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Worming Regimes for Sheep

This article was written by Bob Norquoy, a partner in our neighbouring practice, Northvet, in Orkney.  It summarises how the approach to worming has changed in recent years.


With the emergence of and increased awareness of resistance in sheep worms to anthelmintics,  there have been significant changes in the regimes used for worming sheep compared with several years ago when regular and blanket worming was the norm. The SCOPS (Sustainable Control of Parasites of Sheep) project has been useful in producing a technical manual which is a useful resource for both vets and farmers in planning worming regimes for sheep flocks.

There are several key issues which should be addressed in a developing an appropriate regime.

Worms in refugia. These are the populations of worms which are not exposed to wormers and are important in preventing the emergence of resistant strains. Typically they are represented by the free living stages and those in sheep which have not been wormed eg adult ewes. Adult ewes carry a low burden of worms and should not need regular worming except in the spring time around lambing.

Targeted dosing. Instead of worming all lambs every few weeks, lambs which are not thriving, or are showing signs and symptoms of worms , showed be targeted for worming. The use of weigh crates is useful in assessing growth rates as well as allowing accurate dosing. Similarly blanket dosing of all ewes is not recommended during the summer and autumn, but can be targeted towards thin ewes or those showing symptoms of worms.

Monitor for resistance . The history of which wormers have been used on the farm is important as well as the apparent failure of particular wormers to work effectively. Resistance to a wormer can be starting to emerge long before it is noticed grossly. The use of faecal worm egg counts is a useful tool in both monitoring the need for worming, identifying the species of worm involved, and detecting emerging resistance. Resistance can be detected by measuring the worm egg counts from a selection of sheep before and after dosing. A reduction of less than 95% can be indicative of resistance.

There are now five main groups of sheep wormer classified according to their active ingredient. Particular attention should be paid to how and when wormers from each group are used, and depending on the farm history, it is not always a case of simply changing from the routine use of one type of wormer to the routine use of another. Strategic dosing with some of the newer groups of wormer can have a benefit if they are used within a regime involving other groups.

Quarantine dosing. The aim of quarantine dosing is to prevent the introduction of new types and potentially resistant strains of worms on to the farm. It also allows the new sheep to become populated with the strains of worms which are already on the farm. New sheep should be yarded for 24-48 hrs to prevent their worms contaminating the pasture. Sheep are then treated with a combination of wormers to effectively kill all the worms in them. After treatment, the new arrivals are turned out onto contaminated grass where they can pick up worms from the farm’s existing worm population.

Dose effectively. Failure to dose correctly and effectively is wasteful in both time and money, as well as allowing resistance to develop. Dosing equipment should be checked before use to make sure it is working properly and delivering the correct volume. Doses should be calculated accurately and where there is a mixed group of weights, then the sheep should be dosed according to the heaviest sheep in the group. Oral doses should be delivered over the back of the tongue rather than just squirted into the mouth.

In developing a worming regime for sheep it is important:

  •          To prepare it for each individual farm
  •          To monitor its effectiveness
  •          Be prepared to alter and amend it according to changing circumstances.

Donald S McGregor & Partners Ltd is a company registered in Scotland SC495194 | Registered Address: Veterinary Surgery, Janet Street, Thurso, KW14 7EG

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