Farm Ireland

Friday 19 January 2018

Horses: Staying one step ahead of worms: a silent killer

With knowledge of the effects, horse owners can stifle these parasites and ensure their animals retain good overall health and high levels of performance

Caitriona Murphy

Caitriona Murphy

Horse owners have a constant battle on their hands, fighting against the silent killers that can cause severe internal damage to their horse before they realise it.

Internal parasites or worms can affect performance, create dull and listless animals, lower resistance to infection and starve the horse of essential nutrients.

In some cases they can cause permanent damage to the internal organs and even death.

The most common signs of a worm-infected horse are a dull coat, depression, low stamina, loss of condition, poor growth in young animals, pot belly, colic and diarrhoea.

One of the most effective ways to assess the worm burden of a horse is to use a faecal egg count.

This simple test can be used to determine which worms are present and how heavy the infestation is in each animal.

The subsequent results can be used to create and maintain an effective parasite control programme for each animal.

A faecal egg count involves collecting a few manure balls from the horse and sending it to a laboratory, as recommended by your vet.

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Results are expressed as eggs per gram of manure, but the number of eggs present is not as important as determining presence or absence or each worm type.

However, owners must remember that a negative faecal count does not mean the horse is free of internal parasites -- some types of parasites only produce eggs intermittently, larvae do not produce eggs at all and may be present in large numbers in a horse with a fecal count of zero, and tapeworm eggs may be missed with routine faecal egg-count techniques.

So what are the most common worms and how do they infect the horse?

Large redworms

Large redworms (strongyles) are potentially the most dangerous internal parasite to horses.

Adult large redworms feed off the lining of the gut, causing direct damage to it.

Immature stages (larvae) migrate through the blood vessels supplying the gut, a migration that not only damages the blood vessel walls but can also lead to blood clots and a weakening of the blood vessels (aneurysms).

Disruption of the blood supply to the gut as a result of the larval damage can cause colic and, in rare cases, sudden death.

Small redworms

Small redworms (cyathostomes) are the most common and numerous internal parasites of the horse, and account for many worm-related problems.

The normal lifecycle is for infections to develop over a few weeks from ingestion of larvae to adult, egg-laying worms.

However, during the winter months, some of these worms hibernate deep within the gut wall and the subsequent emergence of such worms in the spring can cause damage to the gut wall leading to loss of condition, diarrhoea, colic and death.

Most horses develop natural immunity to this parasite, which, combined with a suitable worming programme over the summer months, protects the animal from harm.

However, young horses that have spent less than three seasons at grass, and veteran horses, may be at risk. These at-risk groups may require additional treatment with a larvicidal product in the autumn.

Large roundworms

Large roundworms (ascarids) are the largest of the parasitic worms that affect horses.

They are rarely a problem in adult horses because they are usually immune to them.

However, these parasites and their migrating larvae can be a significant problem in foals, causing poor growth, digestive and respiratory problems, and occasional fatalities.

Eggs can survive for many years on pasture or in stables.

Foals can become infected by ingesting eggs from their environment. It is not advisable to graze foals on paddocks that have been occupied by foals in the previous year.


During the first few weeks of life, foals are susceptible to this very small worm that can cause severe diarrhoea. Dormant infections in pregnant mares are transmitted to the foal via the milk. Mares should be wormed around the time of foaling and foals may need worming from four weeks old.


The adult worms migrate to the horse's rectum where they lay their eggs on the skin around the outside of the anus.

This can cause intense irritation; the horse scratches and rubs its anal region.

The eggs are then shed onto the pasture or bedding. Persistent scratching can result in loss of hair from the dock and the development of sores and open wounds.


These commonly affect donkeys but can also affect horses; usually where horses share grazing with donkeys.

Infected horses show obvious clinical respiratory signs, such as persistent coughing.

Larvae are ingested from the pasture and migrate through the blood stream to the lungs where they develop into egg-laying adults. Eggs are coughed up, swallowed and passed out in the dung.


Tapeworms are found in the large intestine and congregate around the narrow junction of the small and large intestine (ileo-caecal valve).

Most infections do not necessarily produce obvious symptoms of ill health but they can cause digestive disturbances, loss of condition and are strongly associated with colic.


Bot flies are a common irritant to horses at grass.

These large flies typically lay their sticky, yellow eggs on the horse's forelegs and around the head.

The eggs are licked up by the horse and eventually reach the stomach where they attach to the lining.

Large numbers can cause digestive disorders and, occasionally, perforation of the stomach.

Worm control

Wormers play an essential part in effective worm control. The active ingredient is the chemical component of the wormer that kills the parasite.

There are dozens of brands of wormer on the market but only four families of active ingredients: Macrocyclic lactones (ML), Benzimidazole, Pyrimidines and Praziquantel.

Most wormers on the market are broad-spectrum, meaning that they are effective against several different types of parasites. It is generally best to use a broad-spectrum dewormer as the basis of your deworming programme. However, if a specific problem is identified, such as tapeworms or encysted small strongyles, a more specific dewormer can be used.

No deworming product is 100pc effective in ridding every horse of all internal parasites. It is not necessary for a product to kill all worms in order to improve the horse's health, minimise the risk of serious disease, improve feed efficiency and reduce pasture contamination with parasite eggs and larvae.

Different wormers also control different stages in the lifecycle of these parasites. Some only control adults while others control both the adult and larval stages.

Wormers have different treatment intervals in order to control pasture contamination. For example, some need to be used every four to six weeks while others can be used every eight to 10 weeks or more.

The introduction of combination wormers containing more than one active ingredient means that more than one worm type can be treated with a single dose.

Horse owners should make it a priority to know the active ingredients of the various wormers used, not just the brand name.

Consult your vet for more information on an effective worming programme to prevent parasites becoming a major problem in your horses.

Irish Independent