24 CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 



a horse at the same time the loss of blood may be considerable and 

 may lead to anemia with the usual consequnces of weakness and of 

 watery swelhngs (edema) in various parts of the body. These worms 

 also produce injuries of various sorts in the parts of the body to which 

 they wander, such as the liver, pancreas, and other organs. 



One species, the single-toothed strongyle, is especially injurious 

 because as an immature form it settles in certain arteries, especially 

 in the anterior mesenteric artery which supplies blood to the large 

 gut. As a result of the presence of the worms in this blood vessel, 

 the wall of the artery, where the worms accumulate, becomes thickened 

 and stretches considerably to form what is known as an aneurism. 

 An aneurism is a spindle-shaped, cylindrical, or globular dilation of a 

 blood vessel and often contains a heavy deposit of fibrin inside. lu 

 the horse it mav attain the size of a child's head (fis;. 16). 



Figure 16. — A small aneurism of the anterior mesenteric artery of the horse, partly 

 cut open to show worms and the heavy deposit of fibrin inside. The worms 

 are immature forms of Strongylus vulgaris. Natural size. 



An aneurism interferes to a considerable extent with the circulation 

 of blood through the affected artery, because the heavy deposits of 

 fibrin inside may almost obliterate the lumen of the blood vessel. 

 This condition results in a diminished blood supply to the large 

 intestine; when the intestine becomes anemic as a result of this it 

 becomes predisposed to colic, twist, and intussusception, the last 

 being a condition in which part of the gut shps into an adjoining part. 

 When a piece of fibrin deposit in the aneurism breaks loose, it may be 

 carried in the circulation to a terminal portion of an artery and may 

 lodge there as a plug. As a consequence, the circulation to a part of 

 the large gut may be completely shut off. Such a condition interferes 

 with the functions of the large gut, produces an anemic condition with 

 the consequences noted above, and in extreme cases may produce 

 death. It is believed that most cases of colic in horses result from 

 such disturbances in the blood circulation of the gut. If the plug 

 forms in a hind leg it may cause a form of intermittent lameness. 

 Aside from the injuries described, mixed parasitic infestation, known 



