The Truth about Worms in Horses – the whole story


Alex Wilson is marketing manager for Ellen Collinson Herbal Products and Ellen Collinson is chairman of the company. Wilson and Collinson have undertaken large amounts of research into the effects of parasite in horses and these are their findings.

The subject of worms and parasites in horses is a complex business and work is carried out both in the research environment and on horse properties around the world to try and win the fight against these pests. THESE PARASITES inhabit horses causing problems as severe as chronic colic. It is hard to discuss the subject in any depth in a single article or paper but, by searching the Internet and reference libraries, there is much to understand. Alex Wilson and Ellen Collinson  have tried to look at some of the literature that currently exists and outline useful information about worms and how best to manage horses to avoid parasites becoming a major problem.

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 Worm 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 and Stronggylus 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 effect 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 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 in more depth later.

The History of Worming

The approach to worming has changed over the years and, at the University of Kentucky, Gene Lyons PhD, Sharon Tolliver BS and Hal Drudge DVM documented the history of worming in the scientific journal Veterinary Parasitology. Their paper threw up some unusual practices.

In the 17th century there was a notion that drawing blood from a horse and getting the horse to drink its own blood was a very good way to kill worms and to cure other equine ailments. How a horse was made to drink its own blood was not described. Other historical ways to resolve worms were equally bizarre and included soap, liquorice, linseed oil, chick or human faeces, eggs, guts of chicken or pigeons, and, worst of all, mercury. Owners were advised against using some of these methods with pregnant mares. Feeding horses tobacco was also practised, but the amount needed to have any effect would more than likely make a horse quite ill. Also many people experimented with herbs without proper knowledge of herbal medicine and so rarely getting the required results. However, today’s leading herbal products are very carefully formulated and trialled to ensure the best possible results.

In April 1891 the first edition of the book Veterinary Country Practice was printed. Written by “Qualified and Experienced Members of the Royal College of Veterinary Surgeons”, it was written expressly for Chemists and Druggists. In this book there were several recipes for worming, some containing small quantities of Arsenic, also herbs, including Aloes, Ginger, Liquorice, Valerian and Garlic; also used were Linseed oil, Turpentine, Gentian and this list goes on. There were 11 different recipes, showing that even in the 1890s they knew that it was beneficial to change the treatment to combat resistance. It has to be said it was also to give the less well off a chance to use cheaper ingredients. The last edition was reprinted in November 1935.

Carbon tetrachloride, best known as a component that is used in dry cleaning, was one of the first chemicals used for worming. Whilst it was semi-effective, it was as one can imagine, very toxic to horses.

The 1940s marked the start of a new era of chemical wormers, but even the early chemicals that were introduced, like phenothiazine, were very toxic. Phenothiazine was, however, the first product that would start to attack the strongyles, which until then were a major issue for horses and their owners. But by the 1960s, both in the UK and the USA there was evidence to show that these parasites were starting to become resistant to chemical worming, a situation that was going to make chemical
worming a major problem from there on in.

The 1950s saw the introduction of some products that seemed to be effective on a number of different parasites; mixing a number of chemicals together in smaller qualities helped combat the toxicity of a large dose of one product. This cocktail had to be administered by a vet by inserting a tube down the horse’s throat, a manoeuvre which also carried risks. Organophosphates, which were commonly used in sheep dip, were popular in the 1970s. They are very toxic chemicals both to horses and to humans who get in contact with with. Today there are
some farmers who used these chemicals in sheep dips, etc., who are suffering from multiple chemical poisoning; in some cases they are totally house bound and cannot live normal lives.

It wasn’t until the 1970s when owners were able to worm their horses themselves without the need to call in the vet to run tubes down a horse’s throat. New products were being marketed as a paste and the first of these was Benzimadazole. This drug had a much larger margin for error and could be administered in much smaller quantities than previous de-worming chemicals. Many drugs from this family of chemicals are still in circulation today. Also in the 1970s Pyrantel was also offered from an alternative family of chemicals. This proved to be very popular when parasites started to get resistant to the benzimadazole products.

In the 1980s Ivermectin was introduced which allegedly kills the larvae of parasites as well as the adults and this product would prove to be very popular with horse owners, especially as it could also be administered as a paste. Finally, in the 1990s we saw the introduction of Moxidectin, marketed on the basis of it being able to kill the pesky small strongyles in the intestines of the horse.

Parasites’ resistance to chemical wormers

Whilst the drug companies would want you to believe that their’s is the answer to exterminating parasites in horses, there is a fundamental problem that exists – resistance. Parasites have had to learn to survive and to adapt to their environments and in doing so they have managed to find ways to adapt to modern chemicals that are meant to stop their reproduction. This is called resistance and these worms are now passing this resistance on to their offspring. There is no single chemical product on the market that stops the resistant parasites and there is evidence to show that the number of resistant parasites is on the increase. Historically it has been believed that rotating the products and using wormers from different chemical families can eliminate most resistant parasites but in a paper published by Blanek, Brady, Nichols, Hutchinson et al. in which extensive studies were conducted in the USA using quarter horses, they concluded that resistance in equine parasites was an ever increasing problem and that more research needed to be done into rotating wormers.

Another problem that blanket rotation needs to consider is the fact that certain classes of drugs are aimed at different families of parasites and unless the horse owner is aware of which parasites their horses carry, they can be using chemicals that can have little or no effect on their horse or horses.

One solution to this problem is to have regular egg counts utilising the dung of the horse. This, however can be costly and very few owners undertake this kind of interest in their horses’ worm problems. The usual thinking is that by carrying out regular worming on their animals that will be sufficient. Another approach that should be considered is using a herbal product as an alternative. There seems little evidence that there are worms that become resistant to the tried and tested herbal products. More details of herbal worming will be discussed later.

Toxic effects of chemical wormers

The one subject that will never have been discussed by the manufacturers of the main brands of wormers is the potential side effects of using chemicals. As has been discussed above, these products are poisonous to worms and unless they are used very carefully, they can equally be poisons for horses. As the resistance to chemicals grows, so stronger dosages need to be used and the safety margins become smaller. The following side effects can be caused by the use of equine wormers: swollen glands, colic, allergies, laminitis, intestinal problems, skin reactions, drooling, hoof problems, internal problems, plus dangers to the horse’s immune system. Let’s face it, horses are not designed to be filled with toxic chemicals!

A good example is toxic hepatitis, which is caused by substances poisonous to the liver. There are three known sources: chemical poisons, plant poisons and metabolic poisons.   Chemical poisons include arsenic, copper, mercury and phosphorous among others.  It should be noted that tetrachlorethylene and carbon tetrachloride, both used as worming agents, are potential causes of non-infectious, toxic hepatitis, when improperly used.

 In addition, there is the environmental impact of these chemicals. Using these wormers mean that these chemicals get into the earth through horses’ urine and faeces. The bigger picture means that other animals, especially wildlife, agricultural animals and pets will be affected. It has been noted that birds that eat horse droppings from chemically treated horses die. These
chemicals stay in the land for over three years and can also find their way into streams and rivers, affecting fish, and they can also get into our drinking water. We have a responsibility to our environment not to pollute it unnecessarily.

Herbal Worming

The use of herbal remedies or herbal medicines is thought of today as alternative, but if we examine the evidence, in reality it is the chemical products that should be termed as alternative. Herbal medicine can be traced much further back than any modern chemical treatment. In fact, a large number of medicines used today have their basis in herbal remedies that can be traced from as far as the tribesmen of Africa, the ancient Egyptians and the Chinese.

The first reports of herbal medicine can be dated as far back as 2800BC in ancient China. By 400BC the Greeks had also adopted the use of herbal medicines and by 100BC the first reference book on herbs was written in Greece. By 50AD, herbal medicine had spread to the Roman Empire and by  500AD herbal medicine first came to Britain and was practised throughout Saxon times. By 800AD, herbs were being grown in monasteries in Britain and monks developed the usage of these remedies. At the time of the black death herbal remedies were used to try and contain the spread of the disease. In the 1500s, and during the reign of Henry Tudor, the first laws were passed to control the practise of herbal medicine to stop them being supplied  by untrained apothecaries. In the 1600s, medicine in Britain was becoming 2 tier, chemical based drugs for the rich and herbs for the poor. During this period and into the 1700s many conventional drugs were being sold over the counter, but the realisation of the terrible side effects was starting to become a reality. As we move into the modern era, there has been a large resurgence of herbal medicine, partially due to the side effects of chemical products, but
also as in many cases natural products are
in harmony with nature, so can be
more effective.

There are many products on the market claiming to be herbal Parasite repellents that will have little if any effect, so it is always worth investigating what research has been conducted to back up their claims.

There are herbs that are classed as Anthelmintics: these are herbs which have the capacity to destroy intestinal worms and parasites. They come in two categories: vermicides and vermifuges. The former are agents that destroy worms without necessarily causing their expulsion from the bowels and this category of herbs should be combined with laxative or cathartic herbs which then cause the expulsion of the destroyed parasites. Vermifuges are agents, usually having cathartic properties, which expel worms from the bowels.

As well as Anthelmintics there are Taeniafuges and Taeniacides. These are herbs that expel (taeniafuges) or kill (taeniacides) tapeworms in the intestinal tract.

For a product to have a good combination it must include these herbs and also a separate cathartic; this helps not only to loosen the bowels and work as a laxative but to clean the gut walls of the eggs and larvae. Also needed is a good demulcent: demulcent herbs act as a soothing agent to calm any inflamed or damaged tissue, and also to prevent any ‘side effects’ that a strong anthelmintic, especially chemical anthelmintics, can cause.  And last, but not least, it would contain a stimulant to prompt the herbs to work as a formulation.

As many anthelmintic herbs are very bitter and cathartics very strong, it is usually a good practice to add a herb that has both a palatable flavour as well as a pleasant aroma; this can be the chosen stimulant or demulcent.

Through in vivo trials conducted at the Institute of Organic Research in Switzerland it has been proven that certain herbs show a significant activity with a 79% reduction in egg output on day five of feeding it and a 100% cessation of egg hatch. The trials also show that the anthelmintic activity either killed or paralysed the larvae so they cannot climb up the grass to be eaten by a new host.  This stops the cycle and also prevents the build up of resistance.

It has also been proven in recent trials conducted in the wild, that herbal parasite repellents do not cause any damage to wildlife, the water or the environment.

It is however, essential that before buying a herbal parasite repellent, you check that it has been properly trialled, and that the results prove that it can be used as either a complete wormer or as part of a worming programme.

Environmental Control of Parasites

Controlling parasites on a property can go beyond a good worming programme and the following ideas can certainly help control parasites.

Any new horses arriving, before they are allowed to graze with the other horses, should be isolated for 48 hours and wormed. It is important to know what worming has been done at the property the horse is coming from and if any infestation is known to be a problem there. Even if you are informed that the horse is worm free; do not take that information as gospel and conduct your own worming. Herbal parasite repellents are often good for this situation as you can be confident of not overdosing a horse or giving them a product that might conflict with what they might have been given recently.

During the 48 hour period make sure that all droppings are removed and either have the dung removed from the property or use it as a garden fertiliser so that it is kept away from other horses.

A general consideration is to make sure that paddocks are kept as free from droppings as possible and that will help stop the parasites’ life cycles. Many yards and stud farms use special paddock vacuum cleaners to remove droppings. Again make sure that dropping are disposed of and do not get back in direct contact with horses. Faecal egg counts can also be a very useful tool, and consider the fact that if you have a horse with a large worm burden it is advisable to isolate it from the rest of the herd.


The subject of equine parasites and their control is very complex and a proper understanding of the subject, leading to good worming practice, should maintain clean pastures and healthy horses. It is very easy to fall into the trap of either performing the same worming time in time out or performing a targeted rotation, but these are not necessarily the answers unless the horseman is aware of particular worms that may be affecting their property. The use of herbal parasite repellents should also be considered for good practice as the way that they work, being very different from the chemical approach, breaks the life cycle of equine parasites.


Natural Healthy Horse Care  – RH Kerrigan B.Sc, MAIAS, MAAC. (Equine Educational)
Parasite Cycle –
Should Horses be Wormed –
Equine Internal Parasites – Don Hudson, Dale Grotelelueschen, Duane Rice
Natural Horse Wormers –
Illustrated Veterinary Encyclopaedia for Horsemen (Equine Research Inc.)
Bad Bug Basics  Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Resistance Worms, do your horses have them? Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Deworming adjuncts –  Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Age-related parasites – Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Control for mature horses Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Environment; development and persistence of parasites – Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Bots and beyond –  Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Tapeworms, an underrated threat  Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
Strongyles, the worst of the worms – Karen Biggs DVM, Craig ReinemeyerDVM PhD, Dennis French DVM, MS, DipL, ABVP and Ray Kaplan DVM, PhD (The Horse Magazine)
The History of Herbal Medicine –
Investigation of anthelmintic resistance and deworming regimens in horses – M Blanek, HA Brady, WT Nichols, DP Hutcheson et al.
Historical perspective of cyathostomes; prevalence, treatment and control programs ET Lyons, SC Tolliver and JH Drudge. Department of veterinary science Gluck Research Centre, University of Kentucky.

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