PARASITES AND PARASITIC DISEASES OF HORSES 



13 



During the summer months the eggs develop to the infective stage 

 in two weeks or less. 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 ascaricl 

 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 larvae 

 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. 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 



B 



Figure 6. — The large roundworm, 

 Ascaris equorum: A, male; 

 B, female. About one-half 

 natural size 



