668 INTESTINAL PROTOZOA AND HOST-PARASITE RELATIONS 



hence be instrumental in the infection of new hosts. Other animals do not seem to be 

 as important as flies as transmitters of amel^a cysts but rats, and possibly other 

 domestic animals, may be infected with human amebae. Cysts are not produced in 

 infected cats and dogs, hence these animals cannot be responsible for the transmis- 

 sion of the amebae. It has been noted that infections with Endamoeba histolytica 

 occur frequently in all of the members of certain families indicating that contamina- 

 tion by association is frequent. It may be pointed out also that transmission is due 

 entirely to the behavior of the host and not to any activities on the part of the para- ^ 

 site. It is man's own carelessness that brings about contamination of food or drink « 

 with ameba cysts and transmission can, therefore, be prevented by control measures 

 based on personal hygiene and sanitation. 



Parasitological and clinical periods. — It is difficult to determine the prepatent 

 period in infections with Endamoeba histolytica since the time when cysts are ingested 

 is usually unknown. Walker and Sellards (1913), however, have provided definite 

 data on this point. These investigators succeeded in infecting eighteen of twenty men 

 who were fed gelatin capsules containing cysts. Many of these men began passing 

 cysts in four or five days, a period which represents the prepatent period. Persons 

 once infected with Endamoeba histolytica apparently remain infected for years and 

 perhaps throughout the rest of their lives, hence the patent period is of long duration. 

 The incubation period as determined by Walker and Sellards ranged from twenty to 

 ninety-five days. 



Distribution and localization in the host. — The cysts of Endamoeba histolytica, 

 when swallowed by the host, are passively carried through the small intestine and 

 into the large intestine which is their definitive habitat and the primary site of infec- 

 tion. The cysts are supposed to excyst in the small intestine, but recent investiga- 

 tions prove that they are capable of excysting in the large intestine and may actually 

 do so in normal infections. The process of excystation has recently been observed by 

 Yorke and Adams (1926) in vitro. Apparently the only factors necessary to bring 

 about excystation are moisture and a temperature of about 37° C. for several hours. 

 Studies of excysted specimens in cultures indicate that the single cjuadrinucleate 

 ameba that escapes from the cyst wall divides into four uninucleate amebulae. 



The primary site of infection. — Up to the year 1925 it was supposed that the 

 trophozoites of Endamoeba histolytica were unable to live without access to tissue 

 elements, but we now know that this species is able to live and reproduce on a diet 

 consisting largely of bacteria. It is, therefore, possible for the amebae to live in the 

 lumen of the large intestine without injuring the tissues of the host. In many cases, ■ 

 however, the amebae attack the intestinal wall. They are supposed to secrete pro- 

 teolytic enzymes that dissolve the tissues and in course of time bring about the pro- 

 duction of ulcers. Studies of autopsy material indicate that these ulcers are most 

 abundant in the dependent portions of the large intestine where movement of the 

 contents is sluggish. 



Secondary sites of infection. — When intestinal ulcers are formed many amebae find 

 their way into the capillaries and are carried probal)ly to all parts of the body, where 

 infections may be set up under favorable conditions. The principal secondary site of 

 infection is the liver in which liver abscesses may be produced. Amebic abscesses 



