Bordetella pertussis Symptoms: The 100-Day Whooping Cough

What Is Bordetella pertussis?
Global Burden of Whooping Cough
How Pertussis Toxin Causes Disease
Adenylate Cyclase Toxin and Adhesins
Who Is Most at Risk?
Epidemiological Cycles and Resurgence
The Three-Stage Illness: An Overview
Diagnosis Overview
When to Seek Emergency Care
Key Research Papers
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What Is Bordetella pertussis?

Bordetella pertussis is a small, gram-negative coccobacillus — meaning it is shaped somewhere between a rod and a sphere under the microscope. It is an obligate human pathogen, meaning it infects only humans and has no animal reservoir or environmental source. Every case of whooping cough comes from another person.

The bacterium is extraordinarily contagious. Its basic reproduction number (R₀) — the number of new people one infected person gives the disease to in a completely unvaccinated population — is estimated between 12 and 17. For comparison, measles runs 12–18 and seasonal flu runs about 1.2–1.4. This extreme contagiousness means that herd immunity requires vaccination coverage above 92–94% of the population, a target many communities do not maintain.

Pertussis spreads almost exclusively by respiratory droplets from coughing. The bacteria attach to the cilia lining the trachea and bronchi, and the toxins they release cause the characteristic paroxysmal cough, immune evasion, and in severe infant cases, life-threatening complications.

Global Burden of Whooping Cough

Despite widespread vaccination, whooping cough remains one of the most common vaccine-preventable diseases worldwide. The World Health Organization estimates approximately 24 million cases occur each year globally, resulting in around 160,000 deaths — the vast majority in children under 1 year of age, particularly in low- and middle-income countries where vaccine coverage is incomplete or inconsistent.

Even in high-income countries with strong vaccination programs, pertussis has resurged since the 1990s. The United States saw a major epidemic in 2012 with over 48,000 reported cases — the highest count since 1955. Australia, the United Kingdom, and many European countries have experienced similar periodic epidemics.

The resurgence is not due to declining vaccination rates alone. The shift from whole-cell vaccines (used through the 1990s) to acellular vaccines, which are safer but provide shorter-lived immunity, has played a significant role. Additionally, improved diagnostic testing (especially PCR) has uncovered cases that previously went undiagnosed.

How Pertussis Toxin Causes Disease

Pertussis toxin (PT) is the most important and distinctive virulence factor of B. pertussis. It is an AB₅ toxin — one active (A) subunit surrounded by five binding (B) subunits. The active subunit enters host cells and performs a biochemical action called ADP-ribosylation: it chemically modifies inhibitory G-proteins (Gᵢ proteins) on the inner surface of cell membranes, permanently locking them in an inactive state.

These G-proteins normally act as a brake on intracellular signaling. With the brake removed, cells become hyperactive. In immune cells — especially neutrophils and monocytes — this disrupts signaling pathways needed for migration to the site of infection. The immune cells cannot move effectively into the lungs to fight the bacteria. The infection is hidden in plain sight.

The most dramatic laboratory sign of pertussis toxin's effect is lymphocytosis: a sharp rise in circulating lymphocytes (a type of white blood cell). In mild adult cases, the total white blood cell count may reach 20,000–30,000/μL. In severe infant cases, it can exceed 50,000 to 100,000/μL. This extreme lymphocytosis is not just a laboratory curiosity — it directly causes the most dangerous complication of infant pertussis: pulmonary hypertension from lymphocyte accumulation in the lung vessels.

Adenylate Cyclase Toxin and Adhesins

Pertussis toxin is not the only weapon in B. pertussis's arsenal. The bacterium also produces adenylate cyclase toxin (ACT), which enters macrophages and neutrophils and floods them with cyclic AMP (cAMP). High cAMP paralyzes these immune cells — they cannot engulf and destroy bacteria the way they normally should. The bacteria effectively turn off the primary cellular cleanup crew.

Attachment to the respiratory epithelium requires specialized adhesion molecules. Filamentous hemagglutinin (FHA) is a long, thread-like protein that extends from the bacterial surface and binds to carbohydrate receptors on cilia — the hair-like projections that normally sweep bacteria out of the airway. Pertactin is another outer-membrane protein that promotes tight bacterial adhesion. Once attached, the bacteria resist being swept away, and toxin production begins in earnest.

The cilia themselves are progressively damaged by the infection. This ciliary dysfunction explains why the cough persists for so long even after the bacteria are killed: the physical damage to the airway lining takes weeks to months to fully repair, and during that time any respiratory irritant can trigger violent coughing fits.

Who Is Most at Risk?

Not all people face equal danger from pertussis. Those at greatest risk include:

Infants under 6 months: Too young to have completed the DTaP vaccination series (typically given at 2, 4, and 6 months). This group accounts for nearly all pertussis deaths in the United States and globally. They also receive the most serious disease — apnea, turning blue, severe pneumonia.
Newborns and young infants 0–2 months: The highest-risk subgroup. Maternal antibodies, if the mother was vaccinated during pregnancy, offer some protection, but this protection is imperfect and wanes.
Adolescents and adults with waned vaccine immunity: Immunity from both the childhood vaccine series and from natural infection fades over 4–12 years. Older children, teens, and adults become susceptible again and often spread the infection to vulnerable infants without knowing they have pertussis.
Immunocompromised individuals: Those on immunosuppressive drugs, with HIV, or undergoing chemotherapy may experience prolonged or unusually severe disease.
Healthcare workers: At occupational risk of exposure and of transmitting the disease to vulnerable patients.

Epidemiological Cycles and Resurgence

Even in highly vaccinated populations, pertussis follows a cyclical pattern, with epidemics occurring every 3–5 years. This reflects the gradual accumulation of susceptible individuals — people whose vaccine immunity has waned and who have not had a recent booster — until a large enough pool of susceptibility allows widespread transmission.

The 2010–2012 epidemic in the United States was particularly striking. California declared a pertussis epidemic in 2010, with 9,159 cases and 10 infant deaths — the worst outbreak since 1955. By 2012, the US reported 48,277 cases nationally. Studies showed that children who had received 5 doses of the acellular DTaP vaccine had substantially lower protection than historical whole-cell vaccine recipients by the time they reached age 8–12.

A landmark 2014 study using a baboon model confirmed what epidemiologists had long suspected: acellular pertussis vaccines prevent disease symptoms but do not prevent infection or transmission. Vaccinated animals could carry and shed B. pertussis without becoming ill — a phenomenon that helps explain ongoing spread even in well-vaccinated communities.

The Three-Stage Illness: An Overview

Whooping cough follows a characteristic three-stage progression that earned it the historical name "the 100-day cough." Understanding each stage is critical both for recognizing the disease and for knowing when treatment is most effective.

Catarrhal stage (1–2 weeks): Looks like a common cold — runny nose, watery eyes, mild cough, maybe a low-grade fever. This is deceptively benign, but it is the most contagious stage. Most transmission happens here, before anyone suspects pertussis.
Paroxysmal stage (2–8 weeks): The disease's signature phase. Explosive bursts of many rapid coughs in a single breath, followed by a high-pitched "whoop" as air rushes in through a partially closed larynx. Vomiting after coughing fits is common. In infants, the whoop may be absent — they may simply stop breathing instead.
Convalescent stage (weeks to months): The cough gradually diminishes, but the airway remains hypersensitive. Any respiratory irritant can trigger a return of paroxysmal coughing.

For a detailed breakdown of each stage, see the Three-Stage Illness page.

Diagnosis Overview

Pertussis is one of the most commonly missed diagnoses in medicine because early symptoms mimic a common cold and the classic "whoop" is often absent in vaccinated patients. The three main laboratory approaches are:

PCR (polymerase chain reaction): The test of choice in the first 3 weeks of cough. Highly sensitive and specific. Requires a proper nasopharyngeal swab — not just a nasal swab or throat swab.
Culture: The historical gold standard, but slow (7–12 days for results) and technically demanding. Mostly used today for surveillance and antibiotic susceptibility testing.
Serology (anti-PT IgG): Useful after week 3–4 when PCR becomes negative. A single high antibody level in someone not recently vaccinated strongly suggests recent pertussis.

For full details on how and when to use each test, see the Diagnosis: PCR, Culture, and Serology page.

When to Seek Emergency Care

Most older children and adults with whooping cough can be managed at home with antibiotics, rest, and supportive care. However, some situations require emergency evaluation:

Any infant under 3–4 months with suspected pertussis should be seen urgently — hospital admission is often appropriate even before the diagnosis is confirmed.
Apnea or turning blue (cyanosis) during a coughing fit: Call 911 or go to the nearest emergency room immediately.
Severe dehydration: Infants and young children who vomit after every coughing fit may become dangerously dehydrated and need IV fluids.
Difficulty breathing between coughing fits: Suggests pneumonia or other complications requiring hospital evaluation.
Seizures: Can occur due to hypoxia (low oxygen) from severe coughing fits.
Extreme lethargy or reduced responsiveness: May indicate hypoxia or a serious systemic complication.

In children under 2 months, any cough at all in a febrile infant with known pertussis exposure warrants immediate medical attention — do not wait to see if it worsens.

Key Research Papers

Mattoo S, Cherry JD. Molecular pathogenesis, epidemiology, and clinical manifestations of respiratory infections due to Bordetella pertussis. Clin Microbiol Rev. 2005;18(2):326–82. PMID 15831828
Cherry JD. Epidemic pertussis in 2012 — the resurgence of a vaccine-preventable disease. N Engl J Med. 2012;367(9):785–7. PMID 22931317
Warfel JM, Zimmerman LI, Merkel TJ. Acellular pertussis vaccines protect against disease but fail to prevent infection and transmission in a nonhuman primate model. Proc Natl Acad Sci USA. 2014;111(2):787–92. PMID 24277828
Paddock CD, et al. Pathology and pathogenesis of fatal Bordetella pertussis infection in infants. Clin Infect Dis. 2008;47(3):328–38. PMID 18558873
Carbonetti NH. Pertussis leukocytosis: mechanisms, clinical implications and treatment considerations. Curr Opin Infect Dis. 2016;29(3):257–64. PMID 26986441
Kilgore PE, et al. Pertussis: Microbiology, Disease, Treatment, and Prevention. Clin Microbiol Rev. 2016;29(3):449–86. PMID 27029594
Patel M, et al. Epidemiology of pertussis in the United States. Pediatrics. 2021;147(3):e2020008946. PMID 33608490
WHO. Pertussis vaccines: WHO position paper. Wkly Epidemiol Rec. 2015;90(35):433–58. PMID 26320265
Yeung KH, et al. An update of the global burden of pertussis in children younger than 5 years: a modelling study. Lancet Infect Dis. 2017;17(9):974–980. PMID 28623146
Burns DL. Secretion of Pertussis Toxin from Bordetella pertussis. Toxins. 2021;13(8):574. PMID 34437443

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