PARASITES AND PARASITIC DISEASES OF HORSES 



21 



although it is somewhat smaller, females being about l l / 2 inches long. 

 The single-toothed strongyle, Strongylus vulgaris (fig. 13, B), is the 

 smallest of the three species, and is from a little 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 larva 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 



Figure 13. — Head ends of the three species of blood strongyles: A, S. edentatus; 

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



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, pro- 

 vided the temperature is favorable. Shade affords 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, clew, or fog, the larvae migrate up the grass blades and this 

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



