12 



CIRCULAR 14 8. 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 equorum, of horses (Fig. 

 6), also known as ascarids, are cylindrical 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 



Figure 5. — Portion of the inner lining of a horse's stomach, showing lesions 

 produced by the small stomach worm, Trichostrongylus axei. (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 eggshell protects the embryo it contains 

 against various unfavorable influences. 



