Habronema sp (Stomach worms)

What you need to know

Stomach worm larvae are ingested by fly maggots in dung. The worms develop inside the maggots. Mature flies then deposit the larvae on the lips, nostrils, wounds and other naturally moist areas of the horse. Stomach worm larvae can expand a wound and prevent healing, causing "summer sores". Larvae deposited in the eyes can cause conjunctivitis. Larvae that are eaten can cause gastritis and tumour-like growths which may rupture.

Effective wormer chemicals: Ivermectin, Moxidectin.

More information

The stomach worms Habronema muscae , H microstoma(majus) , and Draschia(Habronema) megastoma are widely distributed. The adults are 6-25 mm in size. Draschia occurs in tumour-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. (See also cutaneous habronemiasis, Cutaneous Habronemiasis.)

A catarrhal gastritis may result from heavy infections with adult worms. Draschia produces the most severe lesions—tumour-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 ( Rhodococcus equi Lung Abscessation in Foals). 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 faecal examinations. 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 Habronema muscae .