PROPHYLAXIS l8l 



Some parasites leave man by other excretions : 

 Schistosomum hcematobium usually in the urine, Para- 

 gonimus westermani in the expectoration. 



The Filariidae in most cases require the active interven- 

 tion of a blood-sucking intermediate host, as in Filaria 

 bancrofti, F. loa, and probably in the cases of F. perstans, 

 and F. demarquayi, though the intermediate host in the 

 latter cases is not determined. 



Dracunculus medinensis is the only important parasite 

 that itself provides for the discharge of its embryos by 

 perforating the skin and discharging them directly into 

 water. Possibly in Filaria volvulus the embryos find 

 their way through the skin, but this 'is uncertain. 

 Embryos have been found in some of the pustules in 

 one of the many forms of pustular eruption popularly 

 known as " kra kra " in West Africa, but whether these 

 embryos were escaping through the skin, or what the 

 parental form was, is unknown. 



(2) Changes outside the Human Host. The eggs or larvae 

 after leaving man always undergo further development. 

 This development is a limited one, but may take a con- 

 siderable time. When the limit is reached the embryos or 

 larvae may remain alive for indefinite periods without any 

 further change taking place. 



Where no immediate host is required the development 

 is as follows : 



In Ascaris lumbricoides and Trichocephalus dispar the 

 embryo forms inside the egg-capsule, but does not escape 

 from it. 



In Ankylostomum duodenale and Necator americanus 

 the embryo escapes from the egg-capsule and continues 

 to develop after leaving the faeces. Increases in size, 

 living in damp earth or mud, and finally remains quies- 

 cent for prolonged periods enclosed in its last skin, which 

 serves as a sheath. 



In one method of infection by A. duodenale and 

 N. americanus the embryo escapes from the sheath formed 

 by the old integument and re-enters the definitive host 



