By vet Hamish Leslie
Unexplained weight loss in horses can be worrying for any owner. While there are many potential causes, from dental issues to metabolic disease, one of the most common and often overlooked factors is an ineffective worming programme.
Internal parasites are a natural part of a horse’s environment. Grazing animals are constantly exposed to worm larvae on pasture, and without proper management, these parasites can build up to harmful levels. When that happens, weight loss is often one of the first visible signs.
The most significant worms affecting horses in the UK and Ireland are small redworms (cyathostomins), large redworms, roundworms (ascarids, primarily in young horses), and tapeworms. Among these, small redworms are considered the greatest threat. In large numbers, they damage the intestinal lining, impairing nutrient absorption and leading to poor body condition, dull coat quality, and, in severe cases, diarrhoea and colic. Tapeworms, meanwhile, are associated with spasmodic colic and irritation at the junction between the small and large intestine.
Weight loss linked to worm burden occurs because parasites compete with the horse for nutrients and cause gut inflammation. Even if a horse appears to be eating well, it may not be absorbing feed efficiently. Young horses are particularly vulnerable, as are older animals and those under stress.
However, the literature now warns against routine blanket worming without evidence. Overuse of anthelmintic drugs has led to increasing resistance, particularly among small redworms. This means that some wormers are becoming less effective, making strategic parasite control more important than ever.
Veterinary organisations such as the British Equine Veterinary Association (BEVA) recommend targeted worming based on faecal egg counts (FECs). These simple laboratory tests measure the number of worm eggs in a horse’s droppings and help determine whether treatment is necessary. Rather than dosing every horse every few months, owners are encouraged to test first and treat only when counts are above recommended thresholds.
Good pasture management also plays a crucial role. Regular poo-picking, avoiding overstocking fields, and rotating grazing areas can significantly reduce exposure to worms. Quarantining and testing new arrivals before turning them out with the herd further limits spread.
It is important to note that not all weight loss is caused by worms. Dental problems, gastric ulcers, chronic pain, liver disease and inadequate feeding can present in similar ways. If a horse is losing weight despite a suitable diet and worming strategy, veterinary investigation is essential.
Effective parasite control programmes rely on targeted diagnostic testing to detect specific parasites. Tapeworm eggs are shed inconsistently, FWEC aren’t always accurate. Specialised tests, such as blood or saliva antibody tests, are commonly used. These tests detect the horse’s immune response to tapeworm exposure and help determine whether treatment is required.
Testing for cyathostomins is carried out through regular FWEC counts throughout the grazing season. While these tests measure egg shedding rather than the exact number of worms present, they provide a useful indication of parasite burden and whether treatment is necessary. In late autumn and winter, larvae can encyst in the gut wall. It’s recommended targeted treatment even if egg counts may be low. Combining regular testing with seasonal treatments helps control parasite levels while slowing the development of wormer resistance.
Ultimately, effective worm control is about balance, protecting the horse’s health while preserving the long-term effectiveness of worming medications. By combining diagnostic testing with good management practices, owners can reduce the risk of parasite related weight loss and ensure their horses maintain optimal condition year-round.