SMI: Investigation of whooping cough

Types of specimen

• Pernasal swab
• Nasopharyngeal aspirate
• Nasopharyngeal swab
• Serum

Cough plates are not recommended.

Selective media

It contains blood or charcoal or both, along with selective antibiotic supplements – penicillin, cefalexin or meticillin.
It should be able to suppress other nasopharyngeal flora. Meticillin is the least inhibitory, and cefalexin is the most. Cefalexin is the drug of choice for selective media in the UK.

Pertussis selective media –

Regan-Lowe Charcoal Agar (Ref bd.com)

Safety

Containment level 2.
If the procedure is likely to give rise to aerosol, perform it inside a microbiology safety cabinet.

Culture

• Good specificity.
• It should be performed during the first 2 weeks of cough. Sensitivity decreases after that – increasing the risk of false negatives.
• False negative increases if the culture is performed after 3 weeks/ in adults or adolescents/ in cases of processing delay (it is a delicate organism)/ poorly taken specimens/ vaccinated patients. Hence a negative culture may not exclude the infection.
• Report as – Bordetella pertussis isolated (if positive) / Bordetella pertussis not isolated (or B parapertussis if that is the pathogen) (if negative).
• Confirmation can be done using MALDI ToF.

PCR

• More sensitive than the culture.
• Organisms need not be viable.
• It should be performed within 4 weeks of symptom onset.
• The nasopharyngeal swab is preferred, but a throat swab can be used.
• Interpretation is based on the target. Report B pertussis/B parapertussis/B holmesii DNA detected or not detected. (see below)

IS481	ptxS1	hlS1001	plS1001	Report
+	+	–	–	B pertussis DNA detected
–	+	–	+	B parapertussis DNA detected
+	–	+	–	B. holmesii DNA detected
+	+	–	+	B. pertussis and B. parapertussis DNA detected
+	+	+	–	B. pertussis and B. holmesii DNA detected

Serology

• Should be performed after 2 weeks of symptom onset.
• ELISA to detect anti-pertussis toxin (PT) IgG; anti-filamentous hemagglutinin (FHA) can also be tested.
• Serology is useful in older childrens and adults.
• Maternal antibody (in neonate), primary vaccine (up to 10 months) and booster vaccine (up to 3 years) can confound the serology result. It is recommended that serology should be performed at least perform 1 year after the primary or booster dose of the Pertussis vaccine.
• Anti-PT IgG serology test cannot be used to test for immunity.

• A case of pertussis is serologically confirmed when anti-PT IgG concentration is >70 International Units per millilitre (IU/mL) in the absence of recent vaccination (within the past year).
• A case of parapertussis is serologically confirmed when there is a significant anti-FHA IgG increase without an increase in anti-PT IgG, IgM, and IgA antibodies.
• If both anti-PT IgG and anti-FHA IgG are significantly increased, the results are indicative of infection with Bordetella species.

Reference:

https://www.gov.uk/government/publications/smi-b-6-investigation-of-specimens-for-bordetella-pertussis-and-bordetella-parapertussis