Overview of pertussis

The term pertussis, which refers to "a violent cough" and is commonly referred to as whooping cough or "the cough of 100 days," was initially reported during the 1578 outbreak in Paris. It was found that Bordetella pertussis was the causative organism in 1906, and in the 1940s a vaccine was produced.¹ Pertussis, another name for whooping cough, is a highly contagious upper respiratory infection. Usually, it causes extended coughing fits that happen repeatedly. After you start feeling sick, you may experience these coughing fits for weeks or even months at a time. Babies with pertussis may have trouble breathing instead of coughing fits. This covers ailments when there are breathing pauses, such as apnea. In infants, whooping cough can result in severe, sometimes fatal consequences. Roughly one-third of all infants (babies under one-year-old) with whooping cough require hospitalisation for treatment. The illness can be avoided in part with the whooping cough vaccination.² The signs and symptoms of whooping cough usually occur seven to ten days after infection, however, they can occasionally take longer. They typically start mild and mimic those of a typical cold: runny nose, congestion in the nose, teary, red eyes, and cough with fever. The symptoms and indicators get worse after a week or two. Your airways fill with thick mucus, which makes you cough uncontrollably. Prolonged and severe coughing fits may: Cause vomiting to occur, cause a face to become red or blue, induce extreme exhaustion, and Complete with a loud "whoop" sound made with the following breath. Still, a lot of people never acquire the distinctive whoop. Sometimes the only indication that a teen or adult has whooping cough is a persistent hacking cough. Babies might not even cough. Rather, they might have trouble breathing or even stop for a while.³

Methods used to diagnose pertussis

Polymerase chain reaction (PCR), serology, culture, and direct-fluorescent antibody testing (DFA) are common laboratory diagnostic techniques used to diagnose pertussis.⁴

Laboratory diagnosis

Polymerase chain reaction (PCR)

Healthcare professionals will see a greater number of patients who may have pertussis due to the rise in cases documented during the early 1990s. Healthcare professionals are increasingly having access to PCR as a valuable tool for the prompt diagnosis of pertussis. A molecular method called PCR is used to find Bordetella pertussis DNA sequences. PCR does not require the presence of viable (living) bacteria in the specimen, in contrast to culture. Despite these benefits, PCR has the potential to produce false-positive or false-negative results.⁵

End-point PCR (Classical PCR): In 1989, Houard and associates used PCR to amplify particular B. pertussis DNA sections. This was the first PCR run to identify B. pertussis. The first set of primers, PTp1/PTp2, found a 191-bp DNA fragment in the pertussis toxin operon's regulatory region. B. pertussis-specific insertion-like element containing 121-bp of DNA was amplified using a second pair of primers. After several years, primers from the adenylate cyclase gene, repetitive insertion sequences, and a DNA region upstream of the porin gene were employed. Because it is present in several copies, it is evident that the insertion sequence IS481 is said to have the maximum diagnostic sensitivity. Its specificity may be lowered by false positive results, though, as cross-reactions with B. holmesii and B. bronchiseptica have been documented. Meade and Bollen (1994) released a series of guidelines for the use of PCR in the diagnosis of B. pertussis infections to create a standardised procedure that would be broadly applicable. Many papers have now been published that outline various PCR techniques and advocate for their regular application in the therapeutic context.⁴

Quantitative real-time PCR (qPCR): This technique has transformed clinical microbiology laboratories' diagnostic methods and is now essential for confirming Bordetella infections. Generally speaking, this approach uses the same reaction vessel for both PCR chemistry and fluorescence probe detection of the amplified product. In contrast to the traditional PCR amplification approach, qPCR does not involve the use of dangerous chemicals, necessitate extensive and time-consuming post-PCR adjustments, or pose a risk of laboratory contamination. It tracks the "real-time" accumulation of PCR products during the reaction's exponential phase. Additionally, qPCR can measure and identify minuscule amounts of nucleic acids, which increases its sensitivity. Additionally, qPCR holds the potential to be more precise than end-point PCR.⁴

Clinical diagnosis

Heterogeneity in disease expression, particularly in adults and adolescents, vaccination-induced disease modification, mixed infections, and a low index of suspicion among many clinicians confound the clinical diagnosis of pertussis. It is only possible to make a clinical diagnosis when the illness exhibits typical symptoms. Still, Bordetella pertussis or other species may be the cause of the sickness. Furthermore, several other bacteria can mimic the clinical symptoms of pertussis, including adenovirus, parainfluenza virus, respiratory syncytial virus, Mycoplasma pneumoniae, and Chlamydia pneumoniae. The World Health Organisation (WHO) and the Centres for Disease Control and Prevention (CDC) have created a uniform clinical case description. When diagnosing a patient with a mild or unusual illness, a history of interaction with a known case usually validated by a laboratory test can be useful in certain situations.⁴ The illness is rarely recognised since it is well known to be atypical in teenagers and adults compared to the classic whooping cough in newborns and early children.⁶

Serological diagnosis

Serology becomes increasingly significant as the disease progresses because PCR and culture become less effective. As the disease progresses, PCR and culture become less effective; as a result, serology increases in importance. Agglutination, complement fixation, immunodiffusion, indirect immunofluorescence, bactericidal reaction, and other conventional serological techniques are used to assess the immunological response to pertussis. They haven't, however, offered extremely precise or sensitive measurements of the immunological response to vaccination or pertussis infection. Immunoassay techniques have been available since the 1980s. They have been applied to both the assessment of vaccine immunogenicity and the identification of pertussis. The most widely utilised serology currently uses enzyme-linked immunosorbent test (ELISA) techniques. Serological tests typically only yield a late or retrospective diagnosis, while being more sensitive than culture. The usage of paired sera also places restrictions on them.⁴

Fluorescent-antibody direct assay

The straightforward and quick approach of direct fluorescent-antibody assay (DFA) of nasopharyngeal samples depends on the microscopic visualisation of fluorescent antibodies directed towards B. pertussis cells. As this technique has low sensitivity and specificity, DFA diagnosis should always be backed up by serology, PCR, or culture^.7

Culture

Because it allows for strain identification and antibiotic resistance testing, bacterial culture is a highly specific laboratory test that is recommended by the CDC and is widely regarded as the gold standard for pertussis diagnosis. To confirm the diagnosis, patients with suspected pertussis should have a nasopharyngeal (NP) swab or aspirate taken from the posterior nasopharynx. Since viable bacteria are present in the nasopharynx during the first two weeks of sickness, this is the best time to collect a culture specimen. After two weeks, culture specimens have a low specificity and are more prone to yield false-negative results. The sensitivity of bacterial culture lowers if the patient has already been vaccinated against B. pertussis or has received antibiotic therapy. Doctors ought to think about doing concurrent PCR testing.⁸

Comparison of pertussis testing methodologies

Test methodology

PCR

Advantage

Test advised by the CDC 

Increased sensitivity 

Does not need living bacteria to produce a reliable result. 

Disadvantage

For accurate findings, it must be done within the first three weeks of the illness. 

Variations exist in specificity

A higher chance of receiving a false positive 

Test methodology

Culture

Advantage

Test advised by the CDC 

Increased specificity

Permits testing for antimicrobial resistance and strain identification.

Disadvantage

For accurate findings, it must be done within the first two weeks of the sickness. 

Minimal sensitivity

Requires living microorganisms 

Test methodology

Serology

Advantage

Performable up to 12 weeks following the onset of the disease 

Able to identify prior B. pertussis exposure

Disadvantage

Not a test advised by the CDC 

Inaccurate results may arise from recent immunisation, prior infection, and cross-reactivity with different Bordetella species.

It's frequently ineffective for babies under six months. 

Summary

Pertussis, also known as whooping cough, is a highly contagious respiratory infection caused by Bordetella pertussis. It is characterized by severe coughing fits and can lead to complications, especially in infants, who may experience breathing difficulties. The disease can be diagnosed through several methods, including PCR, serology, culture, and direct fluorescent antibody (DFA) testing. PCR is highly sensitive and can detect B. pertussis DNA, though it may yield false positives or negatives. Culture is the gold standard for diagnosis, but it requires viable bacteria and is most effective within the first two weeks of illness. Serology, while useful for late-stage diagnosis, may be affected by prior vaccination or infections. DFA is a quick method but lacks sensitivity. Early diagnosis and vaccination are key in preventing and managing pertussis, especially in high-risk populations like infants.