PARASITES AND PARASITIC DISEASES OF HORSES 



13 



THE LARGE INTESTINAL ROUNDWORMS OR ASCARIDS 



The large intestinal roundworms, Parascaris 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 distin- 

 guishable lips. Closely related worms occur 

 in pigs, cattle, and human beings. Ascarids 

 occasionally pass out of the bowels spontane- 

 ously, in which 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 smaU intestine; they are occasionally 

 found in the cecum and in the stomach. They 

 may occur in large numbers, especially in 

 foals and in young horses. 



Life history (fig. 7). — The female worms 

 produce large numbers of eggs which are 

 microscopic in size. The eggs are deposited 

 in the lumen of the horse's intestine 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 susceptible horse. The ^ 

 thick eggshell protects the embryo it contains 

 against various unfavorable influences. 



During the summer months the eggs develop 

 to the infective stage in about 2 weeks. The 

 low temperatures of cold weather retard the 

 development of the eggs, as does also lack of 

 moisture. Ordinarily sufficient moisture is 

 present in horse manure to favor the develop- 

 ment of the eggs. Balls of manure which 

 appear dry on the surface commonly contain 

 sufficient moisture in the middle to permit 

 the normal development of these eggs. Ex- 

 cessive drying destroys the vitality of ascarid 

 eggs. 



If infective ascarid eggs are swallowed by 

 horses with grass, water, or dry feed which ff 



has become contaminated with horse manure, 1| 



the embryos are liberated from their shells * 



in the horse's intestine and then burrow into Figured— The large round- 

 the wall of the gut and migrate with the blood worm ; Par ? C 1F\ eq T 



,, R. -T-, fe ,, . ,i rum: A, male; B, female, 



stream to the liver. *rom this organ they About one -half natural 

 proceed in the blood stream through the size. 



