8 



CIRCULAR 1-1S, U. S. DEPARTMENT OF AGRICULTURE 



blood red. Roundworms occurring in the horse have a considerable 

 range of size, the large, intestinal roundworm or ascarid being from 

 6 inches to over a foot long, while the small stomach worm is only 

 about one-fifth of an inch long. Roundworms reproduce by means 

 of eggs which are produced by the female worms. The eggs are 

 usually eliminated from the horse's body with the manure. Some 

 species of roundworms must be taken up by an intermediate host in 

 which they undergo part of their development, while other species 

 are transmitted from one animal to another directly by eggs or by 



larvae which have emerged 

 from eggs. Most round- 

 worms which occur in the 

 horse are transmitted directly 

 by eggs and larvae, without 

 the use of an intermediate 

 host. 



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. 



Figure 3. — Large stomach worms of the 

 horse. A. Habronema microstoma; others, 

 H. muscae. Natural size. 



THE LARGE STOMACH WORMS 



The large stomach worms (fig. 3) are relatively long and slender, 

 are whitish in color, and occur either free in the stomach, embedded 

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

 stomach wall, for which tumors these worms are responsible. 



One species, Carter's stomach worm, Habronema muscae, of the 

 horse, is from about one-thud inch to nearly an inch long. This par- 

 asite occurs free in the stomach or attached to the wall of the stomach. 

 A second species, the smaU-mouthed stomach worm, H. majus, syn- 

 onym 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 its wall and causing sores. A third species, the large- 

 mouthed stomach worm, H. megastoma (=Draschia megastoma) , 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. 



