Parasite control: testing-led worming

Why routine worming stopped working

Most horses carry some worms, and a low burden does little harm. A heavy one can make a horse seriously ill. For decades the answer was to give every horse a wormer at regular intervals, but that is exactly what has bred resistance.

The British Horse Society's warning is stark: in the UK there is now resistance to each of the five classes of worming drug, and no new alternatives are being developed. Once a worm population is resistant, it can't be reversed, and a pasture can become unsuitable for grazing horses. If worms become resistant to several drugs at once, you may not be able to keep your horse healthy.

So the advice from the BHS and World Horse Welfare is to use wormers only when needed, guided by tests and a risk assessment, with every horse on the yard following the same approach.

Who to ask

You don't have to work this out alone. Wormers are prescribed by a vet, a pharmacist or a Suitably Qualified Person (SQP), also called a Registered Animal Medicines Advisor. Your prescriber can help you assess your horse's risk, choose tests, read the results and decide whether, when and with what to treat.

The BHS points owners to the CANTER guidelines (Controlling ANTiparasitic resistance in Equines Responsibly) and its "What's your worm risk?" calculator. There is no stigma in a horse coming out as high risk: it just means its management may need to be a little different.

The tests

Faecal worm egg count (FWEC)

A fresh sample, taken from several different balls of one horse's droppings, is sent to a lab, which counts the eggs under a microscope and reports eggs per gram (epg). It estimates how many eggs your horse is passing onto the pasture.

• When: three to four times a year, between spring and autumn. Your prescriber will advise how often for your horse.
• What it finds: adult small and large redworm, ascarids (large roundworm) and threadworm.
• What it misses: tapeworm and encysted redworm larvae (below).

Egg count reduction test

If a count leads to treatment, do a second count 10–14 days after worming. It checks that the wormer actually killed the adult worms, and it is a vital way of spotting resistance on your yard.

Tapeworm test

Tapeworm eggs aren't shed evenly in the droppings, so egg counts are unreliable for them. Instead, a saliva test kit or a blood test from your vet measures the antibodies a horse makes against tapeworm. The BHS advises testing twice a year, ideally in spring and autumn. There's no need to retest after treatment, because the antibodies fall only slowly.

World Horse Welfare notes that healthy adult horses over about four generally develop some natural immunity to tapeworm, but that a heavy burden can block the gut and cause colic.

Encysted redworm

Most small redworm larvae burrow into the gut wall and encyst, often staying there through the winter. Encysted larvae lay no eggs, so an egg count can't see them, even when there are huge numbers.

• In large numbers they reduce how well the horse absorbs nutrients, which can cause weight loss.
• When they emerge en masse, they can badly damage and inflame the gut wall, causing severe illness and even death. The BHS describes this happening as the weather warms in spring. BEVA, the vets' association, warns vets that late autumn and early winter is a particularly high-risk period for the disease the larvae cause.
• Not every wormer kills encysted larvae. BEVA stresses that using only a wormer without that action can leave a dangerous burden in place.

A blood test for antibodies against encysted redworm is available and suits horses at low risk. Your prescriber will advise whether that test or a winter treatment is better for your horse. BEVA adds that a negative egg count doesn't rule out a dangerous encysted burden, and that adult horses are fully susceptible, not just youngsters.

The worms that matter

Bots aren't worms but fly larvae. Bot flies lay sticky yellow eggs on the horse's coat, which it swallows. The BHS says they aren't known to cause disease, and a bot knife can scrape the eggs off.

Source: British Horse Society.

Giving a wormer

When a wormer is needed:

• Give the full dose for the horse's weight. Too little contributes to resistance. A weigh tape helps.
• Worm on a surface you can clean, such as a stable or concrete, so the drug doesn't spread into the wider environment.
• Give it earlier in the day, so you can watch for any gut upset such as diarrhoea or colic. Contact your vet if you're worried.
• Keep dogs away. Horse wormers can make dogs very ill. Don't let them eat any, play with the syringe, or eat the droppings of a treated horse for at least two weeks.
• Protect wildlife. Wormer residues kill dung beetles and aquatic life and stay active for months. Pick up droppings daily after treatment, and store the manure for at least four months before spreading it.

Managing the pasture

Good grazing management cuts the worm burden and the need for wormers. The BHS advises:

• Pick up droppings at least twice a week. It removes eggs from the pasture and breaks the worms' life cycle. Don't harrow: it spreads larvae across the field.
• Don't overstock, and rotate and rest paddocks: pasture rested for several months lets the larvae die.
• Cross-graze with sheep or cattle. They swallow horse worm eggs as they graze, but the worms can't survive in them.
• Graze foals and youngstock on pasture rested from horses, ideally for 12 months, and don't use the same paddocks for youngstock year after year.
• Quarantine new arrivals, and test and treat them as your prescriber advises. An egg count reduction test shows whether they're bringing resistant worms onto your pasture.
• Keep records of tests, treatments and grazing.

Older horses can have a weaker immune response, so keep them on a risk-based plan too. See Senior horse care.