The first anthelmintic wormer was developed in 1961 and played an important role in improving the health and productivity of sheep production. At the time, the aim of this and of other wormers subsequently developed was to reduce pasture contamination with infective worm larvae and, therefore, improve lamb performance. What wasn’t predicted at the time was that the worms themselves would become resistant to the action of the products used against them.
Fast forward to the present day and we are now seeing the full effects of the development of ‘wormer resistance’. First identified in the early 1980’s, surveys show that about 90% of flocks have resistance to the Benzimidazole (White) group of wormers. Resistance to the Levamisole (Yellow) and Macrocyclic-lactones (Clear) are also increasingly common and there are flocks where none of these three wormer groups are fully effective.
It’s not just the UK which has seen sheep worms develop resistance to the action of the chemicals used to treat them. Resistance has been detected globally including New Zealand, Australia and Ireland as well as more exotic locations such as Cameroon, Brazil and South Africa. So, since failure of wormers to effectively kill roundworms is an increasing problem globally will your wormers work this year?
Fortunately, testing to see whether your wormers are working is a simple process. With advice from your vet or animal health advisor, faecal egg counts can be taken before and after a particular wormer group is used. Post-treatment, an effective wormer should reduce the initial egg count by at least 95%.
It’s important when testing for wormer resistance that animals are treated to the correct weight (the heaviest in the group) and the drench gun is dispensing the correct amount of product. On that note, testing of drench guns often shows that they are not delivery the correct amount of wormer so animals are actually being under or over-dosed.
Again this is easy to check. Simply, dispense the contents of a drench gun into the barrel of a syringe. Providing you remember to keep your figure on the bottom of the syringe you can measure how much wormer is actually being given per dose. The process is best repeated a couple of times to confirm the measurements obtained.
Testing for wormer resistance is best carried out in lambs and before you actually begin to suspect that wormers might not be fully effective on your farm. It’s all too common to hear stories where wormer resistance was finally diagnosed following years of reducing growth rates and failure to effectively finish lambs.
Catching the problem early also means that management changes can be put in place to help minimise the problem. The last five years have also seen the development of new wormer types. With trade names Zolvix and Startect, both are valuable strategic tools for reducing the build-up of wormer resistance in a flock as well as preventing the introduction of resistance with new stock entering the flock.
So, while unchecked wormer resistance is one of the biggest threats to many sheep flocks there are lots of simple and cost-effective tools for managing the problem.
We are also fortunate in the UK to have access to guidance from SCOPS (Sustainable control of parasites in sheep) which provides advice resources on worm control. To learn more about SCOPS click here and next time you accidentally step in some sheep muck just think about the valuable information that could be gained from scraping it off and getting it tested.