8 CIRCULAR 148, 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 
larvee which occur on pastures. The larve 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 
A is capable of penetrating the wall 
of this organ and causing sores. 
Ficurn 3.—Large stomach worms of the A third species, the large- 
horse. <A, Habronema microstoma; others, é = 
FL ARAETE, ONE Se 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 wali 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 
larvee 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 
larve. of the large-mouthed stomach worms of the horse develop in 
various species of nonbiting flies, including house flies. These larve 
attain their full growth in the flies when the latter emerge from the 
