Habronema and Draschia spp are vector-borne parasites using muscid flies as intermediate hosts. The adult parasites establish in the stomach upon ingestion of larvae deposited by flies around the mouth or by ingestion of dead flies carrying the larvae. Within the stomach, parasites become adults in about 8 weeks. Adult Habronema are found in close contact with the gastric mucosa, but cause no clinical problems. Draschia sp has become very rare in domestic horses in recent decades.
The gastric stage of Habronema spp infection has been described to cause a catarrhal gastritis, but clinical manifestations have not been reported. Similarly, a pulmonary form of habronemiasis has been described, but the clinical implications are unclear.
Draschia sp has been described to cause large fibrous masses near the margo plicatus, but the clinical significance is unclear, and these lesions are rarely encountered.
Cutaneous habronemosis (summer sores) has clear manifestations.
Fecal flotations or fecal egg counts are not reliable ways to diagnose gastric infections of Habronema spp, because eggs are difficult to see under the microscope. The adult worms are 6–25 mm long, so they could, in theory, be identified via gastroscopy or by gastric lavage, although this has not been described.
Also see pet health content regarding stomach worms in horses.