THE AMOEBAE 



139 



Pathogenesis : As mentioned above, 

 only the large forms of E. histolytica are 

 generally considered pathogenic, altho 

 there are reports of mild disease and 

 slight lesions associated with the small 

 form (Shaffer et al.^ 1958). They may 

 cause diarrhea or dysentery, and may 

 invade the wall of the cecum and colon, 

 forming ragged, undermining or flask- 

 shaped ulcers which may be pinpoint in 

 size or may become large and confluent. 

 The amoebae invade the mucosa at first 

 and multiply to form small colonies. These 

 colonies then extend into the submucosa and 

 even into the muscularis. In the absence of 

 bacterial invasion, there is little tissue re- 

 action, but in complicated infections there 

 is hyperemia, inflammation and infiltra- 

 tion with neutrophiles. 



Some of the amoebae may pass into 

 the lymphatics or even the mesenteric 

 venules. Those entering the hepatic portal 

 system pass to the liver, where they may 

 cause abscesses. Those which enter the 

 lymph ducts are generally filtered out by 

 the lymph nodes. Abscesses may be 

 formed in various other organs, including 

 the lungs, brain, etc. , depending on the 

 host's resistance. 



The relation of parasite strain to 

 pathogenicity has already been mentioned. 

 The species of concomitant bacteria pres- 

 ent may also affect the amoeba's pathoge- 

 nicity, as may the nutritional status of the 

 host and other environmental factors. 

 Dysentery is much more common in the 

 tropics than in the temperate zone. 



In most cases, E. histolytica causes 

 minor symptoms or none at all. Infections 

 may last 40 years or even more. There 

 may be recurring mild to severe gastro- 

 intestinal symptoms, including intermit- 

 tent diarrhea, bowel irregularity, abdom- 

 inal pain, nausea and flatulence. Some- 

 times affected persons tire easily, have 

 headaches or feel nervous. Appendicitis 

 or symptoms resembling it may occur. 

 These symptoms generally clear up after 

 treatment. 



In acute amoebic dysentery, the feces 

 consist almost entirely of blood and mucus 



filled with amoebae and blood cells. The 

 patient is wracked by waves of severe 

 abdominal pain and spends a large part of 

 his time on the stool, straining and passing 

 blood and mucus every few minutes. In 

 contrast to bacillary dysentery, there is no 

 fever in uncomplicated cases. 



Epidemiology : As mentioned above, 

 E. histolytica is primarily a parasite of 

 primates, and man is the reservoir of in- 

 fection for his domestic animals. This is 

 one of the few zoonoses which man gives 

 to his associated animals in return for the 

 many which he receives from them. 



Infection is due to ingestion of cysts. 

 Since trophozoites alone are passed by 

 dysenteric individuals, these are not im- 

 portant sources of infection, while cyst- 

 producing chronic cases and carriers are. 



The cysts are relatively resistant. 

 They are not affected by water chlorina- 

 tion, but can be removed by sand filtration. 

 They survive for at least 8 days in soil at 

 28 to 34° C (Beaver and Deschamps, 1949), 

 but live only an hour at 46 to 47° C and 

 less than a minute at 52° C (Jones and 

 Newton, 1950). They survive longest at 

 refrigerator temperatures (40 days at 2 to 

 6° C according to Simitch, Petrovitch and 

 Chibalitch, 1954; 62. 5 days at 0° C accord- 

 ing to Chang, 1955). They will not excyst 

 after 24 hours at temperatures of -15° C 

 or lower (Halpern and Dolkart, 1954), and 

 die in 7. 5 hours or less in the deep freezer 

 at -28° C (Chang, 1955). 



The cysts are usually transmitted with 

 food or water. Raw vegetables may be a 

 source of infection. Flies may transmit 

 the cysts also. Pipkin (1949) was able to 

 cultivate cysts from the vomitus of filth 

 flies {Musca domestica, Lucilia pallescens, 

 Cochliomyia macellaria, Phormia regina 

 and Sarcophaga ttiisera) 39 to 64 minutes 

 after ingestion and from their feces 172 to 

 254 minutes after ingestion. 



Faulty plumbing and water systems 

 may cause water-borne transmission. The 

 most striking case of this kind occurred 

 during the Chicago World Fair in 1933. 

 An outbreak of amoebic dysentery occurred 



