PARASITES AND PARASITIC DISEASES OF HORSES 23 
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 intermit- 
tent lameness. Aside from the injuries described, mixed parasitic 
infestation, known as strongylidosis, results when blood strongyles 
occur in large numbers and in association with other species of 
roundworms in the large gut, particularly when horses are kept on 
permanent pastures. 
This condition is widespread in horses and is usually more injuri- 
ous to foals and young horses than to older horses. Strongylidosis is 
frequently mistaken for infectious anemia or swamp fever and cases 
diagnosed as swamp fever, in many instances, have cleared up fol- 
lowing the removal of worms by medicinal treatment. The common 
symptoms of strongylidosis are diarrhea, weakness, and emaciation. 
The digestive disturbances result from the irritation to the lining 
FicurE 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 
of the gut produced by the parasites. At first the diarrhea is shght 
and the soft manure has a bad odor and contains poorly digested 
food material; later the diarrhea becomes more pronounced, with 
softer feces. The appetite, which is irregular at first, becomes poorer 
and finally the horses are off feed. As a result of their weakened 
condition, affected horses find difficulty in chewing, may throw out 
mouthfuls of feed, and then reject feed altogether. This leads to 
further emaciation which becomes very marked. As the disease pro- 
eresses the bones become prominent, the coat becomes rough, the 
eyes are sunken, and the animal becomes greatly weakened. 
(Fig. 17.) With these symptoms there are marked changes in 
various tissues and organs of the body. 
The working capacity of horses which are suffering from strongy- 
lidosis is decreased considerably even before the symptoms become 
very pronounced, and in the absence of treatment such horses are 
able to do less and less work as the disease progresses. It has been 
found that when horses are treated for the removal of parasites 
many of the symptoms described above disappear, the animals take 
