What is the likely etiological agent when bright red larval worms are found on the palpation sleeve of a horse with diarrhea and colic?

The presence of bright red larval worms on the palpation sleeve, particularly in a horse presenting with diarrhea and colic, is indicative of a small strongyle infection, specifically cyathostomins. These larvae are often described as being bright red due to their blood-feeding habits and can lead to significant gastrointestinal distress in horses.

Small strongyles, or cyathostomes, are known to evoke a strong inflammatory response in the large intestine, which can manifest as colic and diarrhea when they are present in high numbers. Their life cycle involves both adult and larval stages, and the latter can encyst in the mucosa of the intestines, leading to intermittent or chronic issues once they emerge.

While other parasites like Strongylus vulgaris and Parascaris equorum also affect horses, their larvae and symptoms differ. Strongylus vulgaris larvae, for example, tend to migrate through the blood vessels and are not typically seen on palpation sleeves. Parascaris equorum is a roundworm that primarily affects younger horses, and its appearance and associated signs differ. Oxyuris equi, the pinworm, does not cause such severe gastrointestinal signs typical of colic and diarrhea; instead, it is more associated with tail rubbing