822 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



DISEASES CAUSED BY PROTOZOA* 

 Rhizopoda (von Seibold) 



The amoebae are the most important of the parasites belonging to the rhizopods. 

 Various species of amoeba are parasitic in the intestines of cattle, horses, mice, frogs 

 and fish as well as human beings and most of them, like Entamceba coli of man, are 

 harmless. One species, Entamoeba histolytica {tetragena), produces a very severe dis- 

 ease of man. Entamaba mdeagridis is the cause of a fatal disease of turkeys 

 (page 824). Entamoeba gingivaiis {buccalis) is a parasite which is frequently found 

 in a diseased condition of the gums characterized by peridental abscesses but is 

 also frequent in apparently healthy mouths. The parasitic species lack the con- 

 tractile vacuole which is a feature of the free living species that are commonly 

 encountered so that it is not difficult to distinguish the two types. 



Amosbic Dysentery 



Entamceba histolytica — Schaudinn, 1903 

 Syn.: EtUamceba tetragena — ^Viereck, 1907 



Distribution. — Amoebic dysentery occurs most frequently in tropical, 

 or sub-tropical, countries, but cases of it occasionally occur in Great 

 Britain, in Central Europe, and in the United States. 



Intestinal amoebce; Entamoeba coWL^ch (Fig. 172) and Entamoeba histolytica {tetra- 

 gena) are both parasites in the human intestine. They measure from rs/x to 30/1 in 

 diameter and, when examined in freshly passed fseces, may be seen in active 

 motion. Their cytoplasm contains a nucleus, vacuoles, and food particles. Both 

 may multiply by binary and by multiple division; the appearance of certain of the 

 encysted forms in both species indicates a process of autogamy. Both of the 

 parasitic amoeba: of the human intestine produce a characteristic number 

 of daughter amoebae in the course of their multiplication. E. coli com- 

 monly divides into eight small amoebae so that these organisms may present 

 any number of nuclei below this number and occasionally they contain several 

 more. The encysted forms of this species also divide into approximately eight 

 small amoebse. In E. histolytica the number produced as the result of division 

 is more regular being almost invariably four whether in the division of the 

 motile trophozoite or the encysted forms. The character of the division 

 thus furnishes the most certain criterion in differentiating the two species. 

 Multiplying forms are not always readily found, however, and it is necessary to 

 take other characteristics into consideration. The non-pathogenic species (£. coli) 

 is more sluggish in its movements, is generally larger, dull grayish in appearance, and 

 with no sharp differentiation into ectoplasm and endoplasm. E. histolytica is 

 active, of a greenish hue and the ectoplasm is well defined and very clear in portions 

 extruded as pseudopods. The nucleus in the harmless species, commonly centrally 

 situated, is larger and shows a larger amount of chromatin. The nucleus of the 



* Diseases arranged generically. 



* Prepared by J. L. Todd and revised by E. E. Tyazer. 



