Resistance to horse wormers has now been reported across all drug classes in Australia and poses a significant concern for owners of young horses. A diagnostic-led approach to worming is easily implemented in adult horses but may not be as straightforward for owners of foals and youngstock.
This was confirmed by a recent study in Thoroughbred farms in Australia which highlighted an over-reliance on wormers and a lack of awareness of stud managers on the importance of delaying anthelmintic resistance1. (Resistance refers to the drug no longer being effective against the parasite)
Key parasites affecting foals
- Parascaris equorum (Ascarids) is an important cause of disease in foals and young horses (<18 months)
- Most horses develop a strong immunity to this parasite after 2 years
- Adult ascarids largely reside in the small intestine.
- When ascarid eggs are ingested by foals they travel through the small intestine and migrate through the liver, heart and lungs before returning to the small intestine.
- Migrating larval stages through the lungs can lead to respiratory signs (coughing) and intestinal burdens can lead to a reduced appetite, rough coat and intermittent colic. In severe cases, (potentially fatal) intestinal obstruction may occur.
- Widespread resistance of ascarids to macrocyclic lactone or ‘mectin’ wormers (eg. ivermectin) has been reported across Australia1–3.
- Cyathostomes (small strongyles) may cause disease in older foals and adult horses.
- Younger horses have lower immunity to cyathostome infection and are more susceptible to clinical disease.
- Signs of disease include poor growth, anemia and diarrhoea.
- Emerging resistance of cyathostomes to macrocyclic lactones or ‘mectins’ have been reported worldwide which highlights the need for more targeted strategies to limit the development of further resistance.
Worming strategies for foals
- Blanket worming of foals every couple of months is irresponsible and should no longer be considered acceptable.
- Foals should receive their first worming treatment no earlier than 2-3 months of age4 with a benzimidazole (BZ) wormer (eg. STRATEGY-T®). Treating foals less than 2 months of age is not recommended as there will be very few adult worms present at this age.
- Regular treatment of foals with mectin wormers should be avoided. Ascarids are likely to be resistant to these products.
- Scales should be used to administer an accurate dose of wormer according to bodyweight.
- Faecal egg counts (FEC) should be carried out at weaning and then every 3 months to help decide which wormer is appropriate.
- FECRT will help determine the effectiveness of the worming treatment and is an important tool for monitoring drug resistance on a particular farm. Experts now recommend that FECRT be performed annually.