Presence of larvae in the cardia of the stomach of a Catalan Pyrenees, female, 12 months old horse.

These are diptera larvae (flies) of Gasterophilus gender; G.intestinalis is the most common species. Larvae are usually found in the squamous mucosa of the cardia. Other species are located in different regions: G.nasalis (in the mucosa of the pylorus) and G.heamorrhoidalis (typically, on its way out it can be attached to the rectal mucosa).

The fly lays its eggs in the hair and when the horse licks them the larvae infest the oral mucosa (tongue, cheeks and gums) for 3 or 4 weeks. Later they migrate to the periodontal space, where a purulent exudate can be observed, particularly on the lingual surface of the upper molars. From there they migrate to the base of the tongue and further into the stomach. Larvae can cause small ulcers surrounded by a margin of hyperplastic squamous epithelium and inflammation of the gastric mucosa, but usually do not have much pathological relevance. Complications have been described such as secondary bacterial infections. In summer the larvae are shed within the feces as a pupae.

Gasterophilus intestinalis larvae in the stomach carda of a horse.