A guide to effective wormer use to reduce resistance risk

White wormer accounts for nearly half of all wormers used in the UK, yet resistance to it is present on almost all farms.

This is affecting sheep performance and profitability, says independent sheep consultant Lesley Stubbings, who is trying to get farmers to change their worming habits and stave off anthelmintic resistance through her work with the Sustainable Control of Parasites in Sheep (Scops) group.

Scops’ objective is to slow the development of anthelmintic resistance. Below, we outline how to reduce reliance on anthelmintics and ensure their effective use.

Step one: Map your farm

Using faecal egg count (FEC) testing results you can match the data with a map of the farm and build up a picture of where high worm burden exists.

When you know where the high-burden fields are, you can aim to reduce the presence of parasites by undertaking different management methods including:

  • Rest fields from grazing and use for hay/silage instead
  • Avoid grazing it with vulnerable stock, such as growing or finishing lambs
  • Cross-graze it with non-susceptible stock, such as cattle or dry ewes, to get rid of infective larvae
  • If reseeding, introduce forages less attractive to infective larvae, such as plantain and chicory.

Step two: Keep problems out by following correct quarantine practices

When buying new sheep or bringing sheep back to the farm from grazing away, sheep need treating and quarantining to avoid contaminating the flock with resistant worms.

On arrival, sheep should be yarded or housed away from other sheep and then treated as soon as possible. See the table below to identify which products to use, depending on risk.

Treated sheep should be turned out after 24-48 hours, on pasture that has carried sheep in the current season and they should not be introduced to the rest of the flock for three weeks.


Step three: Avoid highly selective practices

When animals are treated with the right amount of wormer, the only worms that survive are resistant worms.

This means that if all animals are treated and moved to a clean pasture, the only eggs expelled will be those from resistant worms, meaning a resistant population will multiply in that area.

There are two ways to avoid this:

  1. Leave some sheep untreated – this should be at least 10% of the biggest and fittest sheep. This increases the population of susceptible worms, so that the next generation of worms are still partly susceptible.
  2. Treat the whole group but leave them on dirty pasture for four to five days, to pick up dirty worms. Worms on pasture, outside the sheep, are called “in refugia”, meaning they are not exposed to any treatments.

See also: Why delaying fluke treatments could prove costly

Step four: Use anthelmintics effectively

Effective use of wormer is all about using the right product, at the right time, at the right dose and with the right technique.

The right time

It’s important to use wormer only when necessary, because using it more often than that will increase the selection pressure on the worms, meaning you will end up with a resistant population of worms more quickly. It is also unnecessary expenditure, damaging the bottom line.

Conducting a FEC test will help determine whether there is a high worm burden in a particular group that needs treating.

On many farms, ewes are routinely wormed pre-tupping and at lambing – this is often not required as mature, healthy ewes have strong immunity and may not suffer from worms, so it is important to check, rather than treat based on usual patterns.

The right product

There are five groups of broad-spectrum anthelmintics, and resistance to three of them has been seen in the UK (see below). The problem of anthelmintic resistance has gone too far for the historical idea of rotating three groups of wormers to be successful in staving off resistance.


Scops recommends using a narrow spectrum product where possible, to directly target the parasite. This should reduce the need to treat again and decrease your chances of developing resistance to a broader spectrum product.

On the Scops website, there is a comprehensive list of products, detailing which parasites they target, how to use them, what trace elements they cover and the meat withdrawal period. Your vet or the pharmacist or SQP at your local retailer should be able to advise too.

The efficacy of a treatment should be tested to check if it has done the job.

This is possible through an FEC reduction test (FECRT) – if the eggs per gramme have been reduced by more than 90% then the product has worked.

Timings for FECRTs vary, so check the test window for the particular product used.

The right dose

Before treating, the group should be weighed, so that the correct dose can be calculated from the guidelines on the product label.

Using too little will expose the worms to the anthelmintic, but not necessarily kill them, ramping up the development of resistance.

Once the weigh scales have been tested, weigh the heaviest in the group and use that as a benchmark dose rate for the rest of the group. If they are hugely varied, split them into subgroups and calculate the dose for the heaviest in each group.

The right technique

First, check all equipment is in working order and, for the most common method of administering anthelmintic, a drench gun will need to be calibrated.

You can use a pot marked with measurements, or the tube of a syringe without the plunger to do this. Squirt the dose into the receptacle and check that the amount dispensed is what it should be, without any air bubbles.

You also need to make sure sheep are properly restrained so that the whole amount given is swallowed. The nozzle should be slotted in the gap between molar and incisor teeth and over the back of the tongue.

Injections must be administered as per individual product manufacturers guidelines.

Subcutaneous injectable treatments should be given with a clean, sterile, 1.6cm needle, under the skin of the neck, 10-15cm below the ear.

Typically, intramuscular treatments should be given with a 2.5-4cm needle, into the mid neck, aiming inwards and upwards towards the head, but it is important to check for each product used.

Injection sites should be massaged gently. And research has shown that white and clear drenches can work more effectively by withholding food for 12-24 hours before treatment.

Wormers should be stored at 4-25deg, away from direct sunlight and used as per dates and time limits on the packaging. Products should be shaken well before use, particularly those from the white group. Anthelmintics should not be mixed with any other products before administration.