Reducing the use of wormers on sheep and cattle farms can help stop the build-up of resistant worms.

One approach is targeted selective treatment (TST), where wormers are administered to animals on an individual basis, instead of a blanket treatment across the group.

“In a group of animals, the majority of worms are within a few individuals. Most other animals will have a few worms but they will still grow and put on weight,” explained Professor Eric Morgan from Queen’s University Belfast.

“If you can target the treatment at those who need it the most, it’s win-win-win. You will use less wormer, so you reduce your costs, you help those animals that need it by getting rid of their worm burdens, and you maintain a population of worms that aren’t resistant to the wormer,” he said.

The aim of any wormer treatment should not be to wipe out all worms from the animals and pasture. Instead, treatments should lower worm burdens while making sure that the worms which are left are not resistant to the wormer product that was used, so it can be used again in the future.

“You are never going to eradicate the worms so, if you are going to have worms, make sure they are the type that are susceptible to treatment, and not resistant worms,” Morgan said.

The Queen’s academic is currently working with a group of NI farmers to see how the principles of TST can be practically applied on local farms.

The criteria for deciding which animals get a dose under TST can include faecal egg counts of each animal or, more commonly, regular assessment of liveweight gains in individual animals.

Morgan acknowledges that most farmers do not record weights of individual animals on a regular basis, so adopting TST criteria based on liveweight gain will be a stretch on most local farms.

But he said there is scope to use TST “by eye” where wormer treatments are targeted, for example, at lambs that look poorer or show signs of scour. It means clean and fit-looking lambs are left untreated to maintain populations of susceptible (non-resistant) worms.

Group strategies

A recent study has also looked at the use of TST on a group basis, where a subset of lambs in a group had their liveweights monitored and wormer treatments were given to the whole group when weight gains in the sub-group fell below target.

Morgan was involved in the project with scientists at the Moredun Research Institute in Scotland and they found that monitoring 20% of lambs in a group is sufficient to determine the need for wormer treatment across the whole group.

In the ongoing project on NI farms, Morgan has found that use of pooled faecal egg count samples across a group of animals is the easiest TST strategy to implement.

He gave the example of taking dung samples from young cattle at grass to see when the first wormer treatment is needed. In most cases, treatments were then given a few weeks later than usual and led to less wormer use throughout the season.

Little incentive to develop new wormers

It is unlikely that wormers with new active ingredients will be available for farmers who have resistance issues with existing wormers, according to Professor Eric Morgan.

“The financial incentive for new wormer development is very low at the moment. It is very hard to get market penetration if your new product will only be used in times of emergency or just for quarantine,” he said.

At present, five groups of wormers have been manufactured for the control of internal parasites in sheep. But Morgan said there was significant scope for farmers to make more use of the likes of the group four (orange) wormer as it has few reported resistance issues.

It can be used as a late-season drench in lambs to reduce populations of worms which are resistant to other products (groups one, two and three) and have therefore been able to build up during the year.

“We need to integrate the new wormers better to prolong the lifetime of the others. Farmers also need to look at other management practices, such as grass management and longer return intervals between grazings,” Morgan said.


Aside from new wormer development, there is considerable research ongoing in other areas of parasite control. For example, scientists are working on vaccines which aim to protect sheep from a common roundworm.

“I think it will come through, but you will not get the effectiveness that farmers are used to from other vaccines. It isn’t going to be a ‘two shots and forget about it’ vaccine. It won’t be a replacement for anthelmintics [wormers]. It will be complementary to other control approaches,” Morgan said.