Worms & Horses Part 1: Types of Worms

The following is the first of a three-part series on worms and horses, co-authored by Ellen Collinson and Alex Wilson.  You can find the article in it’s entirety, as well as other articles and information on iridology, herbs, and nutrition for horses on Ellen’s website




Why do we need to worm horses?

For years, all the books have told us that it is essential to do a worming treatment every 3 months. This is often by panicking horse owners into fears of what will happen if they don’t: the risk of ill-thrift, dull coats, weight loss and, the worst case scenario, colic.

As is pretty obvious, worms will compete with horses for their food and, in some cases, suck the blood from the horse’s digestive tract, causing damage that will be permanent and can affect the horse secreting the chemicals necessary for it to digest its food properly. In extreme cases, the worms can be so large that they can actually block the digestive tract, causing the horse to become seriously ill or even die. This can be seen in foals or yearlings.

Blanket worming has many drawbacks as it is not an exact science. Many of the household brand wormers work only for one type of worm and not for others. As we will see later, there are few ways of clearing parasites with a single product. Today we are advised to change our products each time, but how many people realise that many wormers have the same ingredients and are just being sold under a different name? It is essential to not only change the brand but to make sure you are changing the drug. Is this the reason we are often told about parasites becoming immune to current worming products and then breeding more immune parasites?

In the wild, horses are found to have less worms than in a domesticated environment, especially where horses are kept in small confined paddocks. If paddocks are kept clean, as we will see later, that can help cut down the problem. The domestication of horses has meant that they are kept in much closer proximity to each other than they would be in the wild and we find that the worm burdens are much higher.


Types of Worms found to be affecting horses

In a paper written by Don Hudson, Dale Grotelueschen and Duane Rice, it is reported that most cases of colic can be put down to parasites. The larvae of bloodworms, from the Strongyles family, cause massive damage to blood vessels. They also reported that as much as 50% of all deaths in horses can be related to internal parasites.

To understand equine parasites properly, it is necessary to remember that there are five major internal parasites. Many horsemen realise that horses get parasites, or worms, but do not tend to look at the subject in any more depth; they will use the same chemicals to remedy the problem year in year out and wonder why their horses are still carrying parasites. The five groups include large and small strongyles, ascarids, bots and pinworms.

Most parasitical worms are passed between horses, via the droppings. Horses that graze near areas where there are droppings can involuntarily pick up infected eggs whilst eating. These eggs, once in the next horse, will hatch, grow into adults and then lay eggs and the process starts all over again … This is a very simplistic description, but of course there is great variety from species to species.


Strongyles (Bloodworms)

The strongyles, or bloodworms, are the most dangerous of the parasites that infest horses and they are common in horses of all ages, barring foals. There are 50 different species of these bloodworms which tend to inhabit the large intestine and they are divided into two groups, large and small strongyles. The three main species of the large species are: Strongylus vulgaris, Strongylus equinus andStronggylus edenatus.

As larvae, they travel to the heart, liver and lungs using the blood vessels as transport, destroying a lot of healthy tissue and leaving scar tissue in their wake. Horses that are carrying bloodworms will deposit the eggs in their dung most of the time. If a grazing horse eats these larvae, they can remain dormant in the intestinal lining for long periods of time. Large amounts of larvae can build up here and eventually they will erupt through the intestinal wall. This can be very damaging to the horse in question. These larvae eventually develop into adults, who in turn lay new eggs (often thousands at a time) and the cycle is repeated. It is more important to control these worms in adult horses than in young animals.

The small strongyle can often be more of a problem than their larger cousins as there are around 40 species and in many cases they are getting more and more immune to conventional chemical wormers. When they get through the gut wall, this is often the cause of colic. Another factor to be aware of is the effect of a bout of chemical de-worming on these parasites because it can act as a trigger for a large number of larvae to emerge through the intestinal wall. This can happen very quickly, within seven to ten days, and in extreme cases this can cause the horse diarrhoea, muscle wasting or weakness and even serious bouts of colic or, worse still, the death of the animal. The large strongyles are easier to control than the smaller variety.

Chemical de-wormers have little effect in breaking this cycle and many of these small strongyles are becoming very resistant to certain classes of anthelmintics. Another point to consider is that the adult worms are less lethal to horses than when they are at the larval stage. With these parasites, the ideal is not to contaminate the environment with them. Once larvae are infecting land, the only way to control them is effective land management, i.e. removing droppings, keeping horses off the land and the hope of warm weather. Another important consideration, if it is practical, is to rotate the animals that use the land. Parasites that effect horses tend not to affect sheep or cattle and vise-versa.

One other effective method is the herbal approach to parasite expulsion that does not cause the same problems that are associated with chemical worming. This method will be discussed later.

It is a fact that not all horses within a herd will be highly contaminated, indeed it is often just a few offenders that infect the rest. One way to help deal with this problem is to carry out faecal egg counts and thus identify the culprits.


Large Roundworms (Parascaris equorum)

Unlike the strongyles, these parasites mainly affect foals and young horses and tend not to be a problem for older animals. Their eggs can remain dormant in the soil for up to 10 years before they are swallowed by horses whilst grazing but once in the animal’s intestine they will start to hatch. Once hatched in the gut system, they move through the walls and into the veins. Their journey takes them up to the lungs, where they go from the alveoli (or air sacks) through the bronchioles until they reach the trachea. Once they are at the back of the throat, the young horse will then swallow them, returning them to the small intestine where they will grow into adults. These adults can grow as large as 50cm long before they lay their own eggs which are in turn excreted in the droppings.

Most foals will be exposed to roundworm eggs and any parasite control programme must be targeting this species when managing immature horses. Good control should look at both killing the worms as well as to stopping them from maturing; thus stopping the egg production and breaking the cycle. A word of caution though: there have been reports from the USA and Canada showing that conventional wormers used for this purpose are starting to encounter major resistance from the worms. This situation is likely to be the same in the UK and Europe.


Bots (Gasterophilus intestinalis)

The bot fly looks similar to a honey bee and is often seen singly or in groups around horses. In the summer, horses can be seen rushing around their paddocks trying to avoid these flies. These are usually females looking to deposit their eggs on the animals’ legs. The flies tend to be striped and the eggs a yellow colour.

The legs are not the final resting place for these eggs and their mission is to get into the horse’s mouth. Within five days, the eggs are ready to hatch; and this happens when the temperature rises due to horses’ faces getting near them as horses use their muzzles to try and stop the itching. The larvae then enter the horse’s mouth and they burrow into its tongue.

The next stage of the process is when they change colour to red, due to production of haemoglobin, which is necessary due to the low oxygen levels in the horse’s tongue. From here, they migrate to the stomach of the horse at which point they become a fully grown bot. They will remain in the stomach for as long as 12 months where there is sufficient oxygen for them to survive.

By late spring, these bots will get excreted by the horse in its droppings and the bots pupate and become adult flies, a process that takes around three weeks, before the cycle starts again.

The early stages of the larvae’s development can have an impact on the host’s health, especially relating to their gums and teeth as well as within their intestines.


Tapeworms (Anoplocephala perfoliata)

The last group of parasitical worms that needs to be discussed is the equine tapeworm, so called as it resembles a measuring tape and is short and triangular and comparatively smaller than those found in cats, dogs and humans. The worm is made up of segments (proglottid) that are like carriages on a railway train. At any point, a segment or segments can break off without killing the worm. For years it was believed that tapeworms were not common in horses and that was because the proglottids tend to dissolve in the large intestine and are rarely seen in the horses’ faeces. It had also been thought that these tapeworms caused little harm to horses, but it is now known that they are responsible for certain types of severe colic.

Tapeworms contain both male and female reproductive organs and, as with other parasitical worms, they produce eggs, but unlike other parasites, they don’t lay those eggs; in fact they hang from the end of the worm and break off once they mature. These eggs are then excreted into the dung.

Before getting into a horse, tapeworms start with an initial host, which is the oribatid mite (Acari:Oribatida). These mites live in large numbers on pastures and the tapeworm eggs tend to be swallowed whilst the mites are feeding on horse dung. Within two to four months, these eggs hatch within the mites and the horse then swallows the mites whilst grazing. Six to 10 weeks later, the worms mature within the horse.

The most accurate way to see if horses are infected, but very impractical, is via post mortem or during surgery. Three surveys conducted in Kentucky (USA), where the world’s largest horse population live, showed that in 1983, 53% were infected, in 1984, 54% were infected and in 1992, 64% were infected.

The effects of tapeworms on horses is still not totally understood, but what we do know is that tapeworms are thought to contribute to major colic attacks. Other hypotheses are that these parasites can cause inflammation of the ileocecal valve and they can contribute to ulcers, plus cause the retention of fluids. These problems can cause the horse to have bowel problems. One problem with diagnosing tapeworms is that there are few symptoms that can be easily identified by horse owners.

With tapeworms, the problem is not always to completely cleanse the horse of the parasite, but to make sure that any infestation is limited, thus stopping the horse from having a large burden of these worms. Another problem is that most conventional wormers won’t tackle tapeworms effectively and it may be necessary to use a large dosage of these drugs, which can be comparatively safe when administered by a vet, but not necessarily when administered by the owner.

Another method, which in most cases is both inhumane and impractical, is keeping horses off pastures. Whilst this will protect horses from the oribatid mite, it makes little sense to keep horses boxed all day, nor is it good for their temperament. A better solution would be to look at herbal alternatives to wormers, marketed as parasite repellents. Many of these products actually expel the parasites from the horse and at the same time cleanse the gut of eggs and larvae. The larvae are killed or paralysed, thus breaking the life cycle of the tapeworm and solving the problem cost effectively. This methodology of clearing worms and other parasites from horses will be discussed more depth later.


Stay tuned for Parts 2 and 3 of this series, which will discuss the history of de-worming, resistance to chemical de-worming, and herbal options.    


Hi! My name is Casie Bazay. I'm a mom, a freelance writer, and a certified equine acupressure practitioner.

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