Equine stomach worms, Habronema muscae, H microstoma, and Draschia megastoma, infect the mucosal lining of the stomach and cause catarrhal gastritis. Draschia can cause tumor-like swellings along the margo plicatus, but this parasite has become rare in domestic horses. In their gastric stage, Habronema parasites rarely cause clinical problems, and their most significant manifestation is the cutaneous condition often referred to as summer sores. Anthelmintic efficacy has not been recently evaluated against these parasites, but macrocyclic lactones are still believed to be effective against the gastric stages.
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.
Clinical Signs of Habronema spp Infection in Horses
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.
Diagnosis of Habronema spp Infection in Horses
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.
Treatment of Habronema spp Infection in Horses
Although no recent anthelmintic efficacy studies exist, the gastric stages of Habronema spp are expected to be susceptible to macrocyclic lactone treatment (ivermectin or moxidectin).
Prevention of Habronema spp Infection in Horses
Fly control could, in theory, reduce infection pressure with this parasite, but this has not been tested experimentally.
Zoonotic Risk of Habronema spp Infection
None.
Key Points
Habronema spp infection is believed to be relatively common in domestic horses.
Clinical implications of the gastric stages are unknown.
Antemortem diagnosis is difficult.
Macrocyclic lactones are believed to be effective against the gastric stages.
For More Information
Also see pet health content regarding stomach worms in horses.
