Amoebiasis

Amoebic dysentery or amoebiasis is an infection of the colon that is present worldwide, but occurs most commonly in warm, moist climates. For people living in many parts of the world, the amoebic agent of dysentery lives and reproduces in the large intestine, and does not always cause disease. In most cases, peaceful coexistence reigns between the parasite and its host, or carrier. Occasionally, however, the amoeba penetrates deeper into the body, damaging tissues and causing amoebic disease or invasive amoebiasis.

The causative agent for amoebiasis, the amoeba Entamoeba histolytica, exists in two forms. One form is a motile, fragile organism; the other is a nonmotile, resistant cyst. The cysts are present in the infected human colon. The cysts are excreted in the faeces and are swallowed by a new host. In this transmission process the cyst wall is destroyed and new motile organisms are produced. Some of the motile organisms are excreted and die quickly. If the motile organism is not excreted, it attacks the lining of the colon and becomes a cyst. The cyst is not easily destroyed and therefore transmits the infection to other humans, and to animals such as cats and dogs and sometimes pigs. The cysts typically survive in soil for at least 8 days and are not destroyed by the usual concentration of chlorine in purified water. Cysts are destroyed by boiling.

Amoebiasis is transmitted through food and drink containing cysts. Food handlers infected with amoebic cysts are primary transmitters of the disease. Raw vegetables and fruit that are unwashed or that are washed in polluted water and food exposed to flies are also sources of infection. The disease can occur 2 to 4 weeks after exposure to the causative organism or it may not occur for years after exposure.

A mild case of amoebiasis would present symptoms of minor abdominal discomfort with intermittent diarrhoea and constipation. The more serious case would be characterized by fever, chills, and frequent semifluid or fluid stools, which can contain blood or flecks of mucus. The intestinal symptoms may persist for weeks, may come and go, and can result in severe weight loss. In most cases the infection is confined to the intestinal tract but in some cases it spreads and causes hepatitis or a liver abscess. A medical examination is necessary for diagnosis and proper drug therapy. General management includes palliative measures for relief of symptoms and replacement of lost fluid, including blood in some cases.

Amoebiasis has existed since remote antiquity and many of the outbreaks of dysentery recorded since the time of Hippocrates were probably of amoebic origin. Clinical descriptions which make it possible to identify amoebiasis with relative certainty have been published since the 17th century, but Fedor Aleksandrovich Lösch actually made the discovery of E. histolytica in 1875 in St Petersburg, now Leningrad, in a patient suffering from dysentery. A few years later the amoeba was also shown to be present in liver abscesses.

Amoebiasis can be found all over the world, including the cold and even polar regions: epidemics of amoebic dysentery have been recorded among Eskimos at the North Pole. However, the frequency of invasive amoebiasis varies greatly from one geographical area to another. In the less developed countries, most of which are located in the tropics and subtropics, the proportion of patients with the invasive form is much higher. This does not mean that a tropical climate favours development of the disease - in Mexico, for example, invasive amoebiasis is more prevalent in the central plateau, at an average height of 2000 feet above sea level and in a temperate climate. It is simply that in the tropics most of the human population live under poorer economic conditions, are less well educated and lack adequate health facilities. It can be said, therefore, that amoebiasis is not a tropical disease, but one of poverty and ignorance. Certain zones within the areas where amoebiasis is prevalent are severely affected, and are referred to as 'homelands of amoebiasis'. They include Southeast Asia, east and west Africa, Mexico and the northwest part of South America. The reasons for the concentration of the disease in these zones is not known.

In 1994 171,000 cases of dysentery with at least 600 deaths were reported in Malawi, Mozambique and Zimbabwe (Holloway 1995).