Design and Development of a Targeted Selective Treatment (TST) Strategy for Gastrointestinal Nematodes (GIN) in Ewes

Abstract

Gastrointestinal nematodes (GIN) are regarded as one of the major threats to productivity, health, and welfare of sheep globally and contribute to significant economic losses within the sheep industry. GIN are mainly controlled by anthelmintic treatments; however, anthelmintic resistance (AR) is becoming an increasing problem following historical excessive application of such treatments and generally inappropriate use. Thus, routine anthelmintic treatment of
sheep is no longer regarded by the industry as being viable and alternative strategies are urgently needed for targeted evidence based anthelmintic administration. One solution proposed is targeted selective treatment (TST), a strategy designed to decrease anthelmintic use by only treating the proportion of the flock that requires treatment. Although extensive research has been conducted into appropriate TST strategies for lambs, limited studies have been conducted on TST of ewes. In this thesis, current ewe GIN control practices in Great Britain (GB) are investigated as are potential indicators for TST against GIN at various points within the ewe’s key production times.

A questionnaire study determined that participating British farmers treat their ewes against GIN 1.68 times per year on average, with 42.3% and 32.1% of participating farmers worming their ewes once or twice a year on average, respectively. 17.2% of participating farmers wormed their ewes more than twice a year, and 8.4% never worm their ewes. The questionnaire also found that participating farmers who devised GIN control strategies based on Sustainable Control of Parasites in Sheep (SCOPS) guidelines treated their ewes significantly less frequently per year (P<0.001), whilst those determining treatment timing based on cleanliness of tail area of the ewe i.e., dag scores or the time of year treated their ewes significantly more frequently (P<0.001). Farmers who devised GIN control strategies in conjunction with their vet had greater odds of using TST (P<0.001), as well as farmers who determined flock treatment timing based on ewe condition (P=0.027). TST indicators in ewe pre-mating and during the periparturient period were also investigated on a total of 8 Welsh farms. At pre-mating, it was found that the reproductive performance of ewes with a BCS < 3
or weighing < 60 kg benefited from anthelmintic treatment (P<0.05). However, ewes with a BCS ≥ 3, or weight ≥ 60 kg did not benefit from receiving an anthelmintic treatment, with regards to scanned litter size, which suggests that ewe BCS or liveweight can be used as TST indicators at this timepoint. During the periparturient period, numerous variables were identified to be associated with ewe FEC. These variables included ewe BCS (P=0.009), age (P=0.029), dag score (P=0.023), and the Suffolk breed (P=0.009), whilst group average FEC was also shown to have power to significantly improve model fit while reducing between farm variances. GIN nemabiome composition was also investigated in periparturient ewes, with eight main species found infecting study ewes, with Teladorsagia circumcincta, and Trichostrongylus axei being the most common GIN species found. Significant relationships were uncovered between specific GIN species within an ewe’s nemabiome and TST indicators which indicates that further consideration of ewe nemabiome is needed when TST strategies are developed. Finally, the thesis investigated the potential for precision livestock farming (PLF) technologies to be used to guide TST in periparturient ewes. The ewe mean number of daily lying bouts and mean lying bout duration were both significantly associated with ewe FEC (P=0.022 and P=0.036, respectively). This demonstrates that ewe behaviours are associated with increasing GIN burden in housed periparturient ewes and PLF technology has potential with further refinement to be a new tool to identify ewes requiring anthelmintic treatment within a TST strategy.

Findings from this thesis suggest blanket treatment of all ewes can be avoided on sheep farms through the application of a TST strategy based on many different characteristics or a combination of characteristics, such as BCS, weight, dag score, age, breed with the possibility of behaviours being used in the future. Application of TST will lead to a reduction in anthelmintic use leading to likely decrease in rate of AR development, along with maintaining productivity and enhancing economic outputs in the long term. Transferring this knowledge, along with further research into the applicability of TST across a range of different farming systems, sheep breeds and varying nemabiome composition is imperative to increase our knowledge of the parasite interaction with the host to apply TST strategies practically in commercial farms to limit GIN and AR threat on livestock health and productivity.

Date of Award	2023
Original language	English
Awarding Institution	Aberystwyth University
Supervisor	Rhys Jones (Supervisor) & Peter Brophy (Supervisor)