Worms and deworming in horses
Targeted deworming in horses based on faecal egg counts: what the evidence says about standard treatment, resistance and additional options.
Short & honest
Strong evidence (grade A): targeted deworming based on diagnostics works better than routine schedules—but there is no proven herbal alternative to chemical anthelmintics.
Veterinary review not yet completed
This page is awaiting review by a registered veterinary surgeon. Until then, treat the text as education based on the sources listed — not as a verified veterinary judgement.
What are horse worms?
Horses are infected by various worm species. The most common are small strongyles (cyathostomins), followed by tapeworms (Anoplocephala perfoliata), large strongyles (Strongylus vulgaris) and in foals roundworms (Parascaris equorum) and lungworm [3]. Each species requires a different approach — and not every anthelmintic works against every species.
How does the current standard work?
The current gold standard is selective treatment based on faecal egg count (FEC). In this process, manure is examined microscopically for worm eggs; if the result is below the threshold value (usually 200–500 eggs per gram, depending on age and season), no treatment is given [2].
A ten-year Swedish monitoring programme showed that 59% of the tested horses did not need any deworming at all [2]. A small group of 'high shedders' produces approximately 80% of all eggs in the environment — targeted treatment of precisely these animals significantly limits resistance pressure.
Routine quarterly rotating deworming without diagnostics is outdated and promotes resistance to available agents [1].
Resistance: a serious problem
Worm resistance is not a theoretical risk — it is current:
- Benzimidazoles (e.g. fenbendazole): resistance in cyathostomins is widespread worldwide [1].
- Pyrethroids (e.g. pyrantel): resistance is increasing [1].
- Macrocyclic lactones (ivermectin, moxidectin): resistance is emerging, particularly in Parascaris in foals [1].
Monitoring efficacy after treatment via a faecal egg count reduction test (FECRT) — measurement before and 14 days after treatment — is the only way to detect resistance in time.
Specific points of attention per worm species
| Worm | Risk | Approach |
|---|---|---|
| Cyathostomins (small strongyles) | Inhibited larvae can emerge en masse → larval cyathostominosis, mortality ~50% [3] | Moxidectin is the only agent with proven efficacy against these larvae (~74% reduction) [3] |
| Anoplocephala perfoliata (tapeworm) | Associated with certain types of colic | Only praziquantel is effective; ivermectin and fenbendazole do not work [3] |
| Strongylus vulgaris | Arterial migration, colic | Ivermectin or moxidectin; has become rarer due to better control |
| Parascaris equorum | Foals; lungworm in young animals | Resistance to ivermectin is increasing; discuss choice with veterinary surgeon |
Supplementary and natural options: what does the evidence say?
There is no clinical evidence that herbal remedies, homeopathy, garlic, papaya seeds, diatomaceous earth or other supplementary agents provide a clinically relevant worm reduction in horses. Controlled studies are almost entirely lacking. Pasture management and pasture rotation lower infection pressure, but do not replace anthelmintics.
Supplementary ≠ replacement. In the case of demonstrable worm burden (FEC above threshold value), treatment with an effective anthelmintic is necessary. Use moxidectin sparingly — it is currently the only agent that kills inhibited larvae and protecting it is of great importance [3].
When to see the veterinary surgeon?
In addition to the red flags below: always ask your veterinary surgeon for guidance when setting up an FEC programme, FECRT after treatment, and choice of anthelmintic per worm species and season.
Why this verdict?
Grade A was assigned based on multiple controlled studies and a ten-year cohort study (n=thousands of horses, Sweden) [2], supported by narrative reviews in peer-reviewed journals on resistance mechanisms [1] and cyathostomin biology [3]. Limitations: much resistance data originates from Europe/Australia and is not always directly applicable to other regions; efficacy figures for additional agents are lacking in controlled research.
Interactions with medication
- Moxidectin has a narrow therapeutic width; combination with other agents or incorrect dosing can cause toxicity—always under veterinary supervision
- Praziquantel is required for tapeworms; ivermectin-praziquantel combination products exist but should be used only on indication
Legal status (Netherlands)
In the Netherlands, anthelmintics for horses are available only on prescription from a veterinary surgeon (UDD medicines). Self-medication without diagnosis is not legally permitted and promotes resistance.
Sources
- [1] Anthelmintic resistance in equine nematodes. International Journal for Parasitology: Drugs and Drug Resistance, 2014. Narrative review doi:10.1016/j.ijpddr.2014.10.003
- [2] Selective Anthelmintic Treatment in Horses in Sweden Based on Coprological Analyses: Ten-Year Results. Animals (Basel), 2023. Cohort study doi:10.3390/ani13172741
- [3] Equine cyathostomins: a review of biology, clinical significance and therapy. Parasites & Vectors, 2009. Narrative review doi:10.1186/1756-3305-2-S2-S1