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 to worm infection as their immune systems take time to mature. They will need a careful schedule of tests, treatments and management to ensure the wellbeing of mum and baby.
Testing/treatment protocol for Foals 0-6 months
- 2-5 weeks: Worm egg count for S. westeri in specific cases (associated disease on the premises or with an individual mare).
- 2-2.5 months: Proactive treatment of all foals with fenbendazole or pyrantel to target ascarid and small redworm. Worm egg count reduction test to check treatment efficacy is recommended
- 4-5 months: Proactive treatment of all foals with fenbendazole or pyrantel. Worm egg count reduction test to check treatment efficacy is recommended
Two months after foaling resume three monthly worm egg counts for the mare, treating as necessary. More about the testing protocol for healthy adult horses here
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. Consider your approach to encysted redworm on a risk basis.
Keep the pasture as clean as possible by poo picking or cross grazing, resting paddocks and taking care not to overgraze the fields. Ascarids (roundworms) are one of the primary parasite risks of foals. Due to the thick sticky outer shell of the ascarid egg these parasites can survive extremes of hot and cold and remain dormant on pasture for up to 10 years. Fresh grazing is recommended for mares and foals; avoid using the same paddock year on year for new foals wherever possible.
Due date
It is no longer deemed necessary to give a preventative treatment to the mare against threadworm (Strongyloides westeri) unless there is a history of associated disease either on the premises or with an individual mare. While infection has potential to cause mild to moderate digestive upset and diarrhoea in some foals, it can be identified and dealt with on a case basis. Proactively treating all mares has a greater risk of contributing to the development of resistance in other parasites.
Instead take a worm egg count between 2-5 weeks old to monitor egg shedding levels of threadworm and other parasites. Your vet will be able to determine if clinical signs combined with the result warrant an appropriate treatment or whether the diarrhoea could be linked to another cause.
If the young foal is scouring and an active infection of threadworm is suspected, a single dose of fenbendazole is the preferred . it is important to consult your vet as dehydration can quickly affect a young foal. Healthy foals should acquire a natural immunity or tolerance to this parasite at around six months of age.
Mares requiring treatment pre foaling should be wormed with a dose of moxidectin 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. 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.
Foals one to six months
The most significant parasite that foals are likely to encounter in this age bracket 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.
Treat the foal at 2-2.5 months old, and again at 4-5 months old with a generous single dose of fenbendazole or pyrantel, alternating chemicals where there is no known resistance. It is difficult to accurately assess the weight of a foal so err on overestimating to ensure an effective amount is given. (These chemicals have high safety margins, with the dose needed to cause toxicity in horses at over 30 times the normal dose for deworming).
General Notes
Moxidectin is not licenced for foals under 4 months old, and moxidectin + praziquantel combination products are not licensed for foals under 6 months.
Ivermectin is not the best choice of product for routine dosing of young horses as there is 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 type of parasite infection and therefore the most appropriate treatment.
Updated Summer 2026 following release of CANTER Guidelines Chapter 11