Equine Worms

The prevalence of worms in horses is high. If horse were roaming free in the wild, the horse herd that deposited worm egg ridden manure would have migrated many miles from the pasture but as horses are generally kept nowadays in fenced in areas, we keep them right on top of the source of infection.

Which worms cause problems in horses?

Small redworms

Cyathostomes or small redworms or small strongyles are the most common equine parasite. The adult worm lives in the large intestine where large numbers of eggs are laid.

A single horse can pass 75 – 100 million eggs daily.

These eggs are passed out in the faeces and become infective larvae on the pasture. These larvae (L3) can crawl up blades of grass to increase their chances of being ingested by grazing horses! Infective larvae then migrate into the lining of the large intestine and then enter a stage of arrested development.

A large number of larvae can be ingested over a grazing season and it is the re emergence of the larvae from the gut lining that can cause huge disruption to gut function. This is known as springtime diarrhoea and although not that common, it can be fatal. Small strongyles are a significant cause of poor growth,loss of condition and poor feed conversion. Young horses are particularly susceptible. Horses do not develop a strong protective immunity and so are a source of pasture contamination throughout their lives. The small strongyle has evolved to make the horse an egg laying machine. Horse only have to pick up infective larvae every 2-3 years for the small strongyleʼs life cycle to function.

Large redworms

Large redworms or large strongyles are also found in the large intestine. Eggs are passed in the faeces and when a third stage larva is ingested, it migrates to the large intestine and penetrates the intestinal wall. The larvae continue their life cycle by migrating to the cranial mesenteric artery, which is the primary blood source for the intestinal tract. The larvae molt here and return via intestinal arteries to the gut , where they reproduce.

All the larvae that migrate to the cranial mesenteric artery cause tremendous damage. Rrely, the artery can rupture, causing rapid death. More commonly what happens is the artery wall is weakened, leading to an aneurysm. This malformation causes abnormal blood flow which in turn can lead to blood clots forming. If a blood clots breaks free , it can block blood flow to the gut. This can cause thrombo embolic colic and death.Adult worms can also lead to ill thrift and anaemia.

Large roundworms

The ascarid roundworm lives in the small intestine of younger horses. Each female can lay 100,000 to 200,000 eggs each day. Older horses are immune. Large worm burdens cause illthrift and in severe infestations total obstruction of the small intestine can occur. The migration phase in its life cycle can cause lung damage due to the passage of larvae through the lungs. Such damage predisposes foals to pneumonia and possibly to pulmonary haemorrhaging in a foal that becomes an athletic horse.The eggs laid by the adult worms are exceptionally hardy and can survive in the environment from year to year.


Pinworms (Oxyuris equi) have a very efficient life cycle whereby the eggs donʼt even have to leave the horse herd to complete their life cycle. When the horse is resting, pinworms crawl out of the horsesʼs rectum , deposit eggs and a sticky glue on the peri anal region of the horse, and crawl back into the rectum. Infective pinworm eggs are ingested orally and end up in the colon. They donʼt migrate through the intestinal wall like other worms and so donʼt cause as much damage.

Pinworms are not life threatening in the way other horse worms can be. They can annoy a horse as the tail head will be itchy. You may see signs of tail rubbing, biting or licking of hindquarters and skin irritation around tail head.


Threadworms or Strongyloides westeri are found in the intestines of foals only as immunity develops after six months. They are passed from mare to foal via the milk. Heavy infestations can cause diarrhoea.


In the Summer months , adult bot flies are a common sight around horses.

Gasterophilus intestinalis lays up to 1000 pale yellow eggs on the horseʼs forelegs and shoulders. After hatching, the larvae are licked into the mouth.

Gasterophilius nasalis lays about 500 eggs around the chin and throat of the horse.

G. nasalis larvae burrow under the skin to the mouth , wandering through the mouth for about a month, before migrating to the stomach.
Bot fly larvae of both species attach themselves to the lining of the horseʼs stomach and remain there during the Winter. After about 10 months they detach and pass out of the body in the manure.

Uusually horses show no clinical signs with bots but they can potentially cause mouth and stomach irritation. The burrowing larvae can cause tiny skin tears.

Horses can be treated for bots after the first frost which kills the adult flies, and again in the Spring, to clear out all the larvae from the stomach.


Three species of tapeworm can infect the horse. Anoplocephala perfoliata is the most common. The tapeworm is different from the other horse worms in that it requires an intermediate host – the forage mite to complete its life cycle. This mite is found on pasture and in hay and straw. The equine tapeworm attaches around the ileocaecal valve in the intestine of the horse. It used to be thought of no consequence but recent studies have pointed to it as a significant risk factor for spasmodic colic and impactions at terminal end of the small intestine.

Diagnosis of a tapeworm infestation is difficult. The eggs occur in small numbers and often in packets rather than individual eggs. They donʼt float well in the testing medium so can be missed in routine faecal egg counts. Sometimes segments of tapeworms can be found in the manure of infected horses after they have been treated with an appropriate anthelminthic for tapeworms.

Treatment for tapeworms should be done every six months, in Autumn and again in late Spring.

With all these worms, how can we control them?

The aim is to stop horses being exposed to high levels of pasture contamination.

Horse wormers are very effective but do not have 100% efficacy against immature larvae. Depending on the wormer, they may have no effect against encysted redworms. Also worm populations have developed resistance to some wormers. For these reasons, a few strategies are required for control.

Prevention of pasture contamination

• Pasture hygiene – Pick up manure piles from fields once or preferably twice a week. Highly effective. Horse manure should be composted.

• Do not overstock – Overstocking causes two problems. A lot more eggs are passed out on the pasture and on overgrazed pasture horses are forced to graze closer to dung pats where there are lots of infective larvae.

• Mixed grazing – If possible graze sheep or cattle with horses as they will clean up pasture. Horse worm eggs are not pathogenic to other species.

• Avoid turnout of horses with high egg counts. Treat any new horse with an appropriate wormer before adding to the herd.

It is not possible or desirable to eliminate horse worms completely but through good management practices and sensible use of anthelminthics, we can control the dangers of worm infestations in our horses.

When should I worm my horse?

There is a wide range of horse wormers available.

For adult horses –

A basic course of wormers for all adult horses should include treatments to target encysted small redworms, tapeworms and bots.
Autumn (Oct/Nov) and Spring (Feb/Mar) doses aimed at encysted small redworm. ( NB. The presence of these larvae do not show up on faecal examination) Annual treatment for tapeworms at end of grazing season.

FEBRUARY Dose for encysted small Redworms
OCTOBER Dose for encysted small Redworms
NOVEMBER Dose for bots and taps

All further treatments over the grazing season should target horses with high faecal egg counts. The faecal egg counts only give information on egg laying adults in the gut. You can use a faecal egg count reduction test in the Summer to check your worming regime. This involves

• collecting a dung sample and get a FEC (faecal egg count) done
• administer anthelminthic
• collect another dung sample 14 days after treatment and repeat FEC
• The results are used to work out whether the wormer you used is working ( ie gives information about drug resistance)

If you are not going to monitor via faecal egg count reduction tests, dose four times a year.

Foals, weanlings and yearlings

Young horses are particularly susceptible to parasite infestation so targeted treatments, based on faecal egg counts, are NOT recommended in this group.

During the first year, foals should receive at least four anthelminthic treatments.

MAR Foal Born
MAY First worming at around 9-12 weeks old to target roundworms
AUG 2nd deworming at around six months or just before weaning
NOV 3rd deworming to mainly target redworms
FEB 4th deworming for redworms and also tapeworms

Recently weaned foals should be turned out on the cleanest pasture available.

Yearlings and two-year-olds are considered ʻhigh sheddersʼ and consequently should get four worm treatments per year.

Some Worming Guidelines

• Worm all new arrivals onto the farm
• Keep all new arrivals housed for 48 hours after dosing to prevent pasture contamination
• Dose accurately according to weight; using less than recommended leads to worms surviving, contaminating pasture and leads to drug resistance
• Worm mares before and after foaling
• Foals are very susceptible to worms so worm regularly from eight weeks old
• Treat all horses twice a year for encysted redworm larvae
• Treat all horses at least once a year for tapeworms and bots (best in late Autumn)
• Manage your pasture – this is a vital component of worm control in horses