Why Deworming Strategy Has Changed
For decades, the standard advice to horse owners across Europe was simple: deworm every horse every six to eight weeks, rotating between different drug classes. This approach has been thoroughly re-evaluated in the light of growing anthelmintic resistance, and the guidance from ESCCAP — the European Scientific Counsel Companion Animal Parasites — now strongly advocates a targeted selective treatment (TST) approach. Understanding why this matters, and how to implement it, is one of the most important things a modern horse owner can learn.
What Is Targeted Selective Treatment?
Targeted selective treatment means treating individual horses based on their actual parasite burden, determined through regular monitoring, rather than treating all horses at fixed intervals regardless of need. The cornerstone of TST is the faecal egg count (FEC) — a laboratory analysis of a fresh dung sample that quantifies the number of parasite eggs being shed per gram of faeces.
Research consistently shows that within any group of horses, a relatively small proportion — roughly 20 per cent — are responsible for shedding 80 per cent of the parasite eggs on the pasture. By identifying these "high shedders" and targeting treatment at them, while allowing low shedders to go untreated or treated less frequently, the overall selection pressure for drug resistance is dramatically reduced.
In practice, horses are classified into three categories based on their FEC results:
- Low shedders: fewer than 200 eggs per gram (EPG) — treatment often not required
- Moderate shedders: 200–500 EPG — treatment recommended
- High shedders: above 500 EPG — treatment required, with follow-up FEC to confirm efficacy
Key Parasites in European Horses
Small Strongyles (Cyathostomins)
Small strongyles are by far the most common and clinically significant group of internal parasites in adult horses across Europe. There are over 50 species, and their larvae can encyst in the gut wall, lying dormant for months or years before emerging. Mass emergence — known as larval cyathostominosis — can cause severe, life-threatening colitis and diarrhoea, particularly in late winter and early spring. Standard FEC measures adult egg-laying worms, so a low count does not always mean a horse is free of encysted larvae.
Large Strongyles
Large strongyles, particularly Strongylus vulgaris, were once the primary equine parasite concern. Modern deworming programmes have dramatically reduced their prevalence, but they should not be ignored. S. vulgaris larvae migrate through the arterial system and can cause severe, potentially fatal colic. Unlike small strongyles, they are not yet showing widespread resistance to available drugs.
Tapeworms
Anoplocephala perfoliata, the equine tapeworm, is associated with certain types of colic, including ileocaecal impaction and spasmodic colic. Tapeworm eggs are not reliably detected by standard FEC methods. A saliva-based ELISA test — the EquiSal Tapeworm Test — or a serum blood test offers a more accurate assessment of tapeworm burden and is recommended as part of the annual monitoring programme.
Bots
Bot flies (Gasterophilus species) lay their yellow eggs on horse hair, particularly on the legs and face. When the horse grooms itself, larvae are ingested and attach to the stomach lining, where they spend several months before passing out in the droppings. While bots rarely cause severe disease, heavy burdens can cause gastric irritation. Treatment is typically given once annually in late autumn, after the first frost has killed adult bot flies, using ivermectin or moxidectin.
Roundworms (Parascaris equorum)
Parascaris is primarily a concern in foals and young horses under two years of age. Adult horses typically develop immunity. Heavy roundworm burdens in foals can cause respiratory disease, weight loss, and intestinal obstruction. Resistance to ivermectin in Parascaris has been reported across Europe, making fenbendazole the preferred treatment in young horses.
Anthelmintic Classes Available in Europe
There are three main classes of anthelmintics licensed for use in horses in Europe:
- Benzimidazoles (fenbendazole, oxibendazole): Effective against adult small and large strongyles, roundworms, and some other species. Widespread resistance in small strongyles means FEC reduction tests are essential to confirm efficacy before relying on this class.
- Macrocyclic lactones (ivermectin, moxidectin): Highly effective against a broad spectrum of parasites including bot fly larvae. Moxidectin has additional efficacy against encysted small strongyle larvae. Emerging resistance to ivermectin in small strongyles has been reported in several European countries.
- Praziquantel: The only drug effective against tapeworms. Usually combined with ivermectin or moxidectin in a single product for convenience.
- Pyrimidines (pyrantel): Effective against adult strongyles and tapeworms at double dose. Resistance is documented in some populations.
The Resistance Problem
Anthelmintic resistance — where parasites survive drug treatment that was previously effective — is now confirmed in small strongyle populations across Europe, including the UK, Germany, France, and Scandinavia. Resistance to benzimidazoles is widespread. Resistance to pyrantel is documented. Reduced efficacy of ivermectin has been reported in multiple countries.
The most effective tool for detecting resistance in your specific yard is the faecal egg count reduction test (FECRT): take a FEC before treatment, administer the chosen drug, and repeat the FEC 14 days later. A reduction of less than 95 per cent for benzimidazoles, or less than 99 per cent for macrocyclic lactones, suggests resistance is present.
Preserving the efficacy of moxidectin — currently the most powerful option against encysted larvae — is a priority. It should not be used as a routine first-line treatment but reserved for situations where its specific properties are genuinely needed.
Seasonal Timing in EU Climates
Climate affects parasite transmission. In warmer, wetter climates such as Atlantic Spain, France, and the British Isles, infective strongyle larvae survive on pasture for much of the year. In Continental climates with cold winters — Germany, Poland, the Czech Republic — larval survival on pasture is significantly reduced during winter, but encysted larvae within the horse remain a concern year-round.
A general seasonal framework for temperate European climates includes:
- Spring (March–May): FEC monitoring as grazing intensifies; treat high shedders
- Summer (June–August): Peak transmission season; FEC every eight to twelve weeks for monitored horses
- Autumn (September–October): FEC before the end of the grazing season; consider tapeworm testing
- Winter (November–February): Treat for encysted larvae (moxidectin) and bots in late autumn; FEC less useful in winter
Pasture Management
Drug treatment alone cannot solve a parasite problem. Pasture management reduces the burden of infective larvae that horses are exposed to. Key measures include:
- Removing droppings from paddocks two to three times per week
- Avoiding overstocking — high horse density dramatically increases larval contamination
- Resting and rotating paddocks where possible
- Harrowing paddocks only in hot, dry weather, which kills larvae; harrowing in cool, damp conditions spreads them
- Co-grazing with sheep or cattle, which graze off horse parasites without being infected by them
Recording Treatments in the Horse Passport
EU legislation requires that all veterinary medicinal products administered to horses be recorded in the horse's passport. This includes all anthelmintic treatments — the date, product name, batch number, and dose administered. Your vet or a suitably trained person can make these entries, but they must be accurate and complete. Horses whose passport records show recent treatment with certain anthelmintics may not enter the food chain, which is a relevant consideration since many horses in Europe are ultimately processed for meat. Always use the correct section of the passport for medication records and never leave entries incomplete.
Implementing a Monitoring Programme
Begin by having a baseline FEC performed on every horse on the yard. This immediately identifies your high, moderate, and low shedders and allows you to target treatments where they are genuinely needed. Repeat FECs two to four times per year depending on season and individual horse risk. Add an annual tapeworm test and a bot treatment in late autumn. Keep detailed records of all FEC results, treatments given, and FECRT outcomes — this information is invaluable for managing resistance over time and for sharing with any new vet who takes over your horse's care.