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Dosing Ewes at Lambing Time

With lambing now underway, farmers' minds should already be focusing on worm control in both their ewes and lambs for the forthcoming grazing season. This letter serves as a reminder to vets and advisors, who should already have established farm health plans for the coming year, and have discussed a worm control plan with their clients.  Of immediate concern is what to advise on choice of wormer, and when and how frequently to treat ewes during, or after lambing.  With the emergence of anthelmintic resistant (AR) nematodes, and the increasing reliance placed on wormers for their control, the continued effectiveness of some of the more traditional worm control strategies has been questioned (Taylor et al., 2009) and may influence recommendations on both product choice and application.

Dosing of ewes at turnout is common practice on most sheep farms with the objective of controlling pasture contamination from the periparturient rise (PPR). It is well established that there is a relaxation in immunity in adult ewes from 2-4 weeks pre- lambing, which persists for 6-8 weeks post lambing (e.g. Kelly 1973, Michel 1985). As ewe lactation commences, adult worms are not expelled and produce more eggs; fewer incoming infective larvae are rejected; and hypobiotic larvae resume development as a consequence of which worm numbers increase and  there is a rise in nematode faecal egg count (FEC).   As lactation ceases, ewes recover their immunity and worm numbers and FECs tend to fall back towards their pre-lambing levels. Several factors are known to influence the PPR. For example, it is less marked in single-bearing ewes than in multiple bearing ewes, and it can be diminished by dietary supplementation.

One of the growing concerns is that treatment during the PPR may have serious consequences for the development of AR, such that the timing of dosing, linked to the choice of wormer are both important. If ewes are still experiencing the PPR when the effect of anthelmintic dosing ceases, they are likely to become re-infected quickly, particularly on permanent pastures that are heavily infective. Under these conditions, selection for AR is minimal, but the benefit of treatment in terms of pasture contamination is also minimal. Therefore, ewes that are treated early in the PPR with  short acting drugs show only a short duration of reduced egg output before resuming the expected, but delayed, periparturient rise in FEC. As a consequence, repeated, or long-acting or persistent treatments have been advocated in order to eliminate the rise in FEC altogether. This strategy should aim to reduce pasture infectivity for the lambs later in the season, whilst also ensuring that the end of the PPR coincides with the period of anthelmintic treatment.  However, with sustained treatments there may be a prolonged period before ewes re-establish a nematode infection from the in refugia population, which can be highly selective for AR worms.

As a consequence of these observations and concerns, guidelines were developed leading to the current SCOPS (Sustained Control of Parasites in Sheep) guidelines (Abbott et al. 2012). The guidelines draw on data from more recent controlled experiments, as well as the field-derived knowledge held by industry and workers in the field.

The strategy that needs to be adopted for ewe treatments is therefore a compromise between reduction in pasture contamination for the subsequent grazing lambs, and avoiding high selection pressure for AR.

SCOPS therefore recommend two possible options:

  • Leave a proportion of the ewes untreated;


  • Treat early in the post-lambing phase to ensure that ewes become re-infected with unselected parasites before their immunity is fully restored.

Both of these approaches increase the risk of parasitic disease for lambs grazing the pastures later in the season so careful planning is necessary to develop strategies that give acceptable levels of worm control without undue selection for AR.  There are also no hard and fast guidelines as to how many ewes to leave untreated.  It has been suggested that leaving about 10% of the flock untreated will be sufficient to provide a large enough dilution effect to delay the development of AR. This can be achieved be leaving single bearing ewes and/or a proportion of ewes in good body condition untreated. Careful consideration has also to be given when using long acting formulations of moxidectin that provide persistent action and long periods of protection against some species of nematodes for up to several months. Wherever possible, the recommendation is to use these products prior to lambing or early in the PPR.

Further information can be found on the SCOPS ( or NADIS ( websites. The latter site contains up-to-date parasite forecasts and recommendations for control based on anticipated parasite disease risks.

Mike Taylor, Wintringham, North Yorkshire

Neil Sargison, University of Edinburgh, Royal (Dick) School of Veterinary Studies

Phil Scott

Mike Howe, NADIS


Abbott K.A., Taylor M.A., Stubbings L.A. (2012). Sustainable worm control strategies for sheep. A Technical Manual for Veterinary Surgeons and Advisors. SCOPS 4th edition, Context Publishing.

Kelly J.D. (1973).Mechanisms of immunity to intestinal helminths. Australian Veterinary Journal 49, 91-97.

Michel J.F. (1985). Chemotherapy of gastrointestinal helminths.  Chapter 3: Epidemiology and control of gastrointestinal helminths in domestic animals. In: Handbook of Experimental Pharmacology (Eds Vanden Bossche H., Thienpoint D, Janssens P.G.), Springer-Verlag Berlin Heidelberg), 77 pp67-123.

Taylor M. A, Learmount J., Lunn E., Morgan C., Craig B. (2009). Multiple resistance to anthelmintics in sheep nematodes and comparison of methods used for their detection. Journal of Small Ruminant Research 86, 67-70.