Doctors usually carry out cholera diagnosis measures clinically and start treatment immediately without waiting for a laboratory confirmation due to the deadly nature of the illness.
However, a cholera case has to be confirmed by isolating the causative agent Vibrio cholerae from the stool (feces) in a microbiology laboratory. A detailed history of the patient is always helpful during the initial stages of the diagnostic procedure, and doctors are likely to provide a questionnaire to investigate:
- The source of infection
- Contact history
- Chemoprophylaxis (treatment) history
- Immunization (vaccine) history
The Steps Involved In Cholera Diagnosis
Step 1: The Sample Is First Collected
Feces or rectal swab (a cotton swab inserted into the rectum, rotated gently, and removed) are collected during the early stages of the disease before administering any antibiotics. Blood may also be collected for the detection of antibodies. This happens when you go for medical advice with cholera symptoms.
Step 2: The Sample Is Transported
Samples collected are immediately transported to the laboratory. Cary Blair transportation media is used to maintain the bacterium in its living state.
Laboratory cholera diagnosis is based on:
- Macroscopic examination of sample
- Microscopic examination
- Bacterial culture identification
- Rapid tests
Step 3: Macroscopic Examination Is Carried Out
Once the laboratory personnel receive the sample, macroscopic examination of the stool is carried out.
The stool of cholera patients has a characteristic ‘rice water’ appearance.
Step 4: Direct Microscopic Examination Is Carried Out
The stool sample is later stained and observed under the microscope for detecting the bacteria that cause cholera.
- Staining: The vibrio bacteria are comma shaped and stain pink (negative) under the Gram staining technique (a widely used staining technique for bacterial identification).
- Motility Tests: Vibrio has a characteristic ‘darting motility’ which can be observed using a ‘hanging drop preparation’, where a drop of the sample is placed in a specially designed inverted microscopic slide and later observed under microscope. Dark field microscopy is also used for observing vibrio motility.
Step 5: The Bacteria Is Cultured
Even though direct microscopy offers a quick presumptive diagnosis of V cholerae, the diagnosis is confirmed by isolating the bacteria in various culture media.
Enrichment media such as alkaline peptone water at pH 8.6 or Monsur’s taurocholate tellurite peptone water at pH 9.2 are used for enriching the bacteria. In simple terms, sometimes the bacteria can’t be identified when they are fewer in number. These components will help them grow and spread, making identification easier.
Even substances like TCBS Agar are used to selectively allow the growth of Vibrio cholera among other intestinal bacteria that may be present in the stool sample. The bacterium forms flat, 2–3-mm-diameter, yellow colonies after 18 hours of incubation.
Step 6: V Cholerae Is Identified
Once the bacterium grows in the culture medium, it is identified and confirmed by:
2. Motility test
3. Biochemical identification
Once the bacterium is isolated and identified, antibiotic susceptibility of the particular isolate can be tested by antibiotic sensitivity tests.
Various rapid test kits such as the Rapid Dip-stick Test are also available commercially, but are used only during an epidemic. This helps in confirming the cholera diagnosis quickly, decreasing the death rate right at the start of the outbreak. The major disadvantages of such tests are that an isolate will not be available for serogrouping or for antibiotic susceptibility testing. For this reason, isolating and identifying the bacteria is still the gold standard for cholera diagnosis and confirmation.