Habronema species and Draschia species

Habronema and Draschia are small nematodes that as adults inhabit the stomach of horses.  They occur in many parts of the world, including warmer areas of the US, but apparently not Canada.  Habronema live on the mucosal surface and Draschia in fibrous nodules beneath the mucosa.  The life cycle of both genera involves the production of delicately-shelled oblong eggs, each containing a first-stage larva.  Once the faeces are voided the eggs hatch, and for development to continue the released larvae must be ingested by a dipteran intermediate host, often Stomoxys calcitrans (the stable fly).  Within the fly the larvae moult twice to form infective third-stage larvae.  Infection of the horse is by ingestion of larvae released by the fly when they land around the lips, or of infected flies.

The adults Habronema and Draschia  in the stomach are usually considered as non-pathogenic.  As the eggs of both genera disintegrate rapidly in faeces, especially if subjected to flotation, diagnosis of the gastric infections is usually by gastric lavage.  If, however, and infected stable fly lands on an open skin wound, the infective larvae of Habronema, and to a lesser extent Draschia, are released and burrow into the wound, causing a granulomatous reaction (cutaneous habronemiasis or "summer sores") that is difficult to treat successfully, and often delays wound healing.

Neither Habronema nor Draschia are known to be zoonotic.