Anthelmintic resistance in sheep
Anthelmintics, otherwise known as wormers are generally used to treat internal parasite infections in animals. Ruminants can be infected with a number of gut worms which can cause clinical disease such as diarrhoea and dehydration, anaemia and in extreme cases death. In less severely affected ruminants, gut worms will slow growth rates in young animals. In most cases, once the animal reaches adulthood it has some immunity to parasite infections and gut worms are less of a concern.
In sheep, there are both production and welfare concerns with gut worm infections. Slow growth rate and clinical disease will delay the time lambs take to reach fat weight or breeding weight in replacement animals. In addition, diarrhoea can cause mucky back-ends in sheep, which will attract flies and lead to fly strike.
One of the biggest challenges currently facing the UK sheep industry is anthelmintic resistance. This is when the parasite develops resistance against the product used to kill it, which means worming treatments aren’t as effective as they should be.
How to slow resistance developing
There are currently 5 groups of wormers; unfortunately, widespread resistance to 3 out of 5 of those wormer groups has been well documented in research. Producing new wormer groups a long process, so we cannot rely on new drugs to tackle anthelmintic resistance. Every time a worming drench is used it increases the chance of resistance developing so we need to be careful about how we administer wormers, which sheep we administer them to and look at other ways to control parasites in our flocks. Some key points include:
QUARANTINE DRENCHING – bringing new animals onto your holding is the easiest way to contaminate your pasture with “resistant” worms. Consult your vet when buying-in to ensure you have a quarantine protocol in place
DRENCH WHEN REQUIRED - the less frequently we use worm drenches, the slower resistance will build. This needs to be carefully balanced with avoiding disease in our flocks. Using worm egg counts can be an extremely useful tool to decide when worming is required
DRENCH CORRECTLY – ensure you have an accurate weight for your sheep and always dose for the heaviest in the group (of the same age group). If using a drench gun, calibrate before each use.
AVOIDANCE – rotating grazing with other species such as cattle or horses can be very useful to “mop up” worms Consult with your vet to create a strategic parasite control plan for your holding; more information about worms and worming can be found at www.scops.org.uk/
Wormer resistance threatens the health and welfare of UK sheep.