AMEBIASIS

Amoebiasis

Amoebae refer to several organisms that belong to the subphylum Sarcodina. These organisms move by cytoplasmic extensions known as pseudopodia. Many species of Amoeba are free-living organisms and a few are parasites of the digestive tracts of vertebrates and invertebrates 
The amoebae vary considerably in their biology. Entamoeba histolytica infects primates; E. invadens is a parasite of reptiles, while E. coli is a harmless species that is found in the colon of man, monkeys and dogs. E. hartmani is a nonvirulent strain that is easily mistaken for E. histolytica.

Those parasitic to man include Entamoeba histolytica, E. hartmani, E. coli, E. gingivalis, Endolimax nana, and Iodomoeba butschlii. E. histolytica is very pathogenic while the rest of the species are non-pathogenic or harmless.

E. Gingivalis inhabits the crevices between the teeth and feeds on bacteria, particles of food and dead epithelial cells. It is transmitted orally by kissing. Other Amoebae are inhabitants of the caecum and the large intestine.

E. coli is the largest intestinal amoeba of man. It feeds on bacteria that abound in the colon and forms eight nuclei, as opposed to the four nuclei of E. histolytica.

E. histolytica

E. histolytica is an anaerobe that lacks a mitochondrion and obtains its energy by glycolysis. It lives in the lower small intestine and the entire colon. The trophozoite stage is motile and measures 12 to 30 µm in diameter. Sometimes larger trophozoites are found in dysenteric faeces. The parasite’s cytoplasm consists of a clear ectoplasm and granular endoplasm. The vacuoles in the endoplasm are filled with ingested red blood cells that are being digested. E. histolytica naturally feeds on the host’s red blood cells and bacteria fauna present in the colon.

E. histolytica is widely distributed both in the temperate and in the tropical regions of the world. It is, however, more prevalent in the tropics where the prevalence in some communities can be as high as 100%.

Life cycle

Cysts of E. histolytica are passed in faeces. Soon after the faeces are voided, the cyst nucleus divides into two. Then each of the two daughter nuclei divides again into two so that the mature cyst has four nuclei. Cysts are susceptible to environmental conditions and are killed by drying, heat, and sunlight. Cysts formed from trophozoites, measure 5 – 20 µm, and usually have four nuclei

Primary amoebiasis

Infection is contracted through the ingestion of cysts in food or water. On reaching the intestine, the cysts divide into active trophozoites. The trophozoite is the feeding stage and it is amoeboid, using pseudopodia for movement and feeding on bacteria and cell debris.

Aided by hydrolytic enzymes, the trophozoites invade the mucosa of the large intestine and proceed to erode the surface of the muscularis mucosae. The characteristic initial lesion caused by the invasive trophozoites is a superficial minute cavity caused by necrosis of the mucosal surface. This lesion enlarges as the amoebae reach the more resistant muscularis mucosae. The parasites may erode a passage through the muscularis mucosae into the submucosa and spread into the surrounding tissues. This invasive stage affects not only the intestinal wall but also the local blood and lymphatic vessels.

Once inside the intestinal tissue, the trophozoites feed on cell debris and whole red blood cells. As the trophozoites feed, they become larger and divide by mitosis, thereby increasing their numbers enormously. In severe cases, the intestinal epithelium is badly damaged, resulting in open wounds. The ulcerated tissue is subject to infection by other pathogens, such as bacteria. A seriously damaged intestinal mucosa leads to amoebic dysentery with discharge of blood, mucus and amoebae into the intestinal lumen.

Repair of the ulcerated bowel lining eventually occurs, but the flexible, absorptive mucosa is often replaced with fibrous scar tissue. Sometimes, this tissue partially constricts the intestine, blocking peristaltic movements of the bowel and interfering with its normal function.

Secondary amoebiasis

Secondary amoebiasis is due to transportation of amoebae via circulation from a primary abscess in the intestine to other tissues. The liver, lungs, and brain develop amoebic abscesses in the given order of frequency. A liver abscess consists of a hollow eroded region that contains a viscous fluid, and mass of dead amoebae, plus blood and tissue detritus. Around the necrotic centre of the abscess, the liver tissue is full of amoebae, which actively invade healthy tissue as they multiply. No fibrous envelope forms around such an abscess and it spreads steadily with age. Amoebic abscesses are usually sterile or bacteria-free.

Lung abscesses develop directly from liver abscesses through the spread of the latter across the diaphragm. Brain abscesses result from amoebae that have lodged and multiplied in the brain. Brain abscesses are less common than lung or liver abscesses. Other sites of amoebic infection have been reported.

Abscesses contain a large number of leucocytes, which have engulfed amoebae, and systemic or secondary amoebiasis usually produces a raised leukocyte count. In some individuals and with certain races of amoebae this defence is so weak that abscesses form and grow in spite of leukocyte activity.

Certain bacteria seem necessary for amoebic virulence, even if the bacteria themselves are harmless. Thus, mutualistic relationships between amoebae and other inhabitants of the intestine are an important part of amoebic pathogenicity.

Symptoms

The infection has an incubation period of a few days to 3 months or more, depending on the strain of amoeba and the nutritional status of the host. In most cases, it is impossible to determine the time of exposure to infection and the appearance of symptoms. Initial symptoms may involve mild abdominal discomfort and passage of soft stools, which may persist for sometime before the patient is compelled to seek medical attention. In some cases, the onset may be sudden, accompanied by dysentery or severe abdominal pain.

The typical clinical symptoms of acute amoebic dysentery are marked colicky pains and severe bloodstained diarrhoea. The stool then contains blood and mucus and the individual feels the urge to open the bowels several times in a day. The disease, if not treated, is fatal.

A hepatic abscess is associated with fever, an enlarged and tender liver. Pulmonary amoebiasis may present with pneumonia and coughing.

Epidemiology

E. histolytica is found all over the world. It is, however, more prevalent and severe in the tropics than in the subtropics. The infection rates are generally high where sanitary conditions are poor such as in mental hospitals, children’s homes and prisons. Asymptomatic carriers of amoeba are common in endemic areas. It is not clear whether this is an indication of an acquired immunity or merely the presence of nonvirulent strains of the parasite.

Endemic amoebiasis may be interrupted by sudden outbreaks of major proportions, resulting from gross contamination of drinking water with viable cysts of amoeba. Besides water and food contaminated with amoebic cysts, another important mode of transmission is hand-to-hand contact, which is possible with people with unclean hands. The parasite may be carried mechanically by houseflies and cockroaches.

Large numbers of cysts may be discharged in the faeces of an individual. The cysts survive for a few weeks to a few months under moist conditions. Drying kills them. Although monkeys, pigs, dogs and cats are naturally infected with E. histolytica, there is no evidence that shows that they transmit the infection to humans.

Diagnosis

Active trophozoites can be detected by direct examination of the faeces and of biopsy material. Typical amoebic faeces contains exudes, mucus and blood. Formed faeces are of no diagnostic value in amoebiasis. It is possible to distinguish the cysts of E. histolytica from those of E. coli by the number of nuclei.

Control

To prevent contracting amoebiasis, drinking water should always be boiled. Water provided by municipalities is usually chlorinated. Boiling drinking water, even piped water in urban centres is important unless one is quite sure that it is safe to drink it unboiled.

Vegetables should not be eaten raw. Salads should be washed thoroughly before serving. Food handlers can transmit the infection if they do not maintain proper personal hygiene.

Food should be covered to prevent insects landing on it especially houseflies and cockroaches. Disposal of human excreta in toilets and maintenance of personal hygiene limit the spread of infection.

The drug of choice is metronidazole. Alcohol should be avoided while taking this drug. Other drugs include emetine hydrochloride and chloroquine.