12 CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 
Prevention.—Preventive measures are similar in a general way to 
those discussed in connection with preventive measures “against blood- 
worms and other strongyles. The similarity in control measures is 
based on similar life histories. 
THE LARGE INTESTINAL ROUNDWORMS OR ASCARIDS 
The large intestinal roundworms, Ascaris eqguorum, of horses (Fig. 
6), also known as ascarids, are cylindrical i in shape, yellowish white 
in appearance, and from about the size of an ordinary lead pencil to 
about a foot long when fully grown. The head is clearly marked 
off from the rest of the body and bears three clearly distinguishable 
lips. Closely related worms occur in pigs, cattle, and human beings. 
Ascarids occasionally pass out of the bowels spontaneously, in which 
FictreE 5.—Portion of the inner lining of a horse’s stomach, showing lesions 
produced by the small stomach worm, Trichostrongylus azei. (Photograph by 
courtesy of the Agricultural and Mechanical College of Texas) 
case they are readily seen, and in horses this is said to take place in 
the spring of the year. 
These worms are located in the upper part, less often in the middle 
and lower parts, of the small intestine; they are occasionally found 
in the cecum and in the stomach. They may occur in large num- 
bers, especially in foals and in young horses. 
Life history—(Fig. 7.) The female worms produce large num- 
bers of eggs which are microscopic in size. The eggs are deposited 
in the lumen of the horse’s intestines and are expelled from the 
bowels in the manure. Under favorable conditions of temperature 
and with an adequate supply of moisture, the eggs develop on the 
ground and on pastures until they reach the infective stage, but the 
embryos remain in their eggshells until they reach the gut of a sus- 
ceptible horse. The thick eogshell protects the embryo it contains 
against various unfavorable influences, 
