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CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 



are eliminated in the manure. The eggs develop very rapidly on the 

 ground and on pastures, and the larvae which emerge from them may 

 either transform directly into infective larvae which serve to infest 

 other foals, or may develop into free-living males and females. As a 

 rule both types of development take place, some larvae becoming 

 infective while others develop into free-living adults. The latter mate 

 on the ground or in manure and the females produce eggs and deposit 

 them on soil or in manure. In a short time, under favorable condi- 



Ft'gure 10. — Type of sanitary watering trough. 



tions, larvae emerge from these eggs. These larvae become infective 

 and are capable of infecting horses. 



These parasites multiply very rapidly, the entire development on 

 the ground taking place in a few days. The infective larvae, whether 

 they hatched from the eggs eliminated with the feces or from the 

 eggs produced by the free-living generation of worms, are taken in 

 by horses with food and are also capable of penetrating the intact 

 skin. Within about 2 weeks after the larvae have been taken up 

 by horses they have developed into mature females which soon begin 

 to discharge eggs into the intestine of the host. 



Symptoms and lesions. — While no definite symptoms have been 

 described in horses as resulting from infestation with these parasites, 

 the worms are probably responsible for diarrhea in foals. These 

 worms make their appearance in foals earlier than do other parasites, 

 and the infestation tends to become heavy as a result of reinfestations 

 which may take place rapidly and often, because of the brief period 

 required for development outside of the host. The parasites are 



