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 
7 of the stomach wall, for which 
tumors these worms are respon- 
sible. 
One species, Carter’s stomach 
worm, Habronema muscae, of the 
- * inch to nearly an inch long. 
This parasite occurs free in the 
horse, is from about one-third 
stomach or attached to the wall 
of the stomach. A second spe- 
cies, the small-mouthed stomach 
worm, //. majus, synonym /. 
microstoma, 1s similar in size 
and appearance to the above 
species. While this form may 
A occur free in the stomach, it is 
capable of penetrating its wall 
FIGURE 8.—Large stomach worms of the and causing sores. A third spe- 
Perse. cai er onema_microsioma;y others Gees athe) lar ce-mouumeG scuonna tim 
worm, H. 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. 
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 
larve 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 
larvee of the large-mouthed stomach worms of the horse develop in 
various species of nonbiting flies, including house flies. These larvee 
attain their full growth in the flies when the latter emerge from the 
