It is becoming widely recognised throughout the sheep industry that sheep, and in particular ewes, have been over-wormed in the past in terms of frequency of dosing. An industry-wide initiative, SCOPS (Sustainable Control of Parasites in Sheep) was developed over a decade ago which created a benchmark from which sheep farmers could work to ensure sheep could be farmed on their land in the future. There are many elements of SCOPS which have, through concerted efforts from supporters of the industry, become key parts of worm management in the UK. Many things have changed in the last decade, including trials, research and development of two new active ingredients. We are also learning some of the key simple things that can be done to ensure the greatest chance of success.


Very generally, fit, healthy adult ewes for most of the year have resilience to keep a worm burden under control. If her immune status is normal, she will ‘handle’ a small worm burden which allows a very small number of eggs to be produced. This means that in the majority of cases on farm, ewes do not need treating at some ‘traditional’ times of year for worms (e.g. autumn tupping dose). Of course, common sense must be used here and decisions made based on individual farm situations, but very generally, if a ewe is not dirty and it is not presenting any other clinical signs (e.g. anaemia, ill thrift etc…) she might very well not need worming for most of the year.


In late pregnancy, which in most cases is in the winter, the ewe’s immunity wanes. This can allow overwintering worms to increase egg production, which in cases of a grazed or partially grazed system can lead to an increased risk for spring lambs resulting from larvae from these eggs. This is known as the peri-parturient rise. This drop-off in the ewe’s immunity is not ‘fixed’ but generally occurs in the last few weeks of gestation and continues into early lactation.


Like dairy cows, ewes suffer from two key areas around birth, which are linked. One is this drop off in immunity and the other is negative energy, often seen in cases of twin lamb disease. Managing both of these well can have a positive influence not just on worms but on other diseases, most of which have greater or lesser effects on the ewe depending on immune status. So in the first instance, it’s a case of very carefully managing ewe immune status, body condition and diet to ensure the best chances of success without using unnecessary treatments.


This is a real balance and demands a fair degree of compromise. This is due to the fact that in the winter, a large proportion of the worm population are inside the animal, and there is a much higher exposure to that population from a wormer. In theory this sounds good, but in reality, we need to maintain a viable population of ‘killable’ (susceptible – not resistant) worms on the pasture that can be picked up.


To put it simply, imagine that because it’s winter time, there were just two worms on a pasture: one susceptible and one resistant. However, because it’s a nicer environment for the worm, there are 100 inside the ewe: 50 susceptible and 50 resistant. If we treated the ewe with a wormer that did not treat the resistant worms, we would have ONLY resistant eggs being passed out onto the pasture, resulting in a pasture with 1 susceptible worm and >51 resistant worms!


SCOPS principles are key here, and we have two choices:
  1. Avoid worming all ewes and treat ‘risk’ ewes only (under pressure, low body condition, triplets etc…). This will ensure a viable population of susceptible worms are available to dilute any resistant worms on the pasture. A good guideline is to leave the best 10% untreated.
  2. Timing. Giving treatments when egg output is high and ewes have access to pasture to be reinfected helps to dilute the worm population. In practice this could mean using a wormer in the crucial few days before and after lambing. This method puts less selection pressure on resistance and most often this is done just after lambing, when the ewe is “penned up” with her lambs.
We look closely at why managing wormer resistance at this time of year is crucial.