Why knowing your horse's worm burden is crucial to wider equine health

All horse owners are aware of the importance of keeping internal parasites under control. However, vets warn that widespread worming practices are actually diminishing our ability to protect horses with potentially fatal consequences.

Rather than blanket worming – that is treating horses every few months without knowing whether they actually require treatment - the key is to understand whether a horse requires worming and, if so, when and how to treat it. Different wormers include different active ingredients that target specific worms and should only be used at certain times of year based on the relevant worms’ life cycles.

The danger of resistance to wormers

The veterinary world has been warning of the dangers of resistance to wormers for several decades and professionals repeatedly plead with horse owners to only worm when a test, such as a faecal worm egg count or blood test, confirms it is necessary.

“Incorrect and over use of wormers will mean they no longer work and all our horses will be at great danger in the future when the threat of internal parasites becomes unmanageable,” warns equine vet Lucinda Ticehurst. “We need to act now to stop this happening.

“There are five main parasites that horse owners need to be aware of: small redworms, large redworms, tapeworms, roundworms and bots,” she says. “However, it must be stressed that a small worm burden is perfectly natural and does not require treatment."

Poo picking is a must

The very first step to parasite control is pasture management: in its most basic form this is poo picking. Worms end up in horses because they’ve grazed in contaminated paddocks. Picking up droppings an absolute minimum of twice a week is essential, more so if the pasture is heavily grazed. If at all possible, don’t have your dung heap close to where horses are grazing and, ideally, turn out with sheep or cows. These are akin to a biological clean-up crew and are able to hoover up equine-specific parasites with no ill effect to themselves.

Faecal worm egg counts (FWEC) are an easy and inexpensive way of identify any horses requiring treatment for some parasites or indicating further tests might be required. Like many tests these are not 100 per cent accurate but provide valuable information.

Several pinches from a pile of droppings needs to be collected in a clean container (greater than 3g) and sent off to a specialist laboratory or given to your vet for processing and analysing under a microscope.

Worm eggs can be counted and an equation used to work out how many eggs per gram are present in the faeces. Less than 200 eggs per gram is not considered to be a problem but over 200 eggs per gram warrants worming. It is important to note that pin worms, bots, tapeworm and encysted red worm are not detected in FWEC.

Other tests for worms

 “Tapeworm can be tested for via a saliva-based test that owners can take themselves (using a specific test kit) or a blood test done by your vet,” explains Lucinda Ticehurst. “Encysted red worm burdens can be identified via a blood test only. Both of these worms could be tested for once or twice a year as detailed below, keeping responsible testing - rather than inappropriate worming - economical.”

Pinworms lay their eggs around the anus of the horse and make them very itchy so they rub their bottoms on stable walls and doors, and then end up re-ingesting the worm eggs when they nuzzle the walls.

“Because of this, horses stabled a lot are most at risk. If your horse is showing clinical signs, such as itching its back end side to side then look under the tail for any toothpaste like residue,” advises vet Lucinda.

Horses that spend a lot of time stabled are more of risk from pinworms

If a pinworm burden is suspected then this are needs to be cleaned daily with a disposable wipe and all reachable surfaces disinfected. To test for them you need to take a strip of sticky tape and repeatedly stick it to the skin around the horse’s bottom to pick up the microscopic eggs. These are far too small to be seen by the naked eye. The strips of sticky tape can be sent off to a lab or examined under a microscope by your vet.

Currently, bots can’t be tested for but Ivermectin and Moxidectin kill them when they are present in the stomach. “This happens specifically in winter after the first frost. The rest of the time, these parasites are flies or eggs laid externally on the horse’s coat,” says Lucinda.

Red worm is the most common and most dangerous parasite for horses, as they reproduce extremely quickly and can pose a serious threat to equine health. Adult redworms damage the gut and are one of the most common causes of spasmodic colic.

When at the larval stage (encysted redworm), small redworms can bury into the gut wall and hibernate for months, sometimes years. When they emerge in late winter and early spring, as the environmental temperature increases, they can cause a large inflammatory response and a  range of issues – some fatal – including not just colic but also diarrhoea and rapid, severe weight loss. If left untreated, small redworm can also significantly reduce an animal’s ability to absorb nutrients.

If a red worm burden is indicated by a blood test or a horse is considered at risk (young and with a poor previous worming history) then treatment requires very specific wormers containing either Moxidectin (single dose) or Fenbendazole (a five-day course).

Parasites are a herd issue; not simply a question of treating individuals

“Your vet will advise on the best option for your horse and how to protect your horse from consequences of the worms dying in your horses gut. The significance of an encysted red worm burden should not be underestimated, always seek veterinary advice. Due to the life cycle of the worm this is best treated for between December and February after a frost,” advises Lucinda.

Time your testing and treatment

Spring and autumn FWEC and tapeworm test

Summer FWEC

Winter blood test for encysted redworm (treatment for this is best between December and February. “Moxidectin or Fenbendazole should not be used at any other time of year unless there is a high-risk case at the advice of your veterinarian,” warns Lucinda Ticehurst.

Parasites are a herd problem and need to be managed as such rather than just treating individuals who will shed their worm burdens on to pastures that other horses will graze. Any new horse, although it may appear very healthy, could have a high worm burden and risk other horses so it is best to test before sharing pastures.

Young horses are particularly susceptible to worms and the most vulnerable alongside any horse that is immunocompromised. “Consider best practice and responsible testing of all horses kept together at the same time to save our wormers so that they continue to work when needed,” entreats Lucinda.

Only treating for worms when absolutely required rather routinely not only helps your horse but also the wider environment. Many of the chemicals in worming products have a detrimental effect on creatures we do want to see in our pastures, such as dung beetles.