20 CIRCULAR 148, 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 ina 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 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 larve, 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, S. edentatus; 
B, S. vulgaris; C, S. equinus. Enlarged. From Looss, 1901 
protection to the eggs and larve and it is likely that direct sunlight 
1s more or less injurious to them. 
The infective larve 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 larvee 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 larve do not penetrate the skin 
and must infect horses by being swallowed with grass, water, or 
dry food which has become contaminated with the larve. 
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 larve, and perhaps none of them, go down 
directly into the cecum and colon and settle there and grow to 
