THE AMOEBAE 



155 



environmental conditions, bursting in 

 water and becoming degenerate in older 

 fecal samples. In order to identify them, 

 smears of fresh feces should be fixed in 

 Schaudinn's fluid containing 10 to 20% 

 acetic acid or in Bouin's fluid and stained 

 with iron hematoxylin. Their morphology 

 has been studied by Wenrich (1936, 1939, 

 1944) and Dobell (1940), whose accounts 

 do not always agree. 



The trophozoites range from 3 to 22 jj. 

 but are usually 6 to 12 jn in diameter. 

 The ectoplasm is distinct from the endo- 

 plasm, which contains food vacuoles filled 

 with bacteria, yeasts, starch granules, 

 and parts of cells. In fresh feces there 

 may be a single clear, broad pseudopod. 

 About 3/5 of the protozoa contain 2 nuclei 

 which are connected by a filament or des- 

 mose. This appears to be one of the first 

 structures to disappear during degenera- 

 tion. Each nucleus is vesicular and has 

 an endosome composed of 4 to 8 granules 

 from which a few delicate fibers radiate 

 to the nuclear membrane. There is no 

 peripheral chromatin. 



Reproduction is by binary fission. 

 There are 4 chromosomes. 



At one time D. fragilis was thought 

 to be non- pathogenic, and this is true in 

 most cases. However, in some persons 

 it causes a mucous diarrhea and gastro- 

 intestinal symptoms. It does not invade 

 the tissues, but may cause low-grade ir- 

 ritation of the intestinal mucosa, excess 

 mucus secretion and hypermotility of the 

 bowel. There may be mild to moderate 

 abdominal pain and tenderness or dis- 

 comfort. There may also be an increase 

 in eosinophiles. 



The mode of transmission of D. fra- 

 gilis is not clear, since there are no cysts 

 and the trophozoites are so delicate. 

 Dobell (1940) was unable to infect himself 

 by mouth or 2 monkeys by mouth or rec- 

 tally and suggested, by analogy with His- 

 tomonas, that D. fragilis might possibly 

 be transmitted by an intestinal nematode 

 such as Trichuris. This idea has been 

 partially confirmed by Burrows and 

 Swerdlow (1956), who found small, amoe- 



boid organisms resembling D. fragilis in 

 the eggs of Enterobius vermicularis and 

 suggested that the pinworm might be the 

 vector. 



D. fragilis can be readily cultivated 

 in the usual culture media. It is sensitive 

 to most amoebicidal drugs, including car- 

 barsone, diodoquin and erythromycin. 



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