Pathogenesis. 549 



In this connection the dampness of the ground is of chief 

 importance. The most severe infection occurs in places where 

 the climate is damp or the rainfall copious, and the temperature 

 comparatively high. Infection is also severe on flood land and 

 during wet seasons. The period during which dangerous land 

 is used as pasture, must also be taken into consideration. The 

 longer the land is used the greater the number of flukes in- 

 gested. A stay of a few hours only, or even less than an hour, 

 has in some instances led to serious outbreaks of the disease. 

 In most cases repeated infection of the same animals is ob- 

 served. 



Susceptibility. Sheep and wild ruminants are the most 

 susceptible, the ox is a little less so and the goat a little less 

 than the ox. Swine, solipeds, and carnivora on the other hand 

 rarely become diseased in spite of being invaded by the parasite. 

 Young adult animals are more susceptible or their powers of 

 resistance are less. 



The wide distribution of the disease among sheep convinces Bailliet that the 

 cercarife are to be found principally on the lower blades of grass which the sheep 

 eat, whereas cattle get only the upper ones. According to Humble and Lush sheep 

 with short lower jaws (parrot-mouthed) escape infection because they can only 

 bite the tips of the leaves. 



Pathogenesis. All the available evidence shows that the 

 parasites, in migrating from the duodenum into the bile ducts, 

 move by elongating their anterior end and flxing the oral and 

 ventral sucker alternately. They spread rapidly through the 

 liver until they reach the smallest bile ducts, where they are 

 either arrested or may pass back again. 



According to Gerlach, May and Spinola, the parasites gain access to the liver 

 by penetration of the capsule after having passed out of the intestine through the 

 wall. Luchs believes that the invasion of the liver is by way of the portal vein. 



The flukes that have penetrated into the smallest ducts, 

 burst through these at numbers of places, and passing into the 

 liver tissue cause destruction of large areas of it and may con- 

 sequently be frequently found under the serous membrane, or 

 they may, by perforation of this, pass into the peritoneal cavity. 

 As a result of the mechanical irritation, due particularly to 

 the spiny covering of the parasites, and to the destruction of 

 the liver tissue, an acute inflammatory condition is set up 

 in both the bile ducts and the liver tissue. There may 

 also be hemorrhages. Under certain conditions abscesses are 

 produced in various parts of the liver. Schaper found strep- 

 tococci and staphylococci in such abscesses. Flukes may pass 

 into the portal or hepatic veins and in the latter case they maj 

 be carried to the right side of the heart and thence to the lungs 

 or even to peripheral parts of the body. 



The majority of distomes that have penetrated into the 



