Cholera
History
During the 19th century, cholera spread across the world from its original reservoir in the Ganges delta in India. Six subsequent pandemics killed millions of people across all continents. The seventh pandemic started in South Asia in 1961, reached Africa in 1971 and the Americas in 1991. Cholera is now endemic in many countries.
Etiology
Cholera is caused by Vibrio cholerae
Vibrio Cholerae strains
There are many serogroups of V. cholerae, but only two – O1 and O139 – cause outbreaks. V. cholerae O1 has caused all recent outbreaks. V. cholerae O139 – first identified in Bangladesh in 1992 – caused outbreaks in the past, but recently has only been identified in sporadic cases. It has never been identified outside Asia. There is no difference in the illness caused by the two serogroups.
What is a cholera endemic area?
A cholera-endemic area is an area where confirmed cholera cases were detected during the last 3 years with evidence of local transmission (meaning the cases are not imported from elsewhere).
Cholera epidemic
A cholera outbreak/epidemic can occur in both endemic countries and in countries where cholera does not regularly occur.
The risk of a cholera epidemic is highest when poverty, war or natural disasters force people to live in crowded conditions without adequate sanitation.
Cholera transmission is closely linked to inadequate access to clean water and sanitation facilities.
Symptoms of Cholera
Diarrhoea - Rice Water stools
Cholera causes a lot of fluid loss
Nausea and vomiting
Dehydration - Dehydration can develop within hours after cholera symptoms start
Because of dehydration, electrolyte imbalance can occur
Electrolyte imbalance can cause -
Muscle cramps - Occurs due to rapid loss of sodium, potassium and chloride
Shock - A serious complication of Dehydration
Cholera toxin is the toxin produced by Vibrio cholerae. Cholera toxin is responsible for diarrhoea and electrolyte imbalance
Detection
Cholera cases are detected based on clinical suspicion in patients who present with severe acute watery diarrhoea. The suspicion is then confirmed by identifying V. cholerae in stool samples from affected patients. Detection can be facilitated using rapid diagnostic tests (RDTs), where one or more positive samples triggers a cholera alert. The samples are sent to a laboratory for confirmation by culture or PCR.
Lab diagnosis of Cholera
SPECIMENS
Watery stool or rectal swab (for carriers)
TRANSPORT
VR medium
Cary-Blair medium
Transport media is used when delay is expected in transport of stool specimen
DIRECT MICROSCOPY
Gram staining of mucus flakes of feces is done
Gram-negative rods, short curved comma-shaped
Hanging drop-demonstrates darting motility
CULTURE
1. Enrichment broth
Alkaline peptone water
Monsur’s taurocholate tellurite peptone water
Enrichment broth is used to inhibit fecal commensals
Fecal specimens are inoculated in enrichment broth and incubated for 4 to 6 hours
2. Selective media
Following enrichment, colonies are plated onto selective media and incubated at 37*C for 24 hours
Bile salt agar
Monsur’s GTTT agar
TCBS agar - Vibrio cholera grows as yellow colonies
MacConkey agar - Produces translucent NLF colonies. Colonies of V. cholerae may become pink on prolonged incubation due to late lactose fermentation
CULTURE SMEAR
Short curved gram-negative bacilli
MOTILITY TESTING
Darting motility
IDENTIFICATION
1. Catalase and oxidase positive
2. ICUT: Indole (+), Citrate (+/–), Urease (–),
TSI:A/A, gas (–), H2S (–)
3. String test positive - When a colony of Vibrio is mixed with a drop of 0.5% sodium deoxycholate on a slide, the suspension loses its turbidity and becomes mucoid. When the suspension is lifted with a loop, it forms a string
4. Hemodigestion occurs on blood agar
5. Automated systems
MALDI-TOF
VITEK
6. Biotyping: To differentiate classical and El Tor
7. Serogrouping: To differentiate O1 and O139
8. Serotyping: To differentiate Ogawa, Inaba and Hikojima serotypes of serogroup O1
9. Cholera dipstick assay - Used for antigen detection. It is useful in fields where laboratory facilities are not available
10. Molecular method
Molecular methods such as PCR can directly detect V. cholerae specific genes in stool
11. Antimicrobial susceptibility testing
Done on Mueller Hinton agar by disk diffusion test
Prevention and control
A combination of surveillance, water, sanitation and hygiene, social mobilisation, treatment, and oral cholera vaccines are used.
Treatment
Cholera is an easily treatable disease. The majority of people can be treated successfully through prompt administration of oral rehydration solution (ORS). Use of ORS helps to replace lost fluids and electrolytes
Severely dehydrated patients are at risk of shock and require the rapid administration of intravenous fluids (Ringer's lactate or Normal saline)
Antibiotics have a minor role in treatment of cholera as the pathogenesis is mainly toxin
mediated
Mass administration of antibiotics is not recommended, as it has no proven effect on the spread of cholera. It may contribute to antimicrobial resistance
Zinc is an important adjunctive therapy for children under 5, which also reduces the duration of diarrhoea and may prevent future episodes of other causes of acute watery diarrhoea.
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