8 CIRCULAR 14 8, U. S. DEPARTMENT OF AGRICULTURE 



Roundworms in the Stomach 



Two kinds of worms are present in the stomach of horses. One 

 kind, designated here as the large stomach worms, is represented by 

 three distinct species in the horse ; these parasites are transmitted by 

 flies. The other kind, designated as the small stomach worm of the 

 horse, is represented by only one species. The small stomach worm 

 is acquired by horses directly as the result of swallowing infective 

 larvae which occur on pastures. The larvae may also be taken in by 

 horses with water and with dry feed. 



THE LARGE STOMACH WORMS 



The large stomach worms (fig. 3) are relatively long and slen- 

 der, are whitish in color, and occur either free in the stomach, em- 

 bedded in mucus, attached to the wall of the stomach, or in tumors 



of the stomach wall, for which 

 tumors these worms are respon- 

 sible. 



One species, Carter's stomach 

 worm, Habronema muscae, of the 

 horse, is from about one-third 

 inch to nearly an inch long. 

 This parasite occurs free in the 

 stomach or attached to the wall 

 of the stomach. A second spe- 

 cies, the small-mouthed stomach 

 worm, H. microstoma, is similar 

 in size and appearance to the 

 above species. While this form 

 may occur free in the stomach, it 

 is capable of penetrating the wall 

 of this organ and causing sores. 

 A third species, the large- 

 mouthed stomach worm, H. me- 

 gastoma, of the horse is the 

 smallest of the three species, ranging in length from less than one- 

 third of an inch to about one-half inch. These worms occur in 

 tumors found in the wall of the stomach; the tumors may become 

 very large, attaining the size of a fist or even of a child's head. 

 Usually, however, they are much smaller. 



Life history. — (Fig. 4.) The eggs of horse stomach worms have 

 very flexible shells ; they are deposited in the lumen of the stomach 

 and are eliminated from the horse's body with the feces. When 

 the eggs are swallowed by maggots of house flies, stable flies, or 

 other flies which breed in horse manure, further development takes 

 place which keeps pace with the development of the maggots. The 

 larvae of Carter's stomach worm of the horse develop for the most 

 part in house flies while those of the small-mouthed stomach worms 

 of the horse have stable flies as their usual intermediate hosts, though 

 they are known to develop also in house flies and other flies. The 

 larvae, of the large-mouthed stomach worms of the horse develop in 

 various species of nonbiting flies, including house flies. These larvae 

 attain their full growth in the flies when the latter emerge from the 



Figure 3. — Large stomach worms of the 

 horse. A, Habronema microstoma; others, 

 //. muscae. Natural size 



