Targeted selective treatment in French dairy sheep flocks

Infection by gastro-intestinal nematodes (GIN) in dairy sheep can continuously impact their health, as immunity is only partial throughout the animals’ life. Over the last decades, GIN infection has mainly been controlled using anthelminthics (AH). However, the use of AHs progressively selects GIN carrying resistance genes. Some practices have been proven to select them quicker than others, such as frequent use of the same family of drugs or doing a “dose and move” [5] during pasture management. On the other hand, refugia-based strategies such as Targeted Selective Treatment (TST) have the potential to slow down the appearance of resistance, yet many questions remain on how to implement them [8]. In this blog article, we report the main insights from a recent widescale field study performed in dairy sheep flocks in France, its objective was to evaluate the feasibility and performance of an original TST approach.

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Why treat dairy sheep selectively?

The principle of selective treatment is to leave part of the worm population non-exposed to the AH used. In this pool of genetically diverse GIN, there is a higher chance some will remain susceptible than if all are exposed to the drug. The use of refugia can therefore slow down the appearance of resistance by dilution of resistant parasites with susceptible parasites, or by limiting the selection of resistant parasites through treatment [9]. There are several ways to generate a population of parasites in refugia, and selective treatment of a flock is one of them.

Classical representation of the principles of refugia applied to worm control in Ruminants

What kind of selective treatment approach can be used?

During the ANTHERIN* project, we evaluated a selective treatment protocol that allowed to maintain refugia when treatment was administered during lactation, while using simple criteria. This protocol was evaluated in 5 dairy farms, located in the French western Pyrenees (farms B and X) or in the Roquefort area (farms A, F and S). These dairy sheep flocks were monitored for 2 or 3 years and had an average of 360 or 330 ewes. Selective treatment was done on the first treatment of the lactation.

The protocol was based on lactation rank. All first lactating ewes were treated, as they are usually the most susceptible population to GIN infection. Multiparous ewes were treated based on their body condition as evaluated by farmers. They treated animals they estimated had a poor body condition, and eventually animals with other clinical signs such as presence of diarrhea, anemia or weakening.

Example of a targeted selective treatment protocol that can be applied in dairy sheep farms

What are the main results?

Efficacy of eprinomectin (EPN) was maintained if it was initially good, which was the case for 3 out of 5 farms. On farm S, initial efficacy was reduced, and EPN treatment was stopped during 2022. On farm B, initial efficacy was good at the beginning of the project, yet further analysis of the larvae showed resistance was already present [12; 2]. Efficacy of EPN was reduced to 77% in 2023.

EPN efficacy at different timepoints during the study. Efficacy measured as described by [4].

The whole flock was sampled upon treatment day, and in both production areas, average fecal egg counts (FECs) of the treated animals were higher than the average FEC of non-treated animals. The fact that, at the time of treatment, the average FECs were higher in the treated group than in the non-treated group across both production areas indicates that the selection criteria successfully targeted individuals with a greater parasite burden. This suggests that the approach was effective in identifying animals most likely to benefit from treatment, and helps validate the use of clinical indicators in guiding targeted selective treatment strategies.

Average FEC values in eggs per gram (+/- SD) of the treated and untreated multiparous ewes at the time of treatment in the French western Pyrenees farms (PA) and the Roquefort region farms (RR)

The treated ewes had a significantly lower Body Condition Score (BCS) (≤2.5 on a scale of 5 and measures every 0.25) than ewes left untreated, indicating farmers were able to properly identify the ewes in poor BCS even without conducting formal BCS scores.

As GIN infection can have an impact upon certain production traits, we measured potential production loss. The treatment regimen did not significantly impact reproduction parameters such as fertility and duration of the mating period in the Pyrenees. In both areas however, non-treated ewes produced on average 8 to 9% less milk than their treated counterparts at the control point after the selective treatment. This effect was no longer significant if the control point was over a month after selective treatment. The impact of non-treatment upon milk yield is similar to the percentage of milk gained when treating during lactation [3; 11].

Some initial assumptions were challenged: our initial hypothesis was that FEC would be higher in first lactating ewes as they were more susceptible to GIN infection. It was however only the case in one farm of each area, farm X and farm A. Similarly, we hypothesized that ewes in poor body condition would be higher egg shedders than ewes in better body condition, this was however only the case in the flocks from the Pyrenees.

What are the main lessons, and the future perspectives?

Overall EPN efficacy was maintained if initially good. Applying the treatment criteria allowed target on average higher egg shedders, and decision on what animal to treat was quick in the field. No single criteria allow for the constant identification of higher egg shedders, hence the importance of combining them. Allowing the farmer to have power of decision over which animals to treat allows a higher chance of adoption over the years. There is a production cost, that however needs to be balanced with the cost of facing resistance in a flock. One further area of research would be to identify which ewes would benefit most from treatment, to limit the negative impact of non-treatment.

TST needs to be part of a sustainable worm control (SWC) program, in which other control measures can be implemented. Among these measures: reinforcement of host resistance through genetic selection of animals less susceptible to GIN infection [1], or various means of pasture management to limit exposure of sheep to infective larvae. Selective treatment of sheep limits the amount of AH in feces and the environment. AH can be detrimental for survival and reproduction of dung beetles [11], key insects of the carbon cycle on a pasture and potentially helpful auxiliaries in decreasing larvae population on a field [13; 7].

Sustainable worm control is a complex approach and requires discussion along with long-term monitoring. It’s an opportunity for veterinarians and health advisors to discuss more with farmers, and to promote evidence-based health advice.

* ANTHERIN: ANTHElminthic Resistance on dairy sheep farms: survey and INnovative solutions. A 3-year (2021 – 2023) project led by the National Veterinary School of Toulouse (ENVT) and the French National Research Institute for Agriculture, Food and the Environment (INRAE) and financed by Ceva Santé Animale and F2E Carnot. Results from this project include improved diagnostic of anthelminthic resistance, and identification of risk factors for the appearance of resistance in the south western French Pyrenees.

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