Resistance to wormers
The topic of wormer resistance has been making a splash in the headlines recently. Professor Martin Nielsen of Gluck Equine Research Centre at the University of Kentucky, published in the April 2017 edition of Equine Disease Quarterly, suggested parasite resistance to at least one drug class is likely to be present in equines across the world.
While wormer resistance is not a new problem – it was first recorded in sheep in the early 1980’s – we are certainly feeling the impact of its effects ever more keenly and this will only increase if horse owners and carers choose not heed the warnings about the sustainable use of drugs. This means using evidence based parasite control programmes, only giving a wormer when the tests indicate and for specific seasonal problems like encysted small redworm, bots etc. Wherever possible pasture management techniques should be employed to help break the lifecycle of worms mechanically, by poo picking, cross grazing and resting paddocks, rather than relying on chemical intervention.
What is wormer resistance?
Top Ten ways to beat resistance
Worming drugs are like antibiotics; the more we use them, the more quickly the worms will develop resistance and the chemicals we have will cease to be effective. There are currently no new wormers in development and so this is a scary prospect for horse owners, farmers and other livestock keepers – because as long as we have animals, they will have parasites.
While we don’t want to over use anthelmintics (that’s wormers to you and me), we do need to keep control of worm populations in our horses. The good news is that chemicals are not the only weapon in our armoury. So what else can we do:
1. Target the drugs; by using worm counts and tests and treating only the animals who need it will go a long way to slowing resistance. Worms need to be exposed to a drug to develop resistance. Low numbers of parasites are a good thing, don’t try to be worm free!
2. Don’t under dose – exposing the worms to a level of wormer chemical that is not enough to kill them is a sure fire way to build resistance. If using a weight tape add 10% to the estimate to cover potential inaccuracies. All licenced wormers on sale in the UK have very safe tolerance levels and it is better to err on the side of heavier rather than lighter for the sake of wormer efficacy.
3. Don’t worm and move; after worming ensure horses stay on the same pasture for a few days so as not to be favouring the development of a resistant worm population.
4. If possible keep horses with the same grazing companions, not constantly changing groups. Rest and rotate grazing as much as possible and don’t overcrowd fields.
5. Poo pick; Worm larvae passed in droppings hatch and become mobile within seven days and wiggle away from the dung piles to re-infect the pasture. Collecting droppings twice a week is a sound investment in your worm control programme. Harrowing is effective if done in dry, bright conditions to allow UV light to kill the worm eggs but conditions in the UK are rarely favourable. On dull wet days it will merely serve to spread the worm eggs and further contaminate pasture.
6. Keep muck heaps well away from grazing land. Worms can travel up to 1 metre in dry conditions and up to 3 metres in wet conditions!
7. Cross graze pasture with other species – except for some tapeworms, lungworm and liver fluke, worms are usually species specific and will not survive when ingested by the ‘wrong’ host.
8. Keep new horses separate until tested and treated accordingly so that you can manage any parasite burdens they may be bringing with them.
9. Be careful of where you allow your horse to graze away from home and consider feeding hay instead. Competition and training venues can be a hotbed for worms and whatever your horse picks up from the land will come home with him. For this reason horses that live in herds with one or more travelling horse are at higher risk from parasites than those in closed groups.
10. Practice good stable management; keep areas where horses hang out like stables and field shelters and their buckets and troughs etc. as clean as possible to prevent worm eggs that have been shed being re-ingested by the horse.
How do I know if I have resistant worms?
Over time worms will build up resistance to the different chemicals they are exposed to. Do some resistance testing to be sure your wormer has worked. This is especially important if you use fenbendazole (Panacur) as there is a huge amount of documented resistance to this product.
A worm count is taken to assess the parasite burden in the horse – if necessary a dose is given. Two weeks after the wormer another sample of dung is sent for analysis. If the result has not reduced by between 85% and 95% then the worms are resistant to the chemical used.
If resistance is identified then we recommend you consult your vet or SQP for an alternative course of action to get on top of the problem.
Contributor: Westgate Laboratories is a family business, founded by husband and wife David and Gillian Booth. Since 1999 Westgate has been leading the field in helping horse owners to manage the parasite control of their horses and other grazing animals. The laboratory offers a quick, easy to use, great value postal worm count service that enables carers and keepers to target the wormers given to horses and keep them as healthy as possible. www.westgatelabs.co.uk