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.