Etiology. 543 



Etiology. Echinococci develop from the embryos of the 

 taenia echinococcus (Siebold) which inhabits the intestine of 

 the dog. These embryos are liberated from ingested eggs in 

 the stomach and pass ont of the intestine into the portal circu- 

 lation. The vehicles of infection are food or water contam- 

 inated with the feces of dogs or with proglottides or tape- 

 worm eggs contained in these. 



Echinococci are cysts of various sizes containing a clear, pale, yel- 

 low liquid the reaction of which is either nentral or slightly acid. 

 The li(iuid contains a little allinmen, a considerable proportion of salt, 

 and not infrequently succinic acid. The wall is composed of two 

 layers. The outer or chitinous M'all is the thicker, white in color and 

 under the microscope appears to be composed of a number of parallel 

 lamina3. The inner or germinal layer is yellowash in color, granular, 

 and sometimes contains in the deeper parts muscle fibres, granules 

 of lime and small blood-vessels. In the livers of cattle and more rare- 

 ly in the livers of pigs and sheep, a second form of parasite occurs, E, 

 multiloeularis or alveolaris, in which there are masses of cysts about 

 the size of peas united together by a fibrous, net-like matrix derived 

 from the outer layer. In this way tumor-like growths of various sizes 

 are formed. The central part is often caseous or calcified, while the 

 peripheral parts are composed of numbers of small cysts resting upon 

 each other, and the whole growth appears to be traversed in all direc- 

 tions by a recognizable fibrous network. According to Ostertag, Mid- 

 ler, Mangold and Posselt, this cyst represents a stage of special tape- 

 worm, the Taenia echinococcus multiloeularis, (see page 469). 



At a certain stage there develop on the inner surface of the cysts small 

 prominences, the soealled brood capsules. Some of the cysts remain attached to the 

 wall and others are free in tlie liquid. They contain the new scoliees. In other cases 

 very small cysts develop between the layers of the wall. Some of these pass inwards 

 and eventually become free in the liquid and later grow into large bladders. There 

 may be large numbers of tliese daughter cysts within the mother cyst. These may 

 contain granddaughter cysts (E. endogenus, altrieipariens or hydatitosus). More 

 frequently the small cysts pass outwards and separating from the parent cyst 

 undergo further development (E. exogenus, scolicipariens simplex, granulosus or 

 veterinorum). Both endogenous and exogenous cysts may later give rise to scoliees 

 or daughter cysts. 



Lichtenfeld found a larger number of fertile cysts in pigs under two years 

 than in older animals and also that the proportion existing between fertile and 

 sterile cysts was in cattle 24:76, in pigs 80:20, in sheep 92.5:7.5 and in the horse 

 38.9:61.1. 



The experimental researches of Bobroff, v. Alexinsky, Deve and Ponomaroflf 

 have shown that echinococci may be transported not only to organs in close con- 

 nection with that primarily infested but also to others, scoliees being set free by 

 the rupture of a fertile cyst. 



The development of echinococci is very slow. According to Leuckart small 

 foci measuring 0.25 to 0..35 mm. in diameter and enclosed in thin fibrous cap- 

 sules are found one month after infection. After two months they measure al;)ont 

 2.5 mm. and are already converted into small cysts. Towards the end of the 

 fifth month the cysts are about the size of hazelnuts, the two layers of the wall 

 are well differentiated and the development of the scoliees and daughter cysta 

 haa begun. 



Anatomical Changes. The liver is enlarged in proportion 

 to the size and number of the cysts present in it, and in the 

 ox may weigh as much as 158 pounds (Ringk). In the pig, in- 



