PARASITES AND PARASITIC DISEASES OE HORSES 



21 



maturity. On the contrary, the larvae, after entering the body of 

 the horse, undergo extensive migrations which bring them to various 

 organs and tissues, such as the liver, pancreas, spleen, lungs, kidneys, 

 and other organs and tissues, from which many of the larvae prob- 

 ably fail to get back to the large gut. However, those larvae which 

 return to this organ become attached to its wall and develop to 

 fertile maturity. The eggs which are produced by the female worms 

 and eliminated from the horse's gut with the manure start the cycle 

 of development once more. 



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Figure 14. — Representation of the life history of one of the blood strongyles, 

 S. equinus. The illustrations of the adult worms are about natural size and 

 those of the eggs and larvae are enlarged about 75 times. This illustration is 

 typical of the life cycle of horse strongyles generally 



From this account of the life cycle of these parasites it is evident 

 that horses infested with blood strongyles contaminate the pastures 

 on which they feed with the eggs produced by the worms, and that 

 the larvae which issue from the eggs and develop to the infective 

 stage may be swallowed by these and other horses. When horses 

 are kept on the same pastures year after year the number of eggs 

 and larvae gradually increases, and this contamination, accumulating 

 from year to year on a given pasture, may be highly damaging to 

 horses which are grazed there. Foals, in particular, suffer from 

 the effects of gross parasitism acquired in this manner. 



