84 ILLINOIS BIOLOGICAL MONOGRAPHS [84 



PATHOLOGY 



The infection in any case of internal parasitism suggests an inquiry as 

 to the effects of the parasite on the host. The injury on the human sub- 

 ject produced by trematode infections has been the subject of numerous 

 observations and records. Notable among these are the contributions of 

 Looss (1913) on Schistosoma haematobium, Katsurada (1914) on Schistosoma 

 japonicum, and Ward (1909) on Fasciolopsis spp. Again, the effect of tre- 

 matodes on their host has been the subject of considerable study in fish infec- 

 tion, on account of the economic importance of the problem. But -where the 

 special incentives to the problem have been lacking, very little study has been 

 made on the pathological significance of trematode infection. 



The helminth parasite causes a two-fold injury to the host, mechanical 

 and chemical. The inclusion of parasites within the organs of the host is the 

 occasion for distension of the organs and consequent irritation; the piercing 

 of organs of the host by the armature of the worm, an actual mechanical, 

 injury. These injuries are accompanied by the formation of fibromata within 

 the organs and, usually, attempts to isolate the parasite by the secretion of a 

 cyst around it, as in schistosomiasis (Bovaird and Cecil, 1914:191). In the 

 ordinary infection a toxin is secreted by the parasite, and frequently an anti- 

 thrombin and a hemolysin are produced. Such injuries as these in higher 

 animals are diagnosed by the blood-picture, where excessive hemocytolysis 

 and eosinophilia are found. 



The infected organs of the molluscan hosts of the Bitter Root Valley are 

 the liver ceca. Altho these Ue next to the testes, the worms have never been 

 found to invade these organs. Thomas (1883:114) found that the cercariae 

 of Fasciola hepatica live normally in the pulmonary chamber of the snail 

 Lymnaea trunculata. Cort (1915) found the infected organs of the Campelomas 

 were the gills, but in other species, the Hver tissues were the seat of the infec- 

 tion. The infected tissue of the Venezuelan snail, Planorhis guadelupensis, 

 is the testicle according to observations made by the writer. 



The liver ceca consist of polygonal lymphocytoidal cells grouped around 

 the liunina of the ceca, with an epithelial Uning surrounding the ceca. Among 

 the ceca are large interstices filled with lymph. These intercecal spaces are 

 the places where the worms are first found, suggesting invasion thru the blood 

 stream. In light infections, the parthenitae lie here, absorbing the nourish- 

 ment from the surrounding liquid in which the worm is bathed. Ths only 

 mechanical injury up to the time of the activity of the cercariae is caused by 

 the agitation of the developing larvae encysted within the host. But in the 

 case of heavy infection, especially where the larva does not encyst within the 

 host, where it works its way out into the water, even a few worms may cause 

 considerable mechanical harm to the host. 



In an examination of living material and sections of infected moUusk Uver 

 tissue, no infection was found to be so light that the host was unharmed. In 



