What Is 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.
Cholera Diagnosed
Cholera
should be suspected in acute massive rice-water diarrhoea, certainly if there
have been several cases in a short time (epidemic). The clinical picture of
severe cholera is so spectacular that differential diagnosis does not present
many difficulties. Milder cholera may be similar to other forms of
gastro-enteritis (but not to dysentery). A child above the age of five years
who develops acute dehydration, or dies as the result of acute diarrhoea, is
always suggestive for cholera.
The
vibrios are very small and can best be seen in a fresh faecal specimen with the
help of dark field microscopy. There is characteristic motility ("star
shooting") which stops immediately after adding anti-O1 antiserum. This
does not give any information on possible toxin production. Confirmation is
best made via a bacteriological culture.
Culturing should preferably be on a
special medium in a bacteriology lab, e.g. TCBS-agar [=
Thiosulphate-Citrate-Bile salts-Sucrose], polymyxin mannose tellurite agar
(PMT) or an other selective medium. In order to identify the serogroup and the
serotype one subsequently finds out to which antibodies (antiserum) the
colonies obtained exhibit an agglutination reaction. It is also possible to
find out whether the vibrios are toxicogenic (produce toxin). Definitive
identification is made in a reference laboratory.
Cholera Treatment
Rehydration
is essential and must be instituted as soon as possible. Two phases are
distinguished. First it is important to replenish what has been lost in the
previous hours or days. Then one must compensate the persistent fluid loss
(e.g. the amount of fluid that is lost every hour). In mild cholera without
vomiting oral rehydration may suffice. In severe forms IV fluids should be
administered.
There are
several possible compositions of rehydration fluids. Solutions containing salt,
sugar, potassium and bicarbonate are recommended. Acetate is also used. Lactate
is also good because it is converted in the body to bicarbonate. In cholera it
is preferable to use Ringer’s lactate ( = Hartmann’s solution). Normal
physiological saline is second choice because it does not correct the acidosis
nor does it contain potassium. Severe hypokalaemia may occur, with cardiac
arrhythmias, kidney damage, paralytic ileus and significant muscle weakness
with reduced or absent tendon reflexes. Dextrose (= glucose) 5 % without
electrolytes is not advised as a rehydration fluid. A reminder: 1 gr KCl = 13
mEq KCl.
Cholera Prevention
In the
West nowadays a patient with cholera will remain a sporadic case. In developing
countries one case can lead to several secondary cases. It is not necessary to
wear special masks, aprons or gloves, but washing hands (hospital staff,
family, visitors) should be obligatory. The contamination of clothing and
bedding is unavoidable. Boiling in water for five minutes is sufficient for
disinfection. Mattresses and blankets can be dried in the sun. It is better to
do this before washing them, to prevent infection of the washing area.
After
surviving cholera a patient is probably immune for more than 3 years. No
cross-immunity between V. cholerae O1
and V. cholerae O139 is seen,
although they produce the same toxin. Immunity relies on antibodies in the
intestinal lumen (the bacteria are not invasive). Babies which are being
breast-fed receive protective antibodies in their mother’s milk.
Vaccination
with dead V. cholerae bacteria (IM
administration) does not lead to the formation of protective antibodies in the
intestinal lumen. Advice to vaccinate was discontinued in 1972 by the WHO
[World Health Organisation]. New oral vaccines are being tested. Vaccination,
mass chemoprophylaxis and cordon sanitaire (= restrictions on travel and trade)
are not effective in preventing or limiting outbreaks.
Adapted from: http://www.itg.be
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