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Amoebiasis

 It is an infection of the large intestine, caused by a protozoan parasite called Entamoeba histolytica, often spreading to various other organs like liver, lungs, brain, etc. It is usually transmitted by fecal contaminated water. Clinically it is characterized by pain abdomen, diarrhoea and even fulminant dysentery. Magnitude: It is an endemic disease in all the developing countries including India, because of poor environmental sanitation. It is the third leading cause of parasitic deaths in the world, next to malaria and schistosomiasis. It is estimated that about 12 percent of the world’s population (500 million) are infected with an annual mortality of about 70,000 people.

Agent Factors of Amoebiasis

Agent

The causative agent is a protozoal parasite, Entamoebahistolytica. This exists in two forms—vegetative (Trophozoite) and cystic forms. Trophozoites dwell in the lumen and wall of the colon, where they multiply and encyst. It divides by binary fission, and grows best in anaerobic conditions. The size of the trophozoite varies from 10 to 50µ in diameter. They have directional mobility with pseudopodia. In the absence of diarrhoea, the trophozoites usually encyst before leaving the gut. The cyst is a round or oval structure of about 10 to 15 µ in size. Immature cysts have only one nucleus and the mature cysts have four nuclei. The cysts are excreted in the stool. They survive for variable period outside the host. Cysts constitute the infective stage of the parasite and are responsible for transmission of the disease. 

The ingested cysts release trophozoites, which colonize the large intestine. Some trophozoites invade the bowel and cause ulceration mainly in the cecum and ascending colon and often in sigmoid colon and rectum. Some may enter a vein and reach liver, lungs, brain, etc. 

The trophozoites live for a short period outside the body. But they are not the infective forms. The quadrinucleate cysts are infective forms, they live for several days in water, sewage and soil in the presence of moisture and low temperature. The cysts are not affected by chlorine in the routine concentration used for purification of water. However, they are readily killed by heat at 55°C. 

POLYCYSTIC OVARY SYNDROME: Causes, Risk Factors, Symptoms, Diagnosis, Management

Reservoir

Humans are the only reservoir of infection and the susceptibility is universal. Healthy and convalescent carrier state occurs. 

Source of Infection of Amoebiasis

Source of infection is the feces containing the quadrinucleate cysts.

Infective Material of Amoebiasis

Infective material is the food and water contaminated with feces containing the cysts. 

Period of Communicability of Amoebiasis

Varies from several days or months to several years. 

Age and Sex Incidence of Amoebiasis

No age and sex is bar from the disease. 

Environmental Factors of Amoebiasis

The various factors responsible for the prevalence of the disease are poor sanitation, poor socioeconomic status, overcrowding, lack of protected water supply, irrigation of agricultural fields by sewage, indiscriminate defecation, etc. Hence amoebiasis is often called a ‘social disease.’

Mode of Transmission of Amoebiasis

The disease is transmitted mainly by feco-oral route, i.e. through the faecal contamination of food and water. Food handlers, who are carriers, also constitute an important source. House flies act as mechanical carriers (Vectors). Sexual transmission has also been reported among homosexuals

Pathology and Pathogenesis of Amoebiasis

The primary lesions is limited entirely to the large intestine and the secondary or metastatic lesions occur in the liver, lungs and brain. 

Intestine

The parasites multiply rapidly, destroy the tissue and even submucous membrane, resulting in necrosis, abscess and ulcers. 

Liver

The trophozoites are carried from the base of the amoebic ulcer, through the portal vein, to the liver, where they multiply, result in enlargement of liver (amoebic hepatitis) and later results in necrosis and amoebic liver abscess. Usually, abscess is located in the postero-superior surface of the right lobe of the liver. 

Lungs

Even though rare, lungs are also affected which the trophozoites from the gut wall, pass through pulmonary capillaries via portal vein, resulting in lung-abscess. Lungs can also be affected as a complication of liver abscess. Thus, pulmonary amoebiasis may be primary or secondary. 

Brain

This is very rare. Cerebral amoebiasis can occur as a complication of hepatic abscess or pulmonary abscess. 

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Incubation Period of Amoebiasis

The incubation period is about 1 week to 3 weeks. 

Clinical Features  of Amoebiasis

Intestinal Amoebiasis

Starts gradually with mild abdominal discomfort, pain, diarrhea, with or without blood and mucus, usually associated with tenesmus. Fever may be present. Abdomen is tender, liver is slightly enlarged and tender. In fulminant colitis, all these features are sudden and severe. 

Amoebic Liver Abscess

Insidious in onset. Pain and tenderness in the right hypochondrium. Pain is often referred to right shoulder due to irritation of phrenic nerve. It may be to left shoulder in a left lobe involvement. Fever is of high grade, associated with nausea, anorexia, and vomiting. Jaundice is unusual. Usually there is single abscess. It may rupture in the peritoneum, pleural cavity or the pericardial cavity. On aspiration, the fluid is odorless, typically described as chocolate syrup/anchovy sauce. 

Investigations for Amoebiasis

Examination of Stool 

Macroscopic (Naked eye):An offensive, dark brown semifluid, stool, mixed with blood and mucus indicates amoebic dysentery. 

Microscopic examination: Fresh sample of stool should be examined. Three types of mounts are prepared. 

1. With normal saline: Amebic trophozoites are easily recognized by their characteristic movement. 

2. With iodine plus saline: This helps to distinguish from other parasites. 

3. With methylene blue: To stain leukocytes and  To differentiate from amebae. 

(Thus a stained preparation is rarely called for). 

Examination of the Blood

Shows moderate leukocytosis.

Serological Tests

Serological tests are often negative. If positive, gives a clue about extraintestinal amoebiasis. 

Liver Aspirate

‘Pus’ is not of suppuration but a mixture of sloughed liver tissue and blood. So thick in consistency and chocolate-coloured, bacteriologically sterile. Trophozoites are not generally found or sparse but can be demonstrated only in the material from wall of the abscess, i.e. about 4 to 5 days after the evacuation of the abscess. 

Treatment 

• Luminal amebicides: Act only on the parasites of the intestinal lumen, e.g. diloxanide furoate (500 mg tid × 10 days), iodoquinol and paramomycin. 

• Tissue amebicides: They are effective in the treatment of invasive amebiasis, e.g. metronidazole, tinidazole, secnidazole, followed by diloxanide furoate. The same drugs have to be used after draining the liver abscess.

Prevention and Control of Amoebiasis

Elimination of Reservoirs 

• By effective treatment of cases and carriers. 

• Carriers working in food and water establishments should abstain from working till they are cured. They are also given education about the periodontal examination and maintenance of high standard of personal hygiene. 

Breaking the Channel of Transmission 

Since the disease is transmitted by feco-oral route, it is broken by construction of sanitation barrier (sanitary latrine and motivating the people to use it) supplemented by provision of protected water supply. Cysts are not killed by chlorine in the concentration used for disinfection of water. Sand-filters are quite effective in removing amebic cysts. Therefore water filtration and boiling are more effective than chlorination of water. 

Food hygienic measures include disinfection of fruits and vegetables with 5 to 10 percent acqueous solution of acetic acid or full strength of vinegar. 

House flies to be controlled by keeping the premises clean in and around the houses. 

Protection of Susceptibles

• By health education: The people are made water conscious, latrine conscious and health conscious.

• At present, no vaccine is available. Studies have shown that purified antigens (amebic proteins) are the viable candidates for vaccine development.