating commenced). In addition, ewe energy utilisation was estimated at the premating timepoint, using methods adapted from a previous study of TST in lambs.19 This allowed the authors to evaluate whether ewes exhibiting traits commonly associated with higher GIN burdens would display varying reproductive performance relative to their anthelmintic treatment status. Overall, Williams and colleagues18 observed no significant difference in scanned litter size between treated and untreated ewes. However, BCS and liveweight at the premating time point proved to be useful indicators of ewe reproductive performance. Specifically, ‘poor-performing’ ewes (ie, those with a BCS of less than 3 and/or a liveweight of less than 60 kg) that did not receive anthelmintic treatment produced significantly smaller litter sizes than poor-performing ewes that received treatment. Conversely, ewes with a higher liveweight and/or BCS produced larger litter sizes irrespective of their anthelmintic treatment status. No significant difference in litter size was observed between treated and untreated ewes when assessing preflushing BCS/liveweight, ewe age, dag score or BCS/liveweight change between the preflushing and premating time points. Similarly, there was no significant difference between treatment groups regarding the proportion of ewes carrying multiple fetuses versus those that were barren or carrying a single lamb.18 Williams and colleagues' findings provide evidence that using ewe BCS and/or liveweight as TST indicators is a viable alternative to blanket treatment for farmers seeking to treat ewes for GINs before mating.18 These results support previous findings by Calvete and colleagues,20 who suggested that only ewes with a BCS of less than 2.75 showed improved conception rates after anthelmintic intervention when compared to their higher performing peers. Similarly, the findings of Cornelius and colleagues21 further support the use of BCS as an indicator for TST-based GIN control strategies in ewes. However, while BCS and liveweight allow for real-time treatment decisions when handling sheep, successful implementation requires that the weighing equipment is correctly calibrated and the scoring of body condition remains consistent (eg, the same operator scoring all individuals in the flock). Assessing data on the parasite burden within the flock before anthelmintic treatment and, where feasible, information on GIN species composition, is also a critical aspect of the treatment decision-making process. Although Williams and colleagues18 conducted pooled faecal egg counts (FECs) before treatment, individual parasite diagnostics were not performed; thus, it was assumed that any improvements in reproductive performance in ewes receiving anthelmintic treatment were due to GIN management. It is unlikely that liveweight and BCS were always correlated with an individual ewe's parasite burden. However, a balance must be struck between the insights gained from individual animal FECs and the additional logistics, cost and labour required at the farm level. In conclusion, the study by Williams and colleagues18 contributes significantly to the literature by providing valuable data on the application of TST in premating ewes. Their analysis revealed that only a small proportion of ewes benefited from anthelmintic treatment when considering reproductive performance, suggesting that blanket treatment at this time point would have been unwarranted. Moving forward, completing similar trials that account for variability in sheep breed, grazing/nutritional management, anthelmintic medicine classes, geographical locations and ram demographics will be essential to inform robust TST decision systems for premating ewes.