Pertussis: Case Study

• Pertussis, also known as whooping cough, has plagued humans for centuries, with the causative organism, B. pertussis, isolated in the early 1900s.

Case Report

• A 12-week-old girl was admitted to the hospital with afebrile peribronchial pneumonia and paroxysms (sudden spasms) of cough after a week of rhinorrhea, sneezing, and mild cough.
• On the day of admission, the patient experienced 15 coughing paroxysms with a staccato series of brief coughs lasting up to one minute, with no inspiratory effort between coughs.
• She was brought to the emergency department by ambulance after experiencing a brief generalized seizure and had received one dose of whole-cell pertussis vaccine combined with diphtheria-pertussis-tetanus (DPT).

Physical Examination

• The patient's temperature was 37.3°C (normal 36-37°C), respiratory rate 30 (normal 20), heart rate 100 (normal 60-90), and blood pressure 70/40 (normal 110/70).
• Clear rhinorrhea and mild conjunctival (inner eyelid) infection were noted during head, eyes, ears, nose, and throat examination.
• Chest examination revealed scattered rhonchi, and the infant was alert and responsive, with no neurological abnormalities.

Laboratory Findings

• The patient's white blood cell count was elevated at 24,000 per cubic millimeter (normal 3200-9800/mm³) with 80% lymphocytes (normal 15-50%).
• Electrolytes, blood urea nitrogen, serum creatinine, serum calcium, serum phosphate, liver function tests, and arterial blood gas were normal.
• Computerized axial tomography scan of the head was normal, and a lumbar puncture showed normal opening pressure with no cells, glucose, or protein present.

Admitting Diagnosis

• The admitting diagnosis was peribronchial pneumonia, with a differential diagnosis including Bordetella pertussis, Mycoplasma pneumoniae, and viral agents like adenovirus.
• The patient was placed on respiratory isolation precautions, and a nasopharyngeal swab was obtained for direct fluorescent antibody (DFA) and culture for B. pertussis.
• Cold agglutinins and mycoplasma acute serology, as well as viral cultures, were procured, and the infant was administered erythromycin therapy.
• The DFA for B. pertussis was reported positive and the acute cold agglutinin titer was negative.
• The patient's course was prolonged and marked by severe paroxysmal cough with whoop, bradycardia, hypoxemia, and apnea requiring hospitalization in the intensive care unit.
• Eventually, the paroxysms diminished, and the patient was discharged from the hospital after 5 weeks.
• The patient's 16-year-old mother was found to have a mild cough and subsequently tested positive for B. pertussis.
• Exposed hospitalized infants and staff were tested for B. pertussis infection (one house officer, two infants, three nurses, and the husband of one of the nurses) and treated with oral erythromycin.
• In the months following hospital discharge, the child had spells of coughing with mild paroxysms associated with apparent viral upper respiratory infections.

Diagnosis

• The first classic description of pertussis may be that of de Baillou in 1578, with the earliest American description in 1822 by Waterhouse.
• The direct proof of the bacterial cause of B. pertussis came from experiments inoculating infants with live B. pertussis organisms.
• B. pertussis is uncommon in the United States due to widespread DPT vaccine use although milder forms without the characteristic whoop are often misdiagnosed.
• Typical pertussis has three stages: catarrhal, paroxysmal, and convalescent.
• The incubation period is 7 to 10 days, and the illness resembles a mild upper respiratory infection, progressing to a dry, hacking cough in the paroxysmal stage.
• The patient is usually afebrile, with physical examination often not revealing, and frequently vomits at the termination of coughing fits.
• The paroxysmal stage lasts 1 to 4 weeks, and paroxysms recur with subsequent upper respiratory viral infections for 4 to 6 months during the convalescent stage.
• In smaller infants, pertussis is particularly severe, with the greatest morbidity and mortality in infants under one year of age, with seizures, pneumonia, apnea, and cyanosis more common in those less than six months old.
• The nosocomial spread of B. pertussis is consistent with the epidemiology of whooping cough which is as contagious as varicella.
• An 80 to 90% attack rate is common in susceptible people in the same household and a 20 to 50% in nurseries.
• Diagnosis is based on a characteristic history and physical examination, with elevated white blood cells and lymphocytosis often present.
• Absolute lymphocyte count typically exceeds 10,000/mm³, and elevated B and T cells are present.
• The gold standard for diagnosis is culture of the organism from a nasopharyngeal swab, but the organism is fastidious and slow-growing, with cultures complicated by the need for direct plating on selective media.
• Direct fluorescent antibody testing of nasopharyngeal smears is a useful rapid method of presumptive diagnosis but both false-positive and false-negative results can occur.
• The measurement of agglutination antibodies is the classic serologic test, used for more than 45 years, while ELISA methods have also been recently applied.
• An elevation of IgA to B. pertussis cell wall protein, filamentous hemagglutinin (FHA) occurs in association with infection and can be used as a single serologic marker.
• Significant titer rises in IgG antibody to FHA and pertussis toxin (PT) are typically seen during convalescence from acute B. pertussis infection.

Pertussis Vaccines and the Continuing Threat from Bordetella Pertussis

• Pertussis attacked over 200,000 children per year in the US in the 1930s, and killed as many as 12,000.
• The introduction of a vaccine containing killed, whole-cell B. pertussis dramatically declined serious pertussis, with approximately 3,000 cases reported per year with few or no deaths currently.
• Adults in the United States constitute an infectious reservoir of the bacterium that put infants at risk and the continuing increase in global travel may increase this threat.
• During the 1980s pertussis killed between 500,000 to a million persons per year outside the United States where there are countries lacking effective vaccination programs.
• The vaccine currently most used in the United States is essentially the original "whole-cell" vaccine, consisting of cells and debris of killed B. pertussis.
• It is frequently combined with diphtheria and tetanus vaccines, referred to as the DTP vaccine; preparations lacking the pertussis component are termed DT vaccines.
• The pertussis component frequently causes redness, swelling, and tenderness at the site of injection along with, fever, fretfulness, drowsiness, vomiting, anorexia, and unusual and persistent crying.
• Reports that the pertussis vaccine might cause permanent neurologic damage, coupled with the perception that the risk of contracting severe or fatal disease was small, led to declines in vaccination rates outside the US, with subsequent epidemics and deaths from the 1980s onwards.
• The US FDA relies on the empirical observation, reported in 1958, that protection in humans correlated to protection of mice from intracerebral challenge for its potency assay.

Pertussis Toxin and the Pertussis Vaccine

• Most acellular vaccines contain multiple antigens, however to date nearly all candidate acellular vaccines, contain a version of the protein termed pertussis toxin (PT).
• Although other strains of Bordetella can cause pertussis-like disease, B. pertussis is likely the predominant cause of severe pertussis in humans: A feature that clearly distinguishes B. pertussis from other Bordetella strains is the production and release of PT.
• Transposon-induced mutations that selectively prevent expression of PT markedly increase the median lethal dose challenge dose of B. pertussis in mice.
• Mice studies suggest antibodies that neutralize PT reduce the severity of pertussis disease.
• In a human trial, vaccines based on formalin-treated PT provided protection from severe, but not enough for from infection.
• Assays measuring anti-PT antibodies that neutralize its activity did not correlate with protection, and antibodies were detected in vaccines against other antigens like a 69-kd protein.
• B. pertussis may express PT not because it is crucial for colonization or growth in the host but because it contributes to the cough that spreads the organism to other humans.
• PT may contribute to the severe undesired reactions associated with the vaccine, including neurological damage.

Pertussis Toxin: Structure and Function

• Pertussis toxin consists of six polypeptides held together by noncovalent interactions and arranged in the A-B architecture typical of many bacterial toxins
• The A protomer consists of a single S1 subunit which possesses the toxin's catalytic ADP-ribosyltransferase activity.
• The B oligomer, which binds the toxin to target cells, contains five subunits: S2, S3, S4, and S5 in a 1:1:2:1 ratio.
• The lack of lysine residues in the S1 subunit may protect it from ubiquitin-mediated proteolytic destruction inside target cells.
• Considerable structural homology is present between sequence data and crystal diffraction data when compared to the Escherichia coli heat-labile and cholera enterotoxins, which also possess ADP-ribosyltransferase activity.
• The G-protein, termed Gᵢ, was the first protein identified to be a functionally relevant substrate for the ADP-ribosyltransferase activity of PT.
• The G-protein derives its name from its first identified function: mediating inhibition of adenylate cyclase by inhibitory hormones such as somatostatin.
• Pertussis Toxin (PT) ADP-ribosylates a cysteine residue on Gᵢ, locks the G-protein into a guanosine diphosphate (GDP)-liganded form, and blocks signal transduction.

Binding, Entry, and Activation of Pertussis Toxin:

• The model shows pertussis toxin binding to target cells via a binding site on the B-oligomer.
• Receptors recognized by the B oligomer lose sialic acid residues through variants of cultured cells that surface glycoproteins and glycolipids.
• B oligomer contains multiple sites with different receptor specificities and can agglutinate erythrocytes.
• PT may also contribute to the adhesion of B. pertussis to the cilia of the lung and the surface of macrophages through the B oligomer.
• Biological activities of PT arise from ADP ribosylations catalyzed by the S1 subunit and occur at nanogram per milliliter concentrations in vitro or from doses of 10 to 400 ng per mouse in vivo.

Other Antigens and Toxic Factors of B. pertussis

• In pertussis vaccines, manufacturers include other antigens which are surface proteins also expressed by other strains of Bordetella, but may include toxic factors of B. pertussis in future tests.
• Two commonly used surface proteins are pertactin, a protein with no well-defined crucial function, and FHA, which can promote adherence of the B. pertussis to eukaryotic cells.
• B. pertussis also produces a toxic protein with two known activities (calmodulin-activated adenylate cyclase activity and a hemolysin activity), termed ACT/HLY, is crucial for colonization in mice and produced by other strains of Bordetella.
• Bordetella also produces a dermonecrotic (causing skin necrosis), heat-labile toxin (DNT or HIT), but its role in colonization and disease is not well defined.
• B. pertussis also produces nonprotein toxic factors, a 921-Da muramyl peptide, termed tracheal cytotoxin (TCT), causes ciliostasis, and disrupts a key defense mechanism of the lung.
• The genes for PT, ACT/HLY, HIT, FHA, and pertactin (but not TCT), are under the control of a gene termed vir.

Therapy

• Erythromycin is effective in reducing contagion if you administer it to the patient early in the course of illness for treatment of B. pertussis, preferably during the catarrhal stage or early paroxysmal stage.
• Supplement oxygen and administer assisted respiratory ventilation, should be appropriate and necessary beyond antibiotic therapy.
• A new hyperimmune globulin prepared from human sera that has been vaccinated with a pertussis toxoid vaccine, reduces the frequency and severity of paroxysmal coughing with associated apnea.