The equine bloodworm, Strongylus vulgaris, was historically considered the most pathogenic equine intestinal helminth parasite and was ubiquitous prior to the introduction of modern anthelmintic products. The frequent use of anthelmintics in recent decades is believed to have significantly reduced or eliminated the parasite from most managed equine populations. However, recent data has demonstrated that it is enzootic. A term often used in veterinary medicine to describe a low-level, steady state of infection rather than a sudden outbreak. As such it still causes serious abdominal disease in horses in some countries.
With this potential, the following myth buster has been prepared on behalf of CANTER by Martin Nielsen – March 2026.
Myth: S. vulgaris is now virtually eradicated in managed horses.
- Reality: The parasite remains enzootic in several countries and persists even in some regularly dewormed horse populations.
Myth: S. vulgaris causes 90% of all equine colics.
- Reality: Modern data show that S. vulgaris-associated disease accounts for only about 1–3% of colic or mortality cases; the 90% figure was a historic exaggeration.
Myth: Disease caused by S. vulgaris typically presents as dramatic, severe colic.
- Reality: Most clinical cases primarily show peritonitis with mild or absent colic signs; the term ‘thromboembolic colic’ is outdated.
Myth: Aorto–iliac thrombosis is commonly caused by S. vulgaris.
- Reality: Evidence does not support this association; larvae are not found in these thrombi, and other causes are more likely.
Myth: Hemomelasma ilei is known to be caused by migrating Strongylus larvae.
- Reality: This remains only a hypothesis; no causal link is proven.
Myth: Coproculture and PCR detect prepatent (migratory) stages.
- Reality: Both methods detect only patent intestinal stages. Migratory arterial stages cannot be diagnosed antemortem using these tests.
Myth: Anthelmintic resistance exists in S. vulgaris.
- Reality: No resistance has been documented; as far as we are aware all major drug classes remain effective, although testing is difficult.
Myth: Pooled coproculture samples maintain diagnostic accuracy.
- Reality: Pooling samples increases false negatives significantly in coproculture; PCR appears to be less affected by pooling samplers, but false negatives might still occur.
Myth: Pasture larval survival similar to cyathostomin larvae.
- Reality: S. vulgaris larvae can survive up to 17 months on pasture—longer than cyathostomins.
Myth: A single all-herd anthelmintic treatment is sufficient to eliminate S. vulgaris.
- Reality: Reinfection from pasture is likely; long-term control requires monitoring, pasture management, and repeated treatments as needed.
Reference
Reference: M. K. Nielsen., 2025. Strongylus vulgaris – an ancient threat revisited. BEVA Equine Veterinary Education Journal.
Available at: https://beva.onlinelibrary.wiley.com/doi/10.1111/eve.70023