What you need to know about worming

Once upon a time worming your horse seemed so simple. Every six weeks or so you would add a sachet of powder to his food and assume that you had the issue sorted. Recent scientific research, however, has led to the belief that blanket worming with anthelmintics (wormers) is both unnecessary and counterproductive as it can lead to a resistance to the very drugs that are intended to rid the horse of his worm burden.

Consult your vets before you start to worm a new horse and they will be able to advise you on the right wormers to use, as well as the frequency at which they should be given. Worms may sound inconsequential, but they can seriously affect the health of your horse, cause lasting damage to his gut, lead to colic and, in some instances, death.

Why counting adds up

Your vet is likely to suggest that you perform regular worm egg counts (WEC), which most veterinary practices will be able to undertake for you. Indeed, relatively few horse owners these days worm their horse without first sending away a small amount of dung for analysis — there are also laboratories and retailers who offer this service online and in store at a cost starting at around £7.50 per horse.

A WEC, which will give an indication of how many large roundworms (ascarids) as well as small adult redworms (cyathastomins) and large adult redworms (strongyles) are actively laying eggs in your horse’s gut, should be undertaken regularly. This is likely to be every eight to 10 weeks during the spring, summer and early autumn months.

You will be sent an analysis following testing which will recommend whether your horse needs worming — this will only be necessary where the number of eggs in the sample exceeds 200epg (eggs per gram of faeces).

Small redworms can account for up to 90% of a horse’s worm burden and are the most common internal parasite. At the end of the grazing season, small redworm larvae become encysted. Should they emerge in large numbers in the spring, the horse can suffer weight loss, diarrhoea, colic and potentially death. Therefore worming with a relevant product between November and December is recommended even if the horse is showing no obvious symptoms. As encysted small redworm do not show up on a WEC the owner may be oblivious that their horse is at risk.

At around the same time of year, it may be necessary to treat for tapeworm. Tapeworm burden can be detected via a blood sample, which your vet will take for you and send off for analysis, or a new saliva test.

Additionally horses can suffer from threadworms — new born foals can be susceptible to these very small worms — pinworms, which lay eggs around the anus prompting intense scratching, lungworms, which can be passed to horses sharing a paddock with donkeys, who are natural hosts, and tapeworm. These can grow up to 8cm in length and occupy the horse’s gut.

While bots are fly larvae and not worms, they can still infect horses and do not show up on a WEC. A one-off early winter treatment should be sufficient for control.

Prevention is better than cure

If you keep your horse’s pasture clean through regular dung collection he will have less chance of developing a worm problem. If you have the luxury of plenty of land, rotating and resting the pasture, or having it grazed by other animals, such as sheep, will help to reduce egg numbers.

If you don’t collect the dung every few days, ensure that you chain harrow the pasture only during warm weather. This will ensure that the larvae will dry out and die. Additionally, avoid turning out too many horses on small patches of land. The BHS recommends a ratio of two horses per hectare on permanent grazing.

A worming schedule can be complicated, so use a yearly chart and make sure that you record any anthelmintics your horse may be given.

With resistance becoming more of a problem, not all horse owners — or indeed all vets — advocate using chemical wormers and today there are also herbal alternatives available on the market.


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