PARASITES AND PARASITIC DISEASES OF HORSES 13 
During the summer months the eggs develop to the infective stage 
in about two weeks. The low temperatures of cold weather retard 
the development of the eggs, as does also lack of moisture. Ordinarily 
sufficient moisture is present in horse ma- 
nure to favor the development of the eggs. 
Balls of manure which appear dry on the 
surface commonly contain sufficient mois- 
ture in the middle to permit the normal 
development of these eggs. Excessive 
drying destroys the vitality of ascarid 
eggs, 
If infective ascarid eggs are swallowed 
by horses with grass, water, or dry feed 
which has become contaminated with horse 
manure, the embryos are liberated from 
their shells in the horse’s intestine and 
then burrow into the wall of the gut and 
migrate with the blood stream to the liver. 
From this organ they proceed in the blood 
stream through the heart to the lungs. If 
many worms go through the lungs at the 
same time they injure this organ and may 
produce pneumonia. This roundabout 
journey from the intestine to the liver and 
thence to the lungs is completed in 
about a week. From the lungs the larve 
crawl up the windpipe until they reach 
the back of the mouth and are then swal- 
lowed. On getting into the small intestine 
from the stomach for the second time, they 
settle down and develop to maturity in 
from about two to two and a half months. 
(Fig. 8.) 
Symptoms and lesions—When ascarids 
are present in large numbers, which is 
likely to be the case in foals and young 
horses, they produce digestive disturb- 
ances of various sorts and may cause colic. 
These parasites frequently become entan- 
gled with one another, resulting in large 
masses of worms which may plug the 
lumen of the intestine. In an extreme con- 
dition of this sort the results may be fatal. 
Such an entangled mass of worms may 
even rupture the wall of the intestine as 
a result of continuous pressure on it. 
These worms have also been reported as 
being capable of perforating the wall of 
the intestine, presumably as a result of 
continually pushing their heads against it. 
Ficure 6.—The large roundworm, 
Ascaris equorum: A, male; 
B, female. About one-half 
natural size 
In either case, a rupture 
of the intestinal wall would usually cause the death of the horse. 
In experimental infestations of horses 
with ascarids, fever and 
a cough have been observed during the early stages when the 
