Worms in Horses; What They Are and How to Treat Them

For as long as horses have existed, there have been parasites specifically evolved to take advantage of them. Frequently-heard questions in veterinary practices include: ‘which wormer do I need this time?’, ‘how do I do a worm egg count?’ and, our favourite, ‘will you look at this disgusting photo of a mangled worm on my stable floor?’.

In this article we will discuss common UK worm species, the problems they can cause and how we can safely keep them in check.

Types of Worms    

Tapeworm– Tapeworm have round disk-like bodies made up of segments. They range from 4-20cm in length and attach themselves to the walls of the small intestine and caecum. They rarely cause disease in horses, although in very large numbers they have been associated with colic.

Roundworm– Roundworms can reach an impressive 40cm long. Thankfully equines develop good natural immunity between 6-12 months, so adults are rarely infected. Once ingested, the larvae migrate through the lungs and liver before arriving at their final destination, the small intestine. Infected horses generally show ill-thrift and occasionally colic.

Large redworm– Don’t let the name fool you, these worms are actually only 2.5 – 5cm long. The adults live in the large intestine and the larvae migrate through the bloodstream as they mature. One subspecies, Strongylus Vulgaris, settles in the large mesenteric arteries within the abdomen, damaging the vessels. This can result in anaemia, gut necrosis and even sudden death. Thankfully, infections are rare.

Small redworm– Despite measuring less than 2.5cm, these guys are not to be trifled with. When a larvae is ingested, it reaches the gut and has two options. In warmer months they mature quickly and begin producing eggs. However when temperatures drop in autumn, the larvae change tactic. They ‘encyst’ in the walls of the gut, becoming inactive until temperatures start to climb in spring. The encysted larvae then emerge from the gut walls en masse, causing widespread gut damage, colic, diarrhoea and collapse. Mortality rates are as high as 50%.

Now that we have identified the baddies, let’s move on to control measures. Wormers contain chemicals which kill internal parasites. They can be divided into those that treat tapeworm, those that treat roundworms and redworms and combination wormers that treat all three types.                

In addition, some wormers are uniquely able to target specific worm lifecycle stages – such as the encysted stage of redworms. Your vets are always happy to advise you and most tack shops employ an SQP (Suitably Qualified Person), who is trained to offer advice on routine treatments such as wormers.

So why don’t we just use a combined wormer every time we want to treat our horses? The answer is that worms can develop the ability to survive exposure to wormers. Resistance has been reported since the 1970s, and likely developed due to the widespread blanket use of wormers. It is impossible to reverse resistance, but we can select our treatments more carefully to reduce the spread. If we fail to do this, then we could end up with certain species of worms that simply cannot be treated.

Only worm horses that require worming. 20% of the equine population carries 80% of the worm burden, as some horses are just more susceptible than others. Therefore most horses shouldn’t need treating for much of the year.

Leave some worms behind. Our goal is not to eliminate every worm – if we tried to achieve this then all we would be left with would be resistant worms. They would have no competition, allowing them to reproduce faster. Instead, the ideal situation is a small population of non-resistant worms which, while posing no risk to our horses, keep the resistant worms outnumbered.

Do not “dose and move”. When we worm our horses, we risk killing off all but the resistant worms. If we then immediately place that horse onto a clean pasture, then the only eggs and larvae on that pasture will have inherited the genes for resistance. The pasture’s worm population will now be made up of 100% resistant worms!

Dose correctly. It is vital to dose your horse to their weight accurately. If horses are under-dosed, then any non-resistant worms in their gut can develop the ability to resist that wormer in future.

Worm egg counting- Microscopically examining faecal samples enables us to calculate the number of roundworm and redworm eggs per gram (epg) of faeces. We can then decide whether treatment is required. Worm burdens below 250epg generally do not need treating, as small burdens pose no risk, whereas burdens above 250epg require treatment to avoid disease. Worm egg counting is widely available and cost-effective. Your vets may offer their own service, or you could use one of the many online laboratories. It is recommended to perform worm egg counts quarterly.

Tapeworm saliva test Historically, testing for tapeworm was challenging and expensive, so most horses were treated routinely every six months – But since 2014 a new saliva-based test, called Equisal, has been available from your vet. You simply use the applicator supplied to collect some saliva, before packaging it up and sending to the lab. The results are sent to your vet, and burdens are classed as low, borderline or high. Low results don’t need treatment, and these account for approximately 75% of test results.It is recommended to test for tapeworm twice a year.      

Encysted small redworm blood test- As encysted redworm cannot be identified on a worm egg count, it is currently advised that all equines are treated with an appropriate wormer each winter to prevent disease in spring. However a new blood test which detects the encysted stage is due to be released next year, and development of a saliva-based test is also underway. These tests should hopefully reduce our reliance on wormers even further, aiding in the battle against resistance.  

In conclusion, modern wormers have been hugely successful in reducing equine worm burdens. However this success has also been a curse, introducing widespread resistance and threatening a resurgence of parasite-related disease. By utilizing lab tests we can accurately assess worm burdens, and select the most appropriate wormer and dosing schedule. This way we will keep our horses disease free, while preserving the effectiveness of our wormers for the future.