The stomach worms Habronema muscae, H microstoma, and Draschia megastoma are widely distributed. The adults are 6–25 mm in size. Draschia are found in tumor-like swellings in the stomach wall. The other species are free on the mucosa. The eggs or larvae are ingested by larvae of house or stable flies, which serve as intermediate hosts. Horses are infected by ingesting flies that contain infective larvae or by free larvae that emerge from flies as they feed around the lips. (Also see Helminths of the Skin: Cutaneous Habronemiasis.)
A catarrhal gastritis may result from heavy infections with adult worms. Draschia produces the most severe lesions—tumor-like enlargements up to 10 cm in diameter. These are filled with necrotic material and a large number of worms and are covered by intact epithelium, except for a small opening through which the eggs pass. Rarely, these nodules rupture and cause fatal peritonitis. Larvae of Habronema spp and Draschia have been found in the lungs of foals associated with Rhodococcus equi abscesses (see Respiratory Diseases of Horses: Rhodococcus equi Pneumonia in Horses). Clinical signs usually are absent except when granulomas associated with Draschia infection lead to mechanical obstruction or rupture.
Antemortem diagnosis is difficult because the thin-shelled eggs or larvae are easily missed in fecal examinations. Molecular methods have recently been developed for this purpose but would not be useful for routine use. Worms and eggs may be found by gastric lavage. Most anthelmintics have not been tested against Habronema spp or Draschia sp, although ivermectin is effective against their cutaneous larvae and against adults of H muscae. Moxidectin is effective against adult H muscae.
Last full review/revision March 2012 by Thomas R. Klei, PhD