Entamoeba Histolytica

Synonyms: Amoeba coli, Amoeba dysenteriae, Entamoeba tetragena, Entamoeba dispar, Entamoeba venaticum.

Disease: Amoebic dysentery.

Hosts: Man, orang-utan, gorilla, chimpanzee, gibbon, many species of macaques, baboons, spider monkeys and other monkeys, dog, cat, pig, rat, possibly cattle. The rat, mouse, guinea pig and rabbit are often infected experimentally.

Location: Large intestine, sometimes liver, occasionally lungs, and rarely other organs including the brain, spleen, etc.

Geographic Distribution: Worldwide. Maps of the world distribution of amoebic dysentery and E. histolytica, together with climatological and other information, were published by Piekarski and Westphal (1952) and Westphal (1955).

Prevalence: E. histolytica is most important as a parasite of man. It also occurs in monkeys and higher primates. According to Belding (1952), it was found in an average of 17.6% of 42,713 persons (range, 0.8 to 50%) in 37 surveys thruout the world from 1941 to 1948. In 10 surveys of 10,867 persons in the United States from 1941 to 1948, it was found in an average of 13.6% (range, 0.8 to 38%).

According to Benson, Fremming and Young (1955), it has given considerable trouble in their chimpanzee colony at the Univ. of Texas. Sporadic cases of amoebic dysentery have been reported in dogs; these animals are generally considered to have acquired their infections from human contacts. Kartulis (1891, 1913) found E. histolytica causing dysentery in 3 dogs in Egypt; in one of these, a liver abscess was also present. Darling (1915) reported a fatal infection in a dog in Panama. Ware (1916) reported an outbreak in a pack of foxhounds in the Nilgiri Hills of India, Boyd (1931) reported an outbreak in another pack of hounds in India, and more recently Ganapathy and Alwar (1957) reported 2 cases of amoebic dysentery in dogs in India. Fischer (1918) reported a case of amoebic dysentery in a dog in China, Bauche and Motais (1920) reported one in Indochina, and Morcos (1936) found 5 cases in Egypt. In the United States, Faust (1930) found 2 dogs in New Orleans with amoebic dysentery, Andrews (1932) found E. histolytica in the feces of a diarrheic dog in Baltimore, and Thorson, Seibold and Bailey (1956) reported a case of systemic amoebosis in a puppy which also had distemper. E. histolytica was found in large numbers in the lungs and amoebae were also seen in the liver, kidneys and spleen.

Species of Entamoeba

In surveys of presumably normal dogs, Kubo (1936) found E. histolytica in 8% of 85 street dogs in Mukden, China, while Yamane (1938) found it in 3% of 60 street dogs from the same city. Chary et al. (1954) stated that amoebic dysentery occurs frequently in dogs in Indochina. Eyles et al. (1954) found E. histolytica in 8.4% of 143 dogs in the Memphis, Tennessee dog pound. The protozoa were so scarce that cultural methods were required to reveal them. This finding suggests that amoebic dysentery may be more common in dogs than is generally believed.

Natural E. histolytica infections are apparently rare in cats, but Kessel (1928) found the protozoon in 3 of 150 kittens in China. E. histolytica is rare in swine. Frye and Meleney (1932) found it in 1 of 127 pigs which they examined in Tennessee; this animal came from a farm where an infected woman lived.

There are 2 reports of what may have been E. histolytica in cattle. Walkiers (1929) saw it in the feces of dysenteric cattle in the Belgian Congo. Thiery and Morel (1956) found it in the lungs of a young zebu in Dakar which was slaughtered on account of generalized streptothricosis.

Natural infections in rats have been reported by a number of workers. Chiang (1925) found E. histolytica in 7 laboratory rats. Brug (1919) found it in 2 of 50 wild rats in Batavia, Nagahana (1934) found it in 3 of 274 wild rats in Mukden, China, and Epshtein and Avakian (1937) found it in 6 of 515 wild Raltus norvegicus in Moscow. In the United States, Lynch (1915) saw it in a wild rat, Tsuchiya and Rector (1936) found it in 2 of 100 wild rats in St. Louis, and Andrews and White (1936) found it in 28 (1.1%) of 2515 wild rats in Baltimore.

Morphology: The trophozoites of the large, pathogenic race of E. histolytica are 20 to 30 u and those of the small race are 12 to 15 u in diameter. They have a thick, clear layer of ectoplasm and granular endoplasm. They move rapidly when warm, usually moving forward in a straight line with a single clear pseudopod at the anterior end. When the feces have cooled, the amoebae stay in one place and throw out large, clear pseudopods from various parts of their body. The trophozoites often ingest erythrocytes, a feature which differentiates them from those of other amoebae. The nucleus is indistinct in living amoebae. When stained with hematoxylin, it has a small, central endosome, a ring of small peripheral granules and a few scattered chromatin granules in between. The cysts of both the large and small races are 10 to 20 u (average, 12 u) in diameter. They have 4 nuclei when mature and often contain rod-like chromatoid bodies with rounded ends. Diffuse glycogen is present in the young cysts.

Life cycle: E. histolytica multiplies in the trophozoite stage by binary fission. It has 6 chromosomes. Before encysting, the amoebae round up, became smaller and eliminate their food vacuoles. They lay down a cyst wall, and the nucleus divides into 2 and then into 4 small nuclei. After the 4-nucleate amoebae emerge from the cyst, both the nuclei and cytoplasm divide so that 8 small amoebulae result. Each then grows into a normal trophozoite.

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. 9 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 muscular is. In the absence of bacterial invasion, there is little tissue reaction, but in complicated infections there is hyperemia, inflammation and infiltration 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 present may also affect the amoeba's pathogenicity, 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 gastrointestinal symptoms, including intermittent diarrhea, bowel irregularity, abdominal pain, nausea and flatulence. Sometimes 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 infection 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 important sources of infection, while cyst-producing chronic cases and carriers are.

The cysts are relatively resistant. They are not affected by water chlorination, but can be removed by sand filtration. They survive for at least 8 days in soil at 28 to 34° C (Beaver and Beschamps, 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 according 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 misera) 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 among guests at two neighboring hotels from which over 1000 cases with 58 deaths were tracked down in 44 states and 3 Canadian provinces (Bundesen et al., 1936). Cross connections between the water and sewage pipes, back siphonage from toilet bowls into the water supply and leakage from an overhead sewage pipe in the kitchen were involved.

Food handlers may play an important role in transmission of amoebae, even tho the cysts rarely survive more than 10 minutes on the hands, except under the fingernails (Spector and Buky, 1934). Thus, Schoenleber (1940, 1941) reported that in a group of Americans living in a Standard Oil Co. camp in Venezuela, the prevalence of amoebic infection was reduced in 3 years from 25.6% to 1.9% and the amoebic dysentery rate from 36.84 to 0.61 per 1000 per year by inspection and treatment of food handlers. Winfield and Chin (1939), in a comparison of the prevalence of amoebic infection with food habits in different parts of China, concluded that transmission by food handlers is probably more important than by other means in that country. E. histolytica is much commoner in North China than in South and Central China. This is correlated with the serving and eating of cold bread with the hands in North China as contrasted to the handling of hot rice with a serving spoon and chopsticks in South and Central China. On the other hand, Sapero and Johnson (1939, 1939a) found no evidence that carriers were important in the transmission of amoebae in a study of 919 persons in the U.S. Navy. The sanitary habits of American sailors probably had something to do with this.

Diagnosis: The laboratory diagnosis of amoebiasis has been discussed in detail by Brooke (1958). Live amoebae can be found in wet smears made with physiological salt solution. These smears may be stained with Lugol's iodine solution diluted 1:5 to bring out the nuclei of the cysts and stain glycogen. However, for accurate identification and differentiation from other species of amoebae, staining with hematoxylin is essential. The smears are generally fixed in Schaudinn's fluid and stained with Heidenhain's iron hematoxylin. Sapero and Lawless's (1953) MIF (merthiolate-iodine-formaldehyde) stain-preservation technic can also be used.

For concentration of cysts, flotation in zinc sulfate solution (Faust et al., 1938) can be used. The cysts are distorted beyond recognition, however, by the other salt and sugar solutions in common use for flotation of helminth eggs. For concentration by sedimentation, the FTE (formalin-triton-ether) sedimentation technic (Ritchie, Pan and Hunter, 1952, 1953) or MIFC (merthiolate-iodine-formaldehyde-concentration) technic (Blagg et al., 1955) can be used.

Cultivation can be helpful in diagnosis of amoebiasis, but only if fresh specimens are used and if the laboratory personnel are expert. Cultivation is not recommended for general use.

E. histolytica cannot be differentiated morphologically from E. hartmanni, and its differentiation from other intestinal amoebae, and especially from E. coli, is not an easy task. There is a surprising amount of discrepancy even among those who should be qualified. Thus, in an evaluation by the USPHS Communicable Disease Center of the diagnostic ability of 42 state health department laboratories (Brooke and Hogan, 1952), an average of 4.1 out of 18 E. histolytica infections was missed among 98 stool samples sent to the laboratories for examination, and an average of 4.4 false positive reports was made among the 80 negative samples. Furthermore, in an analysis of responses by members of the American Society of Tropical Medicine to a questionnaire on the clinical and laboratory diagnosis of amoebiasis, Brooke et al. (1953) found a surprising lack of agreement in statements concerned with the identification of E. histolytica cysts and trophozoites.

Goldman (1959, 1960) was able to differentiate between Entamoeba histolytica, E. hartmanni, E. moshkovskii and E. coli by a fluorescence antibody technic. Three originally invasive strains of E. histolytica which he studied differed significantly from a non-invasive strain.

Cultivation: E. histolytica was first cultivated by Boeck and Drbohlav (1925). Their medium was composed essentially of a coagulated egg slant overlaid with Locke's solution containing serum. Various modifications of this medium are still in use. Cleveland and Collier (1930) used a liver infusion agar slant overlaid with serum and physiological salt solution. Balamuth (1946) introduced an all-liquid egg infusion-liver extract medium.

Treatment: Amoebiasis can be treated with a number of drugs (cf. Balamuth and Thompson, 1955). The old standard drug, emetine, is not used as much as formerly because of its toxicity. Other drugs from which one can choose include (1) the arsanilic acid derivatives, carbarsone, glycobiarsol (bismuth glycoarsanilate, Milibis) and thiocarbarsone; (2) the iodoquinoline derivatives, diodohydroxyquin (Diodoquin), chiniofon (Yatren) and iodochlor hydroxy quin (Vioform); (3) the antimalarial drug, chloroquine; and (4) the antibiotics, erythromycin, fumagillin, tetracycline, chlortetracycline and oxytetracycline.

The particular drug selected depends in part on the type of amoebic infection involved. For acute or subacute dysentery, erythromycin, oxytetracycline or chlortetracycline may be used. Erythromycin is administered to man by mouth at the rate of 15 mg/kg daily in divided doses for 14 days. The usual human course of treatment with oxytetracycline or chlor-tetracycline is 0.5 g 4 times a day by mouth for 10 days.

For chronic cases or to eradicate intestinal infections, one of the arsanilic acid or iodoquinoline derivatives may be used. The usual adult human course of treatment with carbarsone is 0.25 g 2 or 2 times a day by mouth for 10 days. That with glycobiarsol is 0.5 g 3 times a day by mouth for 8 days. That with chiniofon is 1.0 g 3 times a day by mouth for 7 days. That with diodohydroxyquin is 0.65 g 3 times a day by mouth for 20 days.

For amoebic hepatitis or liver abscesses, chloroquine is used. A loading dose of 1 g chloroquine phosphate (0.6 g base) by mouth on each of 2 successive days followed by 0.5 g daily for 2 to 3 weeks is the recommended adult human course of treatment. Diodohydroxyquin has also been recommended as a prophylactic drug for use by travellers in areas of high endemicity.

While relatively little work has been done on the treatment of amoebiasis in domestic animals, the same drugs are in general effective in them. Benson, Fremming and Young (1955) found that for chimpanzees the most successful drugs were carbarsone (0.25 g twice daily for 10 days) and fumagillin (20 to 30 mg twice daily for 10 days) administered in fruit or fruit juice. They also frequently gave a course of emetine hydrochloride (1 mg/kg body weight up to a maximum of 60 mg, injected intramuscularly daily for a maximum of 6 days) prior to carbarsone or fumagillin therapy (see also Fremming et al., 1955). Herman and Schroeder (1939) successfully treated amoebic diarrhea in a 21-lb. orang-utan with carbarsone. They gave 2 courses of treatment 9 days apart, each course consisting of 0.05 g carbarsone in milk or a slice of banana 3 times a day for a week.

Prevention and Control: Infection with amoebae can be prevented by sanitation. Water supply systems should be built without cross connections to sewage systems. Water which may be polluted should be boiled or filtered thru sand, since ordinary chlorination does not kill the cysts. Food handlers should wash their hands thoroughly after using the toilet. Vegetables grown on polluted ground should be cooked, or, if they are to be eaten raw, should be scalded or soaked in vinegar containing 5% acetic acid for 15 minutes at 30° C or in vinegar containing 2.5% acetic acid for 5 minutes at 45° C (Beaver and Oeschamps, 1949). Diodohydroxyquin may also be used prophylactically.