Effective parasite control is a vital part of giving young horses a healthy start in life. Foals are born free of parasites but are often exposed to them within the first few days of life. Youngsters are especially vulnerable as their immune systems take time to mature. They will need a careful schedule of tests and treatment to ensure the wellbeing of mum and baby.

The mare in pregnancy

Good management begins before the foal is born. The mare should be monitored with worm egg counts every three months and tapeworm tests every six months, treating as the results indicate plus a winter dose for encysted redworm. Check with your vet or SQP to ensure any wormers are licenced for use in pregnant mares.

Keep the pasture as clean as possible by poo picking or cross grazing, resting paddocks and taking care not to overgraze the fields. Foals and young stock are especially vulnerable to ascarids. Due to the thick sticky shell of the ascarid egg these parasites can survive extremes of hot and cold and remain dormant on pasture for many years which is why fresh grazing is recommended for mares and foals.

Due date

Threadworm, Strongyloides westeri is the first parasite to be concerned with. The female worm of this species has the ability to penetrate the horse’s skin and, once there, can remain in the body tissue for many years. Threadworm can also be passed through the mare’s milk to infect the foal. There is some debate about whether threadworm is harmful - it was once thought to be a potential cause of chronic diarrhoea in the foal but this is now refuted by leading parasitologists*. New thinking is that strongyloides is actually harmless to the foal - the choice is with the owner as to whether you would prefer to worm as a preventative for it or not.

In order to treat for threadworm the mare should be wormed proactively with a dose of moxidectin (Equest) four weeks before the foaling due date or an ivermectin based wormer around foaling time - our preference if you're going to treat is to use the moxidectin wormer as we prefer not to give chemicals around such a critical time as foaling. Healthy foals should acquire a natural immunity or tolerance to this parasite at around six months of age.

Foaling to one month

The mare should not be wormed until at least two weeks after foaling unless under veterinary supervision - this is because metabolites from the wormer can be passed through the mare’s milk to affect the foal. If the young foal is scouring and you suspect an active infection of threadworm it is important to consult your vet as dehydration can quickly affect a young foal.

One month to six months

Roundworm in poo

The next parasite foals are likely to encounter is the ascarid, Parascaris equorum - these are huge creamy white worms which can grow to 40cm in length, a very large worm for small foals to carry. They reproduce in large numbers and an infected youngster can produce a frightening barrow load of these worms after treatment.

Clinical signs of infection would be poor weight gain, unthriftiness, pot belly or rough coat due to the compromising effect of the parasite on the foal’s growth and development. The size and quantity of worms can form intestinal blockages leading to colic and ruptures of the gut while migrating larvae cause coughing and respiratory damage through pulmonary haemorrhaging.

When the foal is a month old treat with a generous single dose of fenbendazole (Panacur), effective for ascarids – it is difficult to accurately assess the weight of a foal so err on overestimating to ensure an effective amount is given. (This drug has a particularly high safety margin, with the dose needed to cause toxicity in horses at over 50 times the normal dose for deworming).

Continue to worm the foal every 4-6 weeks alternating between pyrantel and fenbendazole until the foal is six months old, monitoring with worm counts when worming is due for best practice. As with threadworm, healthy young horses should develop natural immunity to ascarids at around 2-4 years of age - though cases are not unusual in older horses that have had a poor start in life.

Two months after foaling resume three monthly worm egg counts for the mare, treating as necessary.

Six months to yearling

As the foal gets older and grazes more, the risk of other parasites such as the small redworm, Cyathastomins and  large redworm - Strongylus vulgaris, and tapeworm, Anoplocephala perfoliata, take over. If the foal is grazing with several other horses then a first tapeworm test should be given at 6 months old using the Equisal saliva test. If tapeworm is present they can be dosed using either a double dose of pyrantel or a single dose of praziquantel.

From 6 months of age test every 6-8 weeks until a yearling only worming if needed. Continue testing the mare at three monthly intervals.

Either blood test or worm both mare and foal for the possibility of encysted redworm in winter. Small redworms are one of the most common and harmful parasites found in horses. They are at their most dangerous in their larval stages when they burrow into the lining of the gut and encyst. In this stage they don’t lay eggs and so their presence can’t be detected by a worm egg count. Untreated these encysted small redworm pose a potentially fatal health risk to horses as they can emerge en-masse from the gut wall in spring, causing loss of condition, digestive upsets and colic. Use Panacur 5 day Guard for lean youngsters or Equest if they have a good covering of body fat.

General Notes

  • Moxidectin is not a suitable drug for young foals until they have a sufficient covering of body fat.
  • Ivermectin is not the best choice of product for routine dosing of young horses as there is some known resistance to ascarids.
  • If you have any health concerns about your mare or foal please consult your vet.

While it goes against the recommendations for worming adult horses, young foals need proactive treatment to protect them from parasites. Incorporating worm counts and tests into the programme early on will help to identify the wormy horses and those that are going to need more support, preventing any potential problems developing from unnecessary parasite burdens.

An example worming programme for a mare and foal born on May 1st




Summer of conception

Worm Count


Autumn of pregnancy

Worm Count, Saliva test


Winter of pregnancy

Encysted redworm dose, plus resistance test to check for treatment efficacy


April 1st before due date

Saliva test for tapeworm
Moxidectin for threadworm if choosing to treat


1st May foal is born

June 1st
1 month after foaling

(The mare should not be wormed until at least 2 weeks after foaling unless under veterinary supervision)

Single dose of fenbendazole (Panacur). Then worm count and treat every 4-6 weeks until 6 months old rotating the use of pyrantel and fenbendazole (single doses)

Early July

Worm egg count

Worm count and worm with pyrantel

Mid August


Worm count and worm with fenbendazole

Mid to late Sept


Worm count and worm with pyrantel

End of October

Worm count and tapeworm test

Worm count and worm with fenbendazole

Tapeworm test, treat if necessary

1st November foal is 6 months old

Mid Jan following foaling

Blood test or treatment for possible encysted Redworm dose plus resistance test to check for treatment efficacy

Encysted Redworm dose plus resistance test to check for treatment efficacy

Early March


Worm count

End April

Worm count and tapeworm test

Worm count and tapeworm test

1st May foal is 1 year old


Worm count every 3 months

Worm count every 2- 3 months depending on previous results


* Video: The Parasite Journey of the Horse, Episode 1, Dr. Martin Nielsen, University of Kentucky Maxwell H. Gluck Equine Research Center