92 ONE-CELLED ANIMALS— PHYLUM PROTOZOA 



may carry these organisms in their intestines without showing any 

 symptoms of the disease. In such a person a balance has been struck 

 between the virulence of the organism and the resistance of the body, 

 so that neither is able to overcome the other ; yet the parasites are able 

 to live and produce their cysts which may infect others. Drinking wa- 

 ter may be contaminated from sewage seeping into surface wells or 

 streams which are used as drinking water without proper purification. 

 There was even one famous case where confused plumbers made the 

 wrong connections in a hotel and allowed some sewage to get into run- 

 ning ice water in the hotel rooms. Many cases of amoebic dysentery 

 resulted from this mix-up. 



There are different strains of Endamoeba histolytica which vary in their 

 effects on the body. There are some which are so mild that many persons 

 are infected without knowing it. About 5 to 10 per cent of the people of 

 the United States carry this organism. In tropical countries the percentage 

 of infection is much higher, and serious symptoms may follow an infec- 

 tion. Our soldiers who were held prisoners in Korea were almost 100 

 per cent infected due to the poor sanitary conditions under which they were 

 forced to live, and many died of the disease. 



Any organism that causes a disease is known as pathogenic. Enda- 

 moeba histolytica, therefore, is a pathogenic protozoan. We will study sev- 

 eral of these in this phylum, and it will help the student to be sure that he 

 knows the name of each one studied, the disease it causes, the symptoms of 

 the disease, and the means of spread from one person to another. 



There is a third species of endamoeba that we will mention before we 

 close our discussion of this genus. This is Endamoeba gingavalis which 

 is found in the mouths of as many as 70 per cent of the population. They 

 are especially abundant around the base of the teeth where they join the 

 gums. If you scrape some of the material from this region of your own 

 mouth and stir it up with a 1 per cent salt solution and keep the slide warm 

 while studying it under the microscope, you are very likely to find that you 

 are harboring this organism in your own mouth. It is more sluggish than 

 the free-living amoebae, but you can see them throw out broad pseudopodia. 

 Human customs permit this organism to spread from mouth to mouth in 

 the vegetative condition and it is not known to produce cysts. They are 

 not generally pathogenic, but there is some evidence that they may play 

 some part in the damage done by the disease of pyorrhea. For the average 

 person with a healthy mouth, however, there is no need to worry about the 

 presence of these organisms and, human nature being what it is, there is 

 little likelihood that we shall ever be able to control their spread. 



