42 TROPICAL MEDICINE AND HYGIENE 



Harrison has shown that, from an analysis of 466 cases 

 acquired during the Transvaal War, in 37 all signs of 

 disease ceased in less than 7 years, and the earliest that 

 can be hoped is 5 years. In 164 cases blood and ova 

 were continuously present, including 5 in which the 

 disease had lasted 13 years, 16 for 12 years, and 57 for 

 ii years. 



Etiology. Very little is known of the development of 

 the Schistosomtun hcematobium. The incubation period 

 seems to be a short one, as symptoms have appeared in 

 from one to two months after bathing in ponds believed 

 to be infected. The eggs passed in the urine or with 

 the faeces already contain the miracidium, or ciliated 

 embryo. This is only set free when the urine or faeces 

 are freely diluted ; the ciliated embryo then bursts through 

 the shell and becomes actively motile. It can live in 

 water for two or three days, rather longer in thick 

 mud, but ultimately dies without further development. 



An intermediate host may be necessary, but it has not 

 yet been discovered." Looss suggests that man himself 

 may be the intermediate host, that the miracidium 

 develops in him possibly in the liver. In this case he 

 may possibly reinfect himself. 



It is also unknown how man becomes infected ; the 

 infection is probably through water, but whether through 

 the alimentary canal, by drinking water, or in bathing, by 

 entrance through the skin, or even through the edges of 

 the anus or urethra, is uncertain. Experiments on cats 

 by the Japanese with the Schistosomum japonicum appear 

 to show that the miracidia penetrate the skin and that the 

 worm then develops in the cat. Immersion of cats in 

 water fouled with excrement containing mature eggs is 

 followed by infection of these cats though they are so 

 confined that it is impossible for them to drink the fluid. 

 According to this no intermediate host is required, but 

 the stages in the development have not been traced. 

 Infection in Europeans seems to follow snipe shooting 

 when wading in padi fields, manured with human excre- 



