Approach
Clinical assessment and the use of local epidemiology can still be regarded as the most important initial diagnostic tools when approaching suspected cases of cholera infection. Nevertheless, great advances have been made in developing specific, easy-to-use, rapid assays that allow early identification of cholera outbreaks and mobilisation of personnel and equipment.
In endemic areas during epidemics and in people with risk factors, empirical treatment in clinically suspected cases is necessary while awaiting results from confirmatory laboratory tests.[71][72]
History
About 70% to 80% of all those infected remain asymptomatic. Cholera needs to be considered in the differential diagnosis of diarrhoea in adults with a history of travel to cholera-endemic countries and/or risk factors (e.g., recent ingestion of contaminated water or shellfish, work with refugees in areas with poor sanitation, visits to areas with recent heavy flooding, exposure to affected family members, malnutrition, achlorhydria, blood group O, or HIV infection), particularly when presenting in a volume-depleted state.
The sudden onset of severe watery diarrhoea in a patient presenting for treatment during an epidemic is highly predictive of cholera. Fever may be present infrequently.[51]
Cholera tends to affect children under 5 years of age in endemic areas. This is due to preexisting immunity among adults and older children. In non-endemic areas, adults account for more cases of cholera than children. Vomiting is an early sign of cholera and is extremely common among children.[73] Abdominal pain may also be seen in children.[73] Breastfeeding has been shown to protect babies from the occurrence of cholera, probably due to the reduced likelihood of direct exposure.[36] Breastfed infants are also less likely to be hospitalised due to cholera.[42]
Pregnant women with cholera have an increased risk of adverse pregnancy outcomes, in particular fetal death.[7][8]
[Figure caption and citation for the preceding image starts]: Cup of typical 'rice-water' stool from a cholera patient shows flecks of mucus that have settled to the bottomCDC/Dr William A. Clark [Citation ends].
Physical examination
Diarrhoea with sustained fluid output over 20 mL/kg during a 4-hour observation period is almost pathognomonic of cholera.[74]
Clinical examination may show evidence of volume depletion, with tachycardia, low blood pressure (BP)/weak pulse, or lack of arousal. The degree of volume depletion is generally divided into severe (>10% volume depletion) and mild to moderate (5% to 10% volume depletion).[3]
In mild-to-moderate volume depletion, patients may present with irritability, sunken eyes, dry mouth, evidence of a significant (>20 mmHg) postural drop in BP, mildly decreased skin turgor, and thirst, but be able to take significant amounts of oral fluid.[3]
In severe volume depletion, patients are lethargic or comatose with circulatory collapse (thready pulse, low BP with a systolic BP <80 mmHg), sunken eyes, absent tears, dry mucous membranes, poor capillary return >2 seconds, and poor skin turgor.[3] These patients need to be identified in triage rapidly because they have a high mortality over a few hours if not appropriately managed. [Figure caption and citation for the preceding image starts]: Due to severe volume depletion, cholera manifests itself in decreased skin turgor, which produces the so-called 'washer woman's hand' signCDC/Dr William A. Clark [Citation ends].
General laboratory tests
None of the following tests are essential for the successful management of a suspected or confirmed cholera case in the field, and rehydration should not be delayed while awaiting results. Non-specific laboratory tests may be performed as part of the routine work-up to assess the severity of illness, as guided by clinical findings. Electrolyte and acid-base disturbances reflect the severity of illness and volume depletion.[75] Some or all routine laboratory tests may not be available in the field or in resource-limited settings.
Full blood count may show elevated haematocrit in non-anaemic patients as a result of volume depletion, and a high neutrophil count may be present in severe infection.[76]
Blood chemistry tests are important in assessing the degree of volume depletion; urea and creatinine levels are frequently elevated due to volume depletion.
Arterial blood gas assessment is helpful because cholera infection often results in low serum bicarbonate with accompanying acidosis, as well as elevated lactate with a high anion gap (due to hyperproteinaemia, lactic acid, and hyperphosphataemia).
An electrocardiogram may demonstrate sinus tachycardia. Bradycardia is an ominous sign of severe illness. Hypokalaemia may produce a prolonged PR interval and flattened T waves.
Specific laboratory tests
The definitive diagnosis depends on identifying Vibrio cholerae in the stool of affected people. Several other tests may also be used.
Dipstick tests are useful for detecting early stages of cholera outbreaks, preventing high mortality in locations where the skill level of personnel may be low (remote areas or refugee camp settings). The commercially available dipstick test has a high degree of sensitivity and moderate specificity in detecting V cholerae O1/O139 in epidemic settings. Local laboratory technicians were able to correctly use the dipstick test using written instructions while testing stool specimens during a cholera outbreak in 2008.[77][78] However, confirmation and further specification of the V cholerae strain is strongly recommended through standard laboratory tests such as bacterial culture.
Another useful initial test in the field is dark-field/phase-contrast microscopy, which may yield a large quantity of curved bacilli on examination of saline suspensions from fresh stool samples.[4] V cholerae are usually variable in size from 1 to 3 micrometres in length and 0.5 to 0.8 micrometres in diameter, with a single polar flagellum and typical shooting star motility.[79]
Gram-stain examination of stool culture is cheap and widely available but not a particularly helpful subsequent laboratory test. Stool Gram stain is not often done in practice because there are too many other gram-negative organisms in stool, and wet prep is better and faster. Smears can demonstrate small, curved gram-negative rods.
Confirmation of the presence of V cholerae by bacterial stool culture is the most specific tool. The colonies are typically described as yellow (due to sucrose fermentation) and shiny, 2 to 4 mm in diameter, with selective growth on thiosulfate citrate bile salt sucrose (TCBS) agar or highly selective gelatin-taurocholate-tellurite medium (Monsur medium) from liquid stool, faecal suspension, or rectal swab. V cholerae is also oxidase positive. TCBS is commercially available, easy to prepare, requires no autoclaving, and is highly differential and selective. Alkaline peptone water is the enrichment broth of choice, recommended especially in non-acute or chronic cases where numbers of V cholerae are expected to be low and numbers of competing organisms high (e.g., where samples have been transported for several days). An adequate means of specimen transport to the laboratory is essential to increase yields of positive cultures. Faecal samples should be placed into Cary-Blair or similar commercially available preservation media for transport to the laboratory, especially if there is a delay of more than a few hours.
Following growth and isolation of V cholerae, serogroup confirmation using antisera and antibiotic susceptibility testing (which is carried out according to guidelines on antibiotic susceptibility reference ranges) are performed. A TCBS bacterial colony is inoculated on to a heart infusion or Luria agar slant. After 6 to 24 hours, slide serology using polyvalent O1 or O139 group-specific antisera is performed. Non-O1, non-O139 strains are seen only in non-epidemic, sporadic cases of diarrhoeal illness so can be diagnosed with the appropriate antisera if negative tests do not fit with the clinical presentation of disease.[79][80][81] Antibiotic susceptibility (antibiogram) is usually determined using the disc-diffusion method, but epsilometer (E)-test strips for rapid susceptibility tests are also used if borderline resistance is suspected.
Other diagnostic methods for V cholerae include molecular detection via multiplex polymerase chain reaction (PCR), which provides fast and reliable diagnosis of V cholerae, and enzyme-linked immunosorbent assay (ELISA). However, correlation with local epidemiology, clinical presentation, and bacterial culture results is always strongly recommended.
Multiplex PCR is highly sensitive and specific and is useful in endemic areas due to its ability to detect other pathogens in a rapid and cost-effective way.[82] PCR-based assays are used to test for toxigenic V cholerae O1/O139 and can simultaneously determine serotype, biotype, and other factors by detecting the ctxA, tcpA El Tor/classical, wbe/wbf, and toxR genes.[83]
Loop-mediated isothermal amplification (LAMP) assay is another molecular diagnostic technique that uses nucleic acid amplification, offering faster and simpler diagnosis than conventional PCR in detecting cholera toxin-producing V cholerae.[84][85]
Molecular diagnostics are becoming more widely available in resource-limited settings in public health laboratories, or otherwise can be easily set up in larger government laboratories for reference and surveillance, where they can prevent the rapid spread of disease by early detection of cholera outbreaks. However, the costs of these technologies may still represent a limitation in resource-limited settings; accessible, emerging point-of-care molecular diagnostic tools still have an important role in V cholerae identification.[86]
Radiology
Radiological tests are rarely helpful in the initial stages of managing cholera cases and should not delay rehydration attempts. Chest and abdominal radiographs are non-specific tools to exclude emergencies involving the small intestine following severe volume depletion as a result of secretory diarrhoea. These imaging tests may also be part of the initial routine work-up of a returning traveller with diarrhoea, but in the epidemic setting are rarely helpful. They can be used to try to rule out possible complications from severe diarrhoea (e.g., small bowel obstruction, ileus, and perforation) or non-infective abdominal pathology (e.g., severe constipation causing overflow diarrhoea).[87] However, attempting to perform these tests may delay intensive rehydration therapy.
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