Cholera

Cholera is an acute infectious disease, characterised by profuse watery diarrhoea. It is caused by a Gram-negative bacterium: Vibrio cholerae O1 (the characters O1 indicate the serogroup). It is a very small, motile, curved bacterium (vibrio is the Greek word for comma). There is a single polar flagellum. Various subtypes exist, with classification according to biological and biochemical behaviour (biotypes) and serological characteristics (serotypes). Analysis of rRNA-genes (ribotyping), electrophoretic typing of multiple enzymes (zymovars) and bacteriophage typing are used in epidemiological research. Until 1992 it was thought that bacteria causing cholera must belong to V. cholerae, serogroup O1 and that they must be toxicogenic (must possess and express the genes for toxins). It was known that non-O1 Vibrio cholerae could sometimes cause mild gastro-enteritis or even septicaemia in immunodepression, but not cholera. In October 1992 in Madras (India), a mutated pathogenic bacterium (a new serogroup) was discovered. This new bacterium also causes cholera. The isolate was given the name Vibrio cholerae O139, nicknamed Bengali.

In August 2000 the whole genome (2 circular chromosomes) of V. cholerae O1, biotype El Tor was charted and published. Antibiotic resistance genes in V. cholerae are often positioned on plasmids and can be transmitted to vibrios from non-pathogenic intestinal flora. There are arguments to suggest that V. cholerae recently isolated from the intestine of a patient is much more infectious than bacteria which have been in the outside world for a long time.

Biotypes

There are 2 biotypes: classic Vibrio cholerae and V. cholerae biotype El Tor. Biotype El Tor agglutinates chicken erythrocytes and causes lysis of sheep erythrocytes, unlike the classic biotype. The name El Tor originates from the Egyptian/Saudi border town and quarantine camp El Tor in the Sinai dessert, where the bacterium was isolated for the first time in 1905 (during the 6^th pandemic) from an asymptomatic Hajj pilgrim from Mecca. The importance of this germ was long disputed (until 1961). At present El Tor has replaced the classic variant in most places, except in the Ganges and Brahmaputra delta. El Tor may also survive longer in the environment, is less dependent on transmission via water and produces more asymptomatic infections (symptomatic/asymptomatic infections = 1/90).

Serotypes

V. cholerae O1 may have 2 different antigens on its membrane. If only the first antigen is present, the bacterium is known as serotype Inaba. If only the second antigen is present, the bacterium is known as serotype Ogawa. If both antigens are present, the name Hikojima is given. Serotype shift seldom occurs (from Ogawa to Inaba and vice versa). The difference between these serotypes is only of importance for epidemiological studies. For example: in 1991 all cases of cholera in South America were caused by toxin-producing Vibrio cholerae, serogroup O1, biotype El Tor, serotype Inaba. The cholera epidemic in the Rwandan Hutu refugees in Zaire (July and August 1994) was caused by El Tor, serotype Ogawa. These bacteria were resistant to tetracyclines, cotrimoxazole, chloramphenicol and ampicillin.

Category: Medical Subject Notes , Microbiology Notes