28 TROPICAL MEDICINE AND HYGIENE 



which they occur are dilated and the walls much thick- 

 ened. The gross organic lesions so produced give rise to 

 definite disease in few cases. 



In some cases in these dilated, thickened ducts biliary 

 calculi form, so that sometimes numerous small calculi 

 are found in many of the dilated ducts, and following this 

 condition an acute pyogenic infection of the bile-ducts 

 occurs. Rarely a large solitary abscess forms and this 

 may attain a considerable size and burst into the peri- 

 cardium or elsewhere. 



Clonorchis sinensis has also been found in the pancreas. 



There is no possibility of removing these worms, and 

 the treatment, therefore, must be symptomatic. 



Prophylaxis. In spite of the rarity of dangerous sym- 

 ptoms, it is advisable to prevent as far as possible infection 

 with these flukes. 



The limited range of distribution of the different species 

 of flukes is best explained on the assumption that the 

 distribution of suitable intermediate hosts is also con- 

 fined to special localities. 



The habit in the East of using human faeces for 

 manure, and the fact that rice cultivation is specially 

 adapted for the growth of molluscs and certain fish, 

 probably explains the frequency of trematode infections 

 in China and Japan. 



The eggs of trematodes are resistant and are not 

 destroyed by passage through the intestines of larvae of 

 flies. Nor are they destroyed rapidly when faeces are 

 desiccated or allowed to putrefy. Even if the excrement 

 is stored for a long time living eggs will still remain, 

 and when the excrement is mixed with water and used 

 as manure, particularly in padi fields, the miracidia will 

 escape and infect a suitable host. The complete change 

 in the habits of the people and the economic loss that 

 would result from more complete destruction of the 

 faeces render it improbable that infection will be entirely 

 prevented. 



