HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



the same species does not ; and the many other problems 

 that are involved in the study of host-parasite specificity 

 (seep. 42). 



12. PREVENTION AND CONTROL 



Carriers and transmitting agents. The methods of 

 transmission of histolytica cysts have already been de- 

 scribed (pp. 74-83). Amcebiasis is a preventable disease 

 just as are typhoid fever, bacillary dysentery, cholera, 

 etc. Patients suffering from an acute attack are not ordi- 

 narily dangerous, since only trophozoites appear in their 

 stools and these are probably seldom if ever responsible 

 for new infections (p. 65) ; it is the carrier v^ho is pass- 

 ing cysts that must be guarded against. Such a carrier 

 may discharge as many as 300,000,000 cysts in a single 

 day (Kofoid, 1923). These cysts, as already pointed out 

 (p. 67), cannot be conveyed through the air since they 

 are killed by dessication ; they must enter a new host by 

 way of the mouth in a moist condition. The problems of 

 prevention and control, therefore, concern methods of 

 transmission and the destruction or control of trans- 

 mitting agents. 



The protection of individuals. Individuals are probably 

 usually infected by cysts ingested in food or drink. These 

 cysts may reach drinking water because of soil pollution 

 or contamination in some other way ; they may be trans- 

 ferred to food by infected food-handlers who are passing 

 cysts, by flies or other animals that have fed upon in- 

 fected human feces, or by the use of night soil in the 

 fertilization of vegetable gardens or the use of con- 

 taminated water to wash uncooked vegetables. Trans- 



116 



