548 Distomatosis. 



is from nine to twelve niontlis. On the other hand, Leuckart and 

 Friedberger hold that the parasites coiiiinenee to migrate within three 

 to four weeks after infection or immediately after they have become 

 sexually mature. Schaper expressed the view that, like infection, migra- 

 tion of the parasites may take place during any part of the year. 



The Limnffius minutus or truncatulus is a small snail measuring about V2 <?'"• 

 in length and provitleil with a brown, f^piral shell. It occurs especially on marshy 

 ground, in ditches and also in slow-flowing brooks. In the autumn it enters the 

 water, preferably in a ditch with a clay bottom, and passes the winter there. 



The life-cycle of the D. lanceolatum is in all probability similar 

 but it is not yet known definitely. According to Plana, the pear-shaped 

 larva? penetrate the snail known as Helix carthusiana, and there develop 

 into cercaria?. Leuckart believes that the escape of the em])ryos into 

 the outer world is not spontaneous, but occurs in the intestine of a 

 special intermediate host. 



Besistance of the Parasite. The eggs remain capable of germinating from 

 one year to another but do not resist drying or putrefaction. The larvae, accord- 

 ing to Friedberger, are not killed by night frosts, and the encapsuled cercarine 

 retain their power of development for weeks or even months (Leuckart). Salt 

 solutions are very ]ioisonous to the larvie and cercarias ( Ercolani and Perroncito), 

 A 2% solution is fatal in 5 minutes and a l^r solution in 20 to 35 minutes. 



Natural Infection. As a rule infection results from the in- 

 gestion of green food carrying encysted cercariae. This is es- 

 pecially likely to happen on moist and marshy land, and on 

 land that is frequently flooded. Contaminated water must also 

 be responsible in many cases. According to Lutz, the latter 

 method is the common one in the Sandwich Islands. The cer- 

 cariae becoming detached from water-plants in shallow streams 

 collect at places where the water is shallow, and are then taken 

 up by the cattle when drinking. As some cercariap tend to be- 

 come encysted in the bodies of grass snails, infection may take 

 place through the ingestion at pasture of plants bearing them. 

 Spinola succeeded in infecting healthy sheep by feeding them 

 with grass snails. 



Housed animals may be infected, infection generally tak- 

 ing place through cercaria-infected green food, and sometimes 

 through food from marshy land which has not been stored 

 very long. 



The disease is introduced into districts previously free, 

 either by diseased domestic animals or l)y wild ruminants and 

 hares. 



Infection generally takes place in the summer or autumn, 

 conditions then being favorable. According to Friedlierger and 

 Schaper, the infection may occur at any time of the year but, 

 for reasons easily understood, infection is less severe in w^in- 

 ter. Infection may occur during winter through the ingestion 

 of food cut fairly fresh, or in "open" weather when the ani- 

 mals can go to pasture. 



The degree of infection depends in the first place upon 

 the extent to which the pasture, food and water are contam- 

 inated. 



