20 



CIRCULAR 14 8, U. S. DEPARTMENT OF AGRICULTURE 



over half an inch to about an inch long and less than one twenty-fifth 

 of an inch wide ; this worm contains a single tooth in its mouth cup. 

 This tooth has two prominent, rounded projections and on casual 

 examination two teeth appear to be present. 



Life history. — The eggs of blood strongyles are discharged into 

 the intestine of the host and are eliminated from the body with the 

 feces. (Fig. 14.) Under favorable conditions of temperature and 

 moisture the eggs develop rapidly on the ground and on pastures and 

 hatch in a day or so. The larv a which emerges from each egg which 

 develops normally, feeds on the contents of the manure in which it 

 finds itself and after it undergoes two molts in more or less rapid 

 succession it becomes infective. This entire development on the 

 ground or on pasture takes place in a week or so during the warm 

 months and is delayed considerably during the cold months. Lack 

 of moisture is unfavorable to the developing eggs and larvae, but 

 ordinarily there is sufficient moisture in horse feces to favor normal 

 development, provided the temperature is favorable. Shade affords 



Figure 13. — Head ends of the three species of blood strongyles: A. &. edvntatus; 

 B, 8. vulgaris; C, S. equinus. Enlarged. From Looss, 1901 



protection to the eggs and larvae and it is likely that direct sunlight 

 is more or less injurious to them. 



The infective larvae are very resistant to unfavorable conditions 

 and are probably capable of maintaining themselves on pastures for 

 long periods. They remain ordinarily in or near the manure where 

 they hatch ; however, they are doubtless scattered by rain and wind 

 and may reach places on pastures relatively remote from their 

 original locations. When the air is sufficiently moist so that the 

 grass becomes covered with a film of moisture, as happens in times 

 of rain, dew, or fog, the larvae migrate up the grass blades and this 

 brings them to a favorable situation to be swallowed by horses while 

 grazing. So far as is known, these larvae do not penetrate the skin 

 and must infect horses by being swallowed with grass, water, or 

 dry food which has become contaminated with the larvae. 



The course of development of these parasites after they get into 

 the body of horses has not been definitely ascertained; it is known, 

 however, that not all larvae, and perhaps none of them, go down 

 directly into the cecum and colon and settle there and grow to 



