Bordetella pertussis is a Gram-negative coccobacillus responsible for causing pertussis, a highly contagious respiratory infection also known as whooping cough due to the characteristic "whoop" sound during inhalation after severe coughing fits [1]. First isolated in 1906 by microbiologists Jules Bordet and Octave Gengou, the bacterium spreads through airborne droplets released when infected individuals cough, sneeze, or talk, with peak contagiousness occurring during the early stages of illness [2]. The disease progresses through three distinct phases: an initial catarrhal stage resembling the common cold, a paroxysmal stage marked by violent coughing spells often followed by vomiting and cyanosis, and a prolonged convalescent phase that can last for months—earning it the nickname "the 100-day cough" [3]. In infants, particularly those unvaccinated or partially vaccinated, pertussis can present atypically with life-threatening apnea rather than coughing, leading to severe complications such as pneumonia, seizures, encephalopathy, and even death [1]. The pathogenesis of B. pertussis involves multiple virulence factors, including pertussis toxin, which disrupts immune signaling and promotes bacterial survival, and adenylate cyclase toxin, which impairs phagocyte function [5]. Diagnosis relies on clinical evaluation supported by laboratory methods such as polymerase chain reaction (PCR), considered the most sensitive technique in early disease, and bacterial culture on specialized media like Bordet-Gengou agar, though its use is limited by low sensitivity and long incubation times [6]. Serological testing may aid in diagnosing late-stage infections. Prevention primarily hinges on vaccination with acellular vaccines (DTaP/Tdap), which contain purified antigens such as pertussis toxin, filamentous hemagglutinin, and pertactin, and are administered as part of routine childhood immunization schedules [7]. Despite high vaccine coverage, pertussis has re-emerged globally due to waning immunity from acellular vaccines, antigenic shifts in circulating strains (e.g., pertactin-deficient variants), and suboptimal vaccination rates, particularly among pregnant women and adults [8]. Treatment typically involves macrolide antibiotics like azithromycin to reduce transmission, especially when initiated early, while severe cases in infants often require hospitalization and intensive supportive care [3]. Public health strategies to protect vulnerable populations include maternal vaccination during pregnancy to confer passive immunity to newborns, cocooning strategies targeting close contacts of infants, and booster doses in adolescents and adults to maintain herd immunity [10].

Microbiology and Discovery

Bordetella pertussis is a Gram-negative, coccobacillus-shaped bacterium responsible for causing pertussis, a highly contagious respiratory disease also known as whooping cough [1]. This pathogen is strictly aerobic and fastidious in its nutritional requirements, necessitating enriched media for growth in laboratory settings [12]. It is non-motile, does not form spores, and possesses a capsule that contributes to its virulence by aiding in immune evasion [3]. The bacterium is highly adapted to the human host and has no known animal reservoirs, making human-to-human transmission the sole route of infection [14].

Isolation and Laboratory Identification

The bacterium was first isolated in 1906 by the Belgian microbiologists Jules Bordet and Octave Gengou, after whom it is named [15]. Their pioneering work laid the foundation for understanding the etiology of pertussis. For laboratory isolation, clinical specimens such as nasopharyngeal swabs or aspirates are collected during the early catarrhal stage of the disease, typically within the first two to three weeks of symptom onset, when bacterial load is highest [6].

The traditional culture medium used for isolating B. pertussis is Bordet-Gengou agar, a nutrient-rich medium supplemented with sheep blood (5–10%) and glycerol to support the bacterium's fastidious growth needs [17]. To enhance selectivity and inhibit contaminating flora, antibiotics such as cephalexin or methicillin are added, creating a selective medium. Colonies of B. pertussis appear after 3 to 7 days of incubation at 35–37°C in a humid atmosphere, exhibiting a characteristic pearly-gray, convex, lens-shaped morphology with a partial zone of hemolysis around mature colonies [17].

Definitive identification is achieved through a combination of morphological, biochemical, and immunological methods. The bacterium is catalase-positive and oxidase-positive but does not ferment carbohydrates, a trait known as asaccharolytic metabolism [19]. Agglutination tests using specific antisera confirm the species, while modern laboratories increasingly rely on molecular techniques such as real-time polymerase chain reaction (PCR) to detect species-specific genetic markers like IS481, ptxS1, and prn [6].

Limitations of Traditional Culture

Despite being considered the historical gold standard for diagnosis, bacterial culture has significant limitations. Its sensitivity is relatively low, ranging from 10% to 60%, particularly when samples are collected late in the disease course or after antibiotic treatment has begun [6]. The slow growth rate necessitates prolonged incubation (up to 14 days), delaying diagnosis and public health interventions [22]. Additionally, the Bordet-Gengou medium is difficult to prepare and unstable over time, requiring precise conditions of humidity and temperature to maintain viability [17].

Environmental factors such as drying and temperature fluctuations during sample transport can further compromise bacterial viability, increasing the risk of false-negative results. Due to these constraints, culture has been largely supplanted by more sensitive and rapid methods like PCR in routine diagnostics, although it remains valuable for antimicrobial susceptibility testing and epidemiological surveillance in reference laboratories [24].

Discovery and Historical Context

The discovery of Bordetella pertussis by Jules Bordet and Octave Gengou marked a pivotal moment in the history of microbiology and infectious disease research. Prior to their work, the cause of whooping cough was unknown, and the disease was a leading cause of childhood morbidity and mortality. Their successful isolation of the bacterium enabled the development of the first whole-cell pertussis vaccines, which significantly reduced disease burden in the 20th century [25]. The genus Bordetella was later named in honor of Jules Bordet, recognizing his contributions to immunology and bacteriology, including his Nobel Prize-winning work on the complement system [15].

The identification of B. pertussis also paved the way for understanding the disease's transmission dynamics, which occur exclusively via airborne droplets released during coughing, sneezing, or talking [2]. The average incubation period is 7 to 10 days, though it can range from 4 to 21 days, during which the infected individual becomes progressively contagious [7]. The high transmissibility of the bacterium, with an estimated basic reproduction number (R0) between 12 and 17, underscores the importance of early detection and containment strategies [7].

Today, while culture remains a reference method, the integration of molecular diagnostics has revolutionized the detection and surveillance of B. pertussis. Nevertheless, the foundational work of Bordet and Gengou continues to inform modern approaches to the prevention, diagnosis, and control of pertussis, highlighting the enduring impact of their discovery on global public health.

Pathogenesis and Virulence Factors

The pathogenesis of Bordetella pertussis involves a sophisticated array of virulence factors that enable the bacterium to colonize the respiratory tract, evade host immune defenses, and cause extensive tissue damage, leading to the characteristic clinical manifestations of pertussis. These mechanisms are orchestrated through a combination of adhesins, toxins, and immunomodulatory molecules that act synergistically during infection.

Adhesion to Respiratory Epithelial Cells

The initial step in the pathogenesis of B. pertussis is the attachment to ciliated epithelial cells lining the upper respiratory tract. This critical process is mediated by several surface-exposed adhesins:

  • Filamentous hemagglutinin (FHA): One of the most important adhesins, FHA facilitates bacterial adherence to host respiratory epithelial cells. It contains an Arg-Gly-Asp (RGD) domain that interacts with integrin receptors on host cells, enabling stable anchoring to the mucosal surface [30]. This interaction is essential for establishing colonization [31].

  • Pertactin (PRN): An outer membrane protein that functions as an adhesin, PRN also contains RGD sequences that enhance binding to host cell receptors. It plays a key role in bacterial attachment and interaction with epithelial cells [32]. Mutations in the prn gene can impair adhesion capacity, contributing to antigenic variation [30].

  • Fimbriae (FIM): These filamentous structures promote bacterial aggregation and adherence to specific receptors on respiratory epithelial cells, further supporting initial colonization [32].

Key Toxins in Pathogenesis

The hallmark symptoms of pertussis, including paroxysmal coughing and systemic effects, are largely driven by potent exotoxins produced by B. pertussis:

  • Pertussis toxin (PT): A complex A-B exotoxin central to the bacterium's virulence, PT disrupts cellular signaling by inhibiting G proteins in host cells. This interference impairs leukocyte chemotaxis, phagocytosis, and both innate and adaptive immune responses [5]. PT also induces lymphocytosis by promoting lymphocyte accumulation in the bloodstream—a characteristic clinical finding in pertussis [3]. Furthermore, PT contributes to neurological and respiratory symptoms, including the paroxysmal cough [37].

  • Adenylate cyclase toxin (ACT): This toxin enters host cells—including macrophages, neutrophils, and epithelial cells—and catalyzes the unregulated conversion of ATP to cyclic AMP (cAMP). Elevated intracellular cAMP levels suppress phagocytic functions, inhibit oxidative burst, and block phagolysosome maturation, thereby neutralizing the bactericidal activity of phagocytes [5]. ACT also induces apoptosis in macrophages and stimulates the release of pro-inflammatory cytokines such as IL-6 from tracheal epithelial cells, exacerbating local inflammation [39].

Additional Virulence Factors

Beyond adhesion and toxin production, B. pertussis employs several other factors to enhance its survival and pathogenicity:

  • Lipooligosaccharide (LOS): Functionally analogous to the lipopolysaccharide of other Gram-negative bacteria, LOS activates the host’s immune system via TLR4, triggering the release of inflammatory cytokines such as TNF-α, IL-1β, and IL-6. This response contributes to epithelial damage and respiratory symptoms [40].

  • Tracheal cytotoxin (TCT): A peptidoglycan fragment released by B. pertussis, TCT directly damages ciliated epithelial cells in the trachea. It inhibits ciliary beating and disrupts mucociliary clearance, leading to mucus accumulation and persistent coughing [41].

  • Dermonecrotic toxin (DNT): This toxin induces vasoconstriction and tissue necrosis, potentially contributing to localized airway damage, although its precise role in human disease remains incompletely defined [42].

Immunomodulation and Infection Persistence

B. pertussis employs its virulence factors not only to cause tissue injury but also to actively subvert host immunity. Pertussis toxin and adenylate cyclase toxin suppress leukocyte activation and recruitment, delaying bacterial clearance and allowing prolonged persistence in the respiratory tract—often for several weeks [40]. Additionally, the bacterium interferes with antigen presentation and skews T-cell differentiation, disrupting the balance between Th1/Th17 and Th2 responses and weakening protective immunity [42].

The coordinated action of these virulence mechanisms—adhesion via FHA, PRN, and FIM, combined with the destructive effects of PT, ACT, TCT, and LOS—explains the severe clinical features of pertussis, including paroxysmal coughing, apnea, cyanosis, and vomiting. Understanding these pathogenic processes is crucial for developing improved therapeutic strategies and next-generation vaccines that can overcome the limitations of current acellular formulations [3].

Clinical Presentation and Disease Progression

Bordetella pertussis infection, known as pertussis or whooping cough, progresses through three distinct clinical phases: the catarrhal stage, the paroxysmal stage, and the convalescent stage. The disease typically begins with mild, cold-like symptoms and evolves into severe, prolonged coughing episodes that can last for weeks or months, earning it the colloquial name "the 100-day cough" [3]. The severity and presentation of symptoms vary significantly depending on age, vaccination status, and immune maturity.

Clinical Phases of Pertussis

Catarrhal Stage (1–2 Weeks)

The initial phase of pertussis resembles a common upper respiratory tract infection and lasts approximately 1–2 weeks. Symptoms include rhinorrhea, nasal congestion, sneezing, low-grade fever, and a mild, intermittent cough [47]. This stage is highly contagious, as the bacterium actively colonizes the respiratory epithelium and is readily transmitted through respiratory droplets. However, due to its nonspecific presentation, pertussis is often misdiagnosed at this stage, delaying appropriate isolation and treatment [47].

Paroxysmal Stage (2–6 Weeks or Longer)

This phase is characterized by intense, uncontrollable coughing fits known as paroxysms. These episodes are often followed by a high-pitched inspiratory "whoop" sound, particularly in older children and unvaccinated individuals, resulting from the rapid inhalation of air against a narrowed glottis after forceful coughing [49]. The term "whooping cough" is derived from this distinctive sound. During these paroxysms, individuals may experience facial cyanosis, extreme fatigue, and post-tussive vomiting due to the physical strain of coughing [49].

In vaccinated individuals or adolescents and adults, the "whoop" may be absent, and symptoms are often milder or atypical, manifesting as a persistent cough without the classic paroxysmal features. This atypical presentation can lead to underdiagnosis and continued transmission within the community [51].

Convalescent Stage (Several Weeks to Months)

The recovery phase is gradual, with the frequency and severity of coughing episodes decreasing over time. However, the cough may persist for several weeks or even months, particularly if the individual develops subsequent respiratory infections [3]. This prolonged coughing period is the basis for the nickname "the 100-day cough." While the contagious period diminishes significantly after the first few weeks of illness, especially with appropriate antibiotic treatment, the lingering symptoms can lead to complications such as sleep disturbances, weight loss, and social disruption [53].

Atypical Presentation in Infants and Neonates

In infants, especially those under six months of age and not yet fully vaccinated, pertussis often presents atypically and can be life-threatening. The classic "whoop" is frequently absent, and the most concerning manifestation is the occurrence of apnea—episodes of breathing cessation [54]. These apneic spells can lead to cyanosis, bradycardia, and, in severe cases, respiratory failure or sudden infant death [1]. Other signs in neonates include feeding difficulties, vomiting, and lethargy. Because of their immature immune systems and underdeveloped respiratory musculature, infants are at the highest risk for severe complications, including pneumonia, seizures, and encephalopathy due to hypoxia [56].

The absence of a prominent cough in young infants makes clinical diagnosis particularly challenging, and a high index of suspicion is required, especially in the context of known exposure to an infected individual [14]. Early diagnosis and hospitalization are critical for these vulnerable patients, as they often require intensive supportive care, including oxygen therapy and respiratory monitoring [3].

Symptom Variability in Vaccinated Individuals

Vaccinated individuals, including adolescents and adults, typically experience a milder form of pertussis. Their symptoms may resemble a persistent bronchitis rather than the classic paroxysmal cough. The absence of the "whoop" and reduced severity can lead to delayed diagnosis and treatment, allowing for continued transmission of B. pertussis within households and communities [7]. Despite milder illness, vaccinated individuals can still serve as reservoirs of infection, particularly for unvaccinated or partially vaccinated infants who are at greatest risk for severe outcomes [14].

The waning immunity associated with acellular vaccines contributes to the increasing incidence of pertussis in older age groups. This shift in disease burden underscores the importance of booster vaccinations in adolescents and adults to maintain herd immunity and protect vulnerable populations [7]. Public health strategies such as maternal vaccination during pregnancy and cocooning—vaccinating close contacts of newborns—are designed to bridge the immunity gap in early infancy [10].

The clinical course of pertussis, while variable, consistently poses the greatest danger to infants. Understanding the progression of the disease and its atypical presentations is essential for timely diagnosis, effective treatment, and the implementation of preventive measures to curb transmission and protect public health.

Diagnosis and Laboratory Methods

Accurate diagnosis of Bordetella pertussis infection is essential for timely treatment, effective public health interventions, and the control of outbreaks. Given the atypical or mild presentation in vaccinated individuals and adults, clinical suspicion must be supported by laboratory confirmation. The primary methods used for diagnosing pertussis include polymerase chain reaction (PCR), bacterial culture, and serological testing, each with distinct advantages and limitations depending on the stage of illness and clinical context [63].

Polymerase Chain Reaction (PCR)

PCR is the most sensitive and widely used method for diagnosing pertussis, particularly during the early stages of infection. It detects the DNA of B. pertussis in clinical specimens, typically collected via nasopharyngeal swab or aspirate. The technique is highly effective when performed within the first 3–4 weeks after the onset of cough, when bacterial load is highest [6]. PCR offers rapid results, often within hours, making it ideal for guiding early clinical decisions and implementing infection control measures.

The use of real-time PCR allows for the specific identification of B. pertussis and differentiation from related species such as Bordetella parapertussis, although cross-reactivity due to shared genetic sequences (e.g., IS481) can occasionally complicate interpretation [65]. PCR's high sensitivity surpasses that of bacterial culture, especially in patients who have already received antibiotics or in whom the organism is no longer viable [66]. However, a positive PCR result may persist after clinical recovery, so results must be interpreted in conjunction with clinical findings to avoid overdiagnosis [63].

Bacterial Culture

Bacterial culture remains the historical gold standard for pertussis diagnosis due to its high specificity (approaching 100%), but it suffers from low sensitivity and practical limitations. Culture requires the isolation of B. pertussis from nasopharyngeal specimens on specialized media such as Bordet-Gengou agar, enriched with sheep blood and glycerol, and often supplemented with antibiotics like cefalexin to inhibit contaminating flora [17].

Colonies of B. pertussis are slow-growing, typically appearing after 3–7 days of incubation at 35–37°C in a humid environment. They exhibit a characteristic "pearl-like" appearance with a partial zone of hemolysis [22]. Identification is confirmed through morphological, biochemical (catalase and oxidase positivity, saccharolytic negativity), and serological agglutination tests [19].

Despite its diagnostic value, culture has several drawbacks: sensitivity ranges from 10% to 60%, particularly when specimens are collected late in the illness or after antibiotic administration [6]. The organism is fastidious and sensitive to environmental conditions, including temperature fluctuations and drying, which can compromise viability during transport [22]. For these reasons, culture is now primarily reserved for reference laboratories conducting antimicrobial susceptibility testing and epidemiological surveillance, rather than routine clinical diagnosis [24].

Serological Testing

Serology plays a crucial role in diagnosing pertussis during the later stages of illness, when direct detection methods like PCR and culture are likely to yield negative results. It measures the host’s immune response by detecting antibodies, particularly IgG against pertussis toxin (IgG-PT), which is highly specific for B. pertussis infection [74].

Serological testing is especially useful for confirming suspected cases with prolonged cough (beyond four weeks), atypical presentations in adolescents and adults, and in epidemiological studies assessing disease burden and seroprevalence [63]. However, interpretation is complicated by prior vaccination, which can induce antibodies that cross-react with vaccine antigens, potentially leading to false-positive results [76]. To improve accuracy, some guidelines recommend using standardized cutoff values or demonstrating seroconversion between acute and convalescent serum samples collected 2–4 weeks apart [77].

Comparative Use and Clinical Context

The choice of diagnostic method depends on the clinical phase of the disease:

  • PCR is preferred during the acute phase (within 4 weeks of cough onset) due to its high sensitivity and rapid turnaround. It is particularly valuable in infants and young children with classic symptoms such as paroxysmal cough, post-tussive vomiting, and inspiratory "whoop" [63].
  • Culture is less commonly used in routine practice but remains important for public health surveillance, strain characterization, and monitoring of antimicrobial resistance, especially in light of emerging macrolide-resistant isolates [79].
  • Serology is most appropriate for late or retrospective diagnosis, particularly in cases of chronic cough where pertussis is suspected but direct detection methods are negative [80].

Integration of clinical, epidemiological, and laboratory data is essential for accurate diagnosis. The World Health Organization (WHO) and national health agencies emphasize the need for improved case definitions and enhanced surveillance systems to address underdiagnosis, particularly in adults and vaccinated populations where symptoms may be subtle [81]. Advances in molecular diagnostics, including multiplex PCR panels for respiratory pathogens, continue to improve the detection and management of pertussis in clinical settings [63].

In summary, while PCR is the cornerstone of modern pertussis diagnosis, a combination of methods—tailored to the timing of specimen collection and patient population—ensures optimal detection. Ongoing research into novel biomarkers and point-of-care tests may further enhance diagnostic accuracy and public health response in the future [83].

Treatment and Management

The management of Bordetella pertussis infection, the causative agent of pertussis, involves a combination of antibiotic therapy, supportive care, and public health interventions to reduce transmission and prevent complications, particularly in vulnerable populations such as infants and young children. Early diagnosis and prompt treatment are critical to minimizing disease severity and limiting community spread [3].

Antibiotic Therapy

Antibiotic treatment is a cornerstone of pertussis management, primarily aimed at eradicating the bacterium, reducing contagiousness, and preventing further transmission. The most effective outcomes are achieved when antibiotics are initiated early in the course of illness, ideally during the catarrhal stage, before the onset of severe paroxysmal coughing [85].

The antibiotics of choice belong to the macrolide class, which are bacteriostatic agents that inhibit protein synthesis in B. pertussis. The preferred macrolides include:

  • azithromycin
  • clarithromycin
  • erythromycin

Among these, azithromycin is often favored due to its favorable side effect profile, shorter treatment duration, and better gastrointestinal tolerance. Erythromycin, while historically used, is associated with an increased risk of infantile pyloric stenosis, particularly in neonates under six weeks of age, necessitating careful monitoring [14].

For patients who cannot tolerate macrolides or in cases of suspected macrolide-resistant strains, alternative antibiotics such as trimethoprim-sulfamethoxazole (cotrimoxazole) may be considered, although evidence for its efficacy is less robust [87]. tetracycline is effective in adults but is contraindicated in children under eight years of age due to the risk of permanent tooth discoloration and enamel hypoplasia [88].

The duration of antibiotic therapy varies by agent: azithromycin is typically administered for five days, clarithromycin for seven days, and erythromycin for 14 days. Regardless of the antibiotic used, treatment reduces the period of infectivity from approximately three weeks (untreated) to just five days after initiation of therapy, underscoring its importance in infection control [87].

Supportive Care and Hospitalization

Beyond antibiotics, supportive care is essential, particularly in severe cases. Management focuses on ensuring adequate hydration, nutrition, and respiratory support. Frequent small sips of fluids are recommended to prevent dehydration, especially in infants who may experience vomiting after paroxysms of coughing [3]. Nutritional intake should be carefully monitored, with fractional feeding to minimize post-tussive vomiting.

In severe cases, particularly among infants under six months of age, hospitalization is often required. Neonates are at high risk for life-threatening complications such as apnea, pneumonia, and respiratory failure, necessitating intensive monitoring and intervention [1]. Hospitalized patients may require supplemental oxygen, continuous cardiorespiratory monitoring, and in some cases, mechanical ventilation to support breathing during apneic episodes [19].

Post-Exposure Prophylaxis

To prevent secondary cases, especially among high-risk individuals, post-exposure antibiotic prophylaxis (PEP) is recommended for close contacts of confirmed pertussis cases. This includes household members, caregivers, healthcare workers, and particularly infants, pregnant women, and immunocompromised individuals [93].

The same macrolide antibiotics used for treatment are recommended for prophylaxis, with azithromycin being the preferred agent due to its safety and tolerability. PEP should be administered as soon as possible, ideally within 21 days of exposure, to prevent the development of symptomatic infection and interrupt transmission chains [94].

Management of Complications

Pertussis can lead to a range of complications, particularly in unvaccinated or partially vaccinated infants. These include pneumonia, which is the most common complication, as well as seizures and encephalopathy resulting from prolonged hypoxia during coughing fits or apnea [95]. In adults, complications may include rib fractures, hernias, and syncope due to the physical strain of severe coughing [14].

Neurological complications such as encephalopathy are rare but serious, often linked to cerebral hypoxia or direct neurotoxic effects of pertussis toxin [97]. Management of these complications requires a multidisciplinary approach, including neurology consultation and supportive therapies such as anticonvulsants for seizures.

Emerging Challenges: Antibiotic Resistance

An increasing concern in the treatment of pertussis is the emergence of macrolide-resistant strains of B. pertussis. Resistance is primarily mediated by a point mutation (A2047G) in the 23S rRNA gene, which reduces the binding affinity of macrolides to the bacterial ribosome [98]. While most circulating strains remain susceptible, resistance has been documented in several countries, including China, where near-complete resistance to erythromycin has been reported [99].

This development has significant implications for both treatment and prophylaxis strategies. In regions with documented resistance, alternative antibiotics such as trimethoprim-sulfamethoxazole may need to be used more frequently. Enhanced surveillance of antimicrobial susceptibility is essential to guide clinical decisions and public health policies [100].

Integration with Vaccination Strategies

Effective treatment and management of pertussis must be integrated with broader prevention strategies, particularly vaccination. The limitations of current acellular vaccines—such as waning immunity and reduced effectiveness against transmission—highlight the need for continued booster doses in adolescents and adults to maintain herd immunity and protect infants who are too young to be fully vaccinated [101].

In conclusion, the treatment and management of Bordetella pertussis infection require a multifaceted approach that combines timely antibiotic therapy, vigilant supportive care, post-exposure prophylaxis, and integration with vaccination programs. As antibiotic resistance and antigenic shifts in circulating strains pose new challenges, ongoing surveillance and adaptation of clinical guidelines are essential to ensure effective control of this re-emerging infectious disease [10].

Vaccination and Prevention Strategies

Vaccination remains the cornerstone of prevention against pertussis, a highly contagious respiratory disease caused by Bordetella pertussis. The primary strategy involves the administration of acellular vaccines (DTaP for children and Tdap for adolescents and adults), which contain purified antigens such as pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (PRN), and fimbrial proteins (Fim2/3) [7]. These vaccines are typically administered as part of combination formulations that also protect against diphtheria and tetanus, forming the DTaP or Tdap vaccines widely used in national immunization schedules.

Acellular vs. Whole-Cell Vaccines and Immune Response

The transition from whole-cell pertussis vaccines (DTPw) to acellular formulations (DTaP) was driven by the superior safety profile of the latter, with significantly fewer adverse reactions such as fever and local inflammation [104]. However, this shift has implications for the quality and duration of the immune response. Whole-cell vaccines, which contain inactivated whole bacteria, stimulate a broad and balanced immune response involving both Th1 and Th17 T helper cells, resulting in longer-lasting immunity—estimated at over 10 years [101]. In contrast, acellular vaccines predominantly induce a Th2-type response, which is less effective at generating long-term mucosal immunity and wanes significantly after 7–8 years [101]. This decline in protection contributes to the resurgence of pertussis in older children and adults, who may serve as reservoirs for transmission to vulnerable infants.

Infant Vaccination and Booster Doses

The recommended vaccination schedule includes a primary series of three doses administered during infancy—at 3, 5, and 11 months of age—followed by booster doses at 5–6 years and around 12 years to maintain immunity [107]. Despite high initial coverage, delays in vaccination timing increase the risk of infection in early life, particularly in infants under 6 months who have not completed their primary series and are at highest risk for severe complications such as apnea, pneumonia, and encephalopathy [108]. Booster doses in adolescence and adulthood are critical not only for individual protection but also for sustaining herd immunity and reducing community transmission.

Maternal Vaccination and Cocooning

To protect newborns during their most vulnerable period, maternal vaccination during pregnancy is strongly recommended. The Tdap vaccine is administered between the 27th and 36th weeks of gestation to maximize the transplacental transfer of protective IgG antibodies to the fetus [10]. This strategy has been shown to reduce the risk of pertussis in infants by up to 91% in the first two months of life and by 69% during the first year [110]. The effectiveness of maternal vaccination surpasses that of the "cocooning" strategy, which involves vaccinating close contacts of the infant—such as parents, siblings, and caregivers—to create a protective barrier. While cocooning is conceptually sound, its real-world impact is limited by low adherence; studies show vaccination rates among family members often fall below 50%, reducing its overall efficacy [111].

Challenges to Vaccine Effectiveness

Despite high vaccination coverage, pertussis has re-emerged globally due to several interrelated factors. First, the waning immunity associated with acellular vaccines allows for silent circulation of B. pertussis in vaccinated populations, particularly among adolescents and adults who may experience mild or atypical symptoms [112]. Second, evolutionary pressure from vaccine-induced immunity has led to the emergence of antigenic variants, including strains that lack expression of pertactin (PRN-negative), a key component of acellular vaccines [8]. These "vaccine escape" variants may have a selective advantage in highly vaccinated populations, potentially reducing vaccine effectiveness over time [114].

Additionally, while herd immunity is essential for protecting unvaccinated or partially vaccinated individuals, the extremely high transmissibility of B. pertussis—with an estimated basic reproduction number (R0) between 12 and 17—makes it difficult to achieve and sustain [7]. Even small gaps in coverage can compromise population-level protection, especially in regions where vaccination rates have declined due to pandemic-related disruptions or vaccine hesitancy [116].

Public Health Strategies and Future Directions

Public health efforts must integrate multiple layers of protection. These include maintaining high infant vaccination coverage (>95%), ensuring timely administration of boosters, promoting maternal immunization, and implementing targeted prophylaxis for high-risk contacts. Post-exposure antibiotic prophylaxis with macrolides such as azithromycin is recommended for close contacts of confirmed cases, especially neonates, pregnant women, and healthcare workers, to prevent secondary transmission [93].

Looking ahead, research is focused on developing next-generation vaccines that induce more durable and broad-spectrum immunity. Candidates under investigation include live attenuated nasal vaccines, novel adjuvants, and formulations designed to elicit stronger Th1/Th17 responses similar to those induced by natural infection or whole-cell vaccines [118]. Such innovations aim to overcome the limitations of current acellular vaccines and provide longer-lasting protection against both disease and transmission.

In conclusion, while existing vaccination strategies have dramatically reduced pertussis mortality, ongoing challenges such as waning immunity, antigenic drift, and suboptimal coverage necessitate a multifaceted approach. Strengthening vaccination programs, improving adherence to maternal and booster immunizations, and advancing vaccine technology are all essential to controlling the persistent threat of pertussis, particularly among the most vulnerable populations [119].

Epidemiology and Resurgence Trends

The epidemiology of pertussis has undergone significant shifts over the past century, shaped by the introduction and evolution of vaccination programs, changes in diagnostic practices, and the adaptive evolution of Bordetella pertussis itself. While the widespread implementation of vaccines drastically reduced the incidence and mortality of pertussis in the mid-20th century, the disease has experienced a notable resurgence in many high-income countries since the 1990s, characterized by cyclical outbreaks and a shifting burden of disease toward adolescents, adults, and, most critically, vulnerable infants [25].

Historical Impact of Vaccination and Shift to Acellular Vaccines

Before the advent of vaccination, pertussis was a leading cause of childhood morbidity and mortality, with regular epidemic cycles occurring every 3–5 years [121]. The introduction of the whole-cell pertussis (wP) vaccine, part of the combined diphtheria-tetanus-pertussis (DTP) vaccine, led to a dramatic decline in cases, demonstrating the profound impact of mass immunization on disease control. However, concerns over the reactogenicity of wP vaccines prompted the development and global adoption of acellular pertussis (aP) vaccines in the 1990s, which contain purified antigens such as pertussis toxin, filamentous hemagglutinin (FHA), and pertactin (PRN) [122].

While aP vaccines are safer and better tolerated, they confer a less durable immunity compared to wP vaccines, with protection waning significantly within 7–8 years after the primary series [101]. This shorter duration of immunity has fundamentally altered the epidemiological landscape, reducing the circulation of the bacterium in the general population but simultaneously allowing for the accumulation of susceptible adolescents and adults who can serve as reservoirs for transmission to unvaccinated or incompletely vaccinated infants [7].

Global Resurgence and Recent Outbreak Trends

Despite high childhood vaccination coverage in many countries, pertussis has re-emerged as a significant public health concern. In Europe, over 25,000 cases were reported in 2023, with more than 32,000 cases in the first quarter of 2024 alone, signaling a clear upward trend [121]. Italy has been particularly affected, with an 800% increase in hospitalizations for pertussis among infants and young children in 2024 compared to previous years [126]. Between January and May 2024, Italy reported at least 110 hospitalizations for pertussis in neonates, including several deaths, highlighting the severe consequences of the disease in the most vulnerable populations [127].

These recent outbreaks are not isolated to Europe. The resurgence is a global phenomenon, driven by a confluence of factors including waning immunity, suboptimal vaccination coverage in certain age groups and regions, and disruptions to routine immunization services, such as those caused by the COVID-19 pandemic [116]. The cyclical nature of pertussis epidemics appears to be intensifying, with peaks occurring more frequently and affecting larger populations of susceptible individuals who have lost their vaccine-induced protection [129].

Factors Driving the Resurgence

The resurgence of pertussis is attributed to a complex interplay of biological, immunological, and sociological factors:

  1. Waning Immunity from Acellular Vaccines: The primary driver is the limited duration of protection offered by aP vaccines. As immunity wanes in adolescents and adults, this creates a large pool of susceptible individuals who can contract and transmit the infection, often with mild or atypical symptoms that go undiagnosed [101].
  2. Vaccine-Driven Evolution of the Pathogen: The selective pressure exerted by aP vaccines has led to the emergence and spread of B. pertussis strains with antigenic differences from the vaccine strains. The most prominent example is the increasing prevalence of pertactin-deficient (PRN-negative) strains, which are thought to have a selective advantage in evading the immune response induced by aP vaccines that include PRN as a key antigen [8]. Additionally, the expansion of the more virulent ptxP3 genotype has been associated with increased bacterial fitness and toxin production [132].
  3. Suboptimal Vaccination Coverage: While childhood vaccination rates in many countries are high, they often fall short of the >95% threshold needed for robust herd immunity. Regional disparities and pockets of low coverage, particularly among adults and pregnant women, create vulnerabilities. For instance, in 2023, the vaccination coverage among pregnant women in Italy was estimated at only around 50%, far below the level needed to provide adequate passive protection to newborns [101].
  4. Transmission from Vaccinated Individuals: Individuals with waning immunity who contract pertussis often present with mild, prolonged coughs that are not recognized as pertussis. This allows for silent transmission within communities, especially from parents, siblings, and caregivers to unvaccinated infants, who are the primary source of infection for this vulnerable group [14].

Limitations of Surveillance and Implications for Control

The effectiveness of public health responses is hampered by the limitations of current surveillance systems. Sentinel surveillance networks, while valuable, are prone to underestimating the true incidence of pertussis. Many cases, especially in adolescents and adults with atypical presentations, are not diagnosed or reported, leading to a significant gap between reported cases and the actual burden of disease [81]. Furthermore, delays in diagnosis and reporting can impede the timely implementation of control measures during an outbreak.

To manage the resurgence, public health strategies have evolved to include:

  • Timely Infant Vaccination: Ensuring the primary vaccine series is administered according to the recommended schedule to protect the most vulnerable as early as possible.
  • Booster Doses for Adolescents and Adults: Implementing routine booster vaccinations (Tdap) to maintain herd immunity and reduce transmission to infants.
  • Maternal Vaccination: Recommending Tdap vaccination during each pregnancy (ideally between 27 and 36 weeks' gestation) to provide passive immunity to the newborn during the critical first months of life [10].
  • Enhanced Surveillance and Genomic Monitoring: Strengthening laboratory-based surveillance to track circulating strains, detect the emergence of antigenic variants like PRN-negative strains, and monitor antibiotic resistance, which is an emerging concern with increasing macrolide resistance [137].

The ongoing resurgence of pertussis underscores the need for a dynamic and multifaceted approach to control, combining high vaccination coverage across all age groups with innovative strategies to address the limitations of current vaccines and surveillance.

Complications and Prognostic Factors

Infection with Bordetella pertussis can lead to a range of complications, particularly in vulnerable populations such as infants, neonates, and individuals with compromised immune systems. The severity of complications is closely tied to age, vaccination status, and the timeliness of diagnosis and treatment. These complications arise from the intense paroxysmal coughing characteristic of the disease, the systemic effects of bacterial toxins, and secondary infections. Prognostic factors, including age and access to medical care, significantly influence clinical outcomes.

Complications in Infants and Neonates

Infants, especially those under six months of age and not yet fully vaccinated, are at the highest risk of severe and life-threatening complications. The most common and serious complications in this group include:

  • Pneumonia: This is the most frequent complication of pertussis in infants and can be caused either by secondary bacterial infection or direct damage to lung tissue by B. pertussis [95]. Pneumonia significantly increases the risk of hospitalization and mortality [1].
  • Apnea: Periods of respiratory arrest, or apnea, are particularly dangerous in neonates and may be the primary or only symptom of pertussis in this age group [54]. Apnea can lead to hypoxia, requiring immediate medical intervention and often mechanical ventilation [49].
  • Seizures and encephalopathy: Neurological complications such as seizures and encephalopathy can occur due to cerebral hypoxia during prolonged coughing spells or apnea [1]. Encephalopathy, in particular, may result in long-term neurological sequelae, including developmental delay and cerebral palsy [97].
  • Vomiting and dehydration: Frequent post-tussive vomiting can lead to dehydration and electrolyte imbalances, further complicating the clinical picture [3].
  • Malnutrition: Difficulty feeding due to coughing and vomiting can result in poor weight gain and malnutrition in affected infants [145].

The combination of these complications contributes to the high rates of hospitalization and mortality observed in unvaccinated or partially vaccinated infants [146].

Complications in Adolescents and Adults

While pertussis is generally less severe in older children, adolescents, and adults, it can still lead to significant morbidity. Common complications in these age groups include:

  • Rib fractures and abdominal hernias: The forceful nature of paroxysmal coughing can cause physical trauma, including fractured ribs and the development of abdominal or inguinal hernias [14].
  • Syncope: Temporary loss of consciousness during coughing episodes, known as cough syncope, can occur due to increased intrathoracic pressure and reduced cerebral perfusion [14].
  • Chronic bronchitis and pneumonia: Persistent respiratory symptoms may progress to chronic bronchitis or secondary bacterial pneumonia [14].
  • Sleep disturbances and chronic fatigue: The prolonged duration of the cough, often lasting for weeks or months, can severely disrupt sleep and lead to chronic fatigue, impacting quality of life [14].

Adults, despite often having milder symptoms, can serve as reservoirs for the bacterium, transmitting the infection to more vulnerable individuals, particularly unvaccinated infants [14].

Neurological Complications and Pathogenic Mechanisms

The most severe neurological complication of pertussis is hypoxic encephalopathy, which results from prolonged or repeated episodes of hypoxia during apnea or intense coughing spells. This condition can lead to permanent brain damage, seizures, and long-term neurodevelopmental disabilities [3]. The pathogenesis of encephalopathy involves both hypoxic-ischemic injury and the direct neurotoxic effects of pertussis toxin (PT) [153]. PT is capable of crossing the immature blood-brain barrier in neonates, where it disrupts neuronal signaling and induces inflammation, contributing to cerebral edema and neuronal death [154].

Prognostic Factors

Several factors influence the prognosis of pertussis:

  • Age: Young age, particularly under six months, is the most significant risk factor for severe disease and poor outcomes [1].
  • Vaccination status: Unvaccinated or incompletely vaccinated individuals face a higher risk of complications and death. Conversely, vaccination, especially maternal vaccination during pregnancy, significantly improves prognosis by providing passive immunity to newborns [110].
  • Timeliness of diagnosis and treatment: Early administration of macrolide antibiotics such as azithromycin can reduce the duration of contagiousness and may lessen the severity of symptoms if given during the catarrhal stage [3].
  • Access to intensive care: For infants with severe disease, access to neonatal intensive care units (NICUs) with capabilities for respiratory support is critical for survival [158].
  • Presence of comorbidities: Preexisting conditions such as prematurity or immunodeficiency increase the risk of adverse outcomes [146].

Long-Term Outcomes and Post-Infectious Syndromes

While most individuals recover fully from pertussis, some may experience prolonged symptoms or post-infectious sequelae. Chronic cough and fatigue can persist for months, resembling post-viral syndromes such as Long COVID [160]. Although not as well-documented as in other infections, these long-term effects underscore the importance of prevention and early intervention.

In conclusion, the complications of pertussis are most severe in infants and neonates, with pneumonia, apnea, and encephalopathy posing significant risks. Prognosis is heavily influenced by age, vaccination status, and access to timely medical care. Preventive strategies, including infant and maternal vaccination, remain the most effective means of reducing morbidity and mortality associated with this disease [10].

Public Health and Control Measures

The public health management of Bordetella pertussis involves a multifaceted approach combining vaccination, surveillance, outbreak control, and targeted interventions to protect vulnerable populations, particularly infants. Despite high vaccine coverage in many countries, pertussis has re-emerged as a significant public health concern due to waning immunity, antigenic shifts in circulating strains, and suboptimal adherence to vaccination schedules [8]. Effective control strategies must therefore be dynamic, evidence-based, and responsive to evolving epidemiological trends.

Vaccination Strategies and Immunity of Herd

Vaccination remains the cornerstone of pertussis prevention and is the most effective means of achieving immunity of herd, a critical factor in reducing transmission and protecting those who cannot be vaccinated, such as very young infants [14]. The shift from whole-cell (wP) to acellular (aP) vaccines in many national programs, including Italy’s, was driven by the superior safety profile of aP vaccines, which cause fewer adverse reactions [164]. However, while both vaccine types are effective in preventing severe disease, aP vaccines induce a less durable and less comprehensive immune response compared to wP vaccines or natural infection [104].

The immunity conferred by aP vaccines tends to wane after approximately 7–8 years, leading to increased susceptibility among adolescents and adults, who then become reservoirs for transmission to unvaccinated or partially vaccinated infants [101]. This decline in immunity undermines the effectiveness of herd protection, especially given the extremely high transmissibility of B. pertussis, with an estimated basic reproduction number (R0) between 12 and 17 [7]. To counteract this, national immunization schedules include booster doses: primary vaccination in infancy (at 3, 5, and 11 months), followed by boosters at 5–6 years and around age 12, with decennial Tdap (tetanus-diphtheria-acellular pertussis) boosters recommended for adults [107].

Maternal Vaccination and Cocooning

To protect newborns during their most vulnerable period—before they complete the primary vaccination series—two key strategies are employed: maternal vaccination and the cocooning approach. Maternal vaccination with Tdap during the third trimester of pregnancy (ideally between 27 and 36 weeks of gestation) is the primary strategy for preventing severe pertussis in infants [10]. This approach allows for the transplacental transfer of maternal antibodies, providing passive immunity that reduces the risk of pertussis by up to 91% in the first two months of life [110]. Despite its proven efficacy, maternal vaccination coverage in Italy was estimated at only around 50% in 2023, significantly below the levels needed for optimal population protection [101].

The cocooning strategy involves vaccinating close contacts of the newborn—such as parents, siblings, grandparents, and caregivers—to create a protective "shield" around the infant [7]. While conceptually sound, cocooning has proven less effective in practice due to logistical challenges and low adherence; studies show vaccination rates among parents often fall below 50%, limiting its real-world impact [111]. Consequently, maternal vaccination is now considered the superior and primary method, with cocooning serving as a complementary measure [10].

Surveillance and Outbreak Detection

Robust surveillance systems are essential for monitoring pertussis trends, detecting outbreaks, and guiding public health interventions. In Italy, the Istituto Superiore di Sanità (ISS) coordinates sentinel surveillance through networks like EpiCentro, which track reported cases and analyze epidemiological data [175]. However, these systems face significant limitations, including underreporting and delayed diagnosis, as many cases present with atypical or mild symptoms, especially in vaccinated individuals [81].

The case definition used by the World Health Organization (WHO) may fail to capture a substantial number of infections, particularly in older children and adults with prolonged cough, leading to an underestimation of true incidence [81]. Additionally, delays in laboratory confirmation—due to the time required for culture or the need for paired serum samples in serology—can hinder the timely detection of outbreaks [178]. To improve surveillance, there is a growing need for more sensitive case definitions, rapid diagnostic testing, and integration of laboratory and hospitalization data.

Antibiotic Prophylaxis and Infection Control

In addition to vaccination, antibiotic prophylaxis plays a critical role in controlling pertussis transmission, particularly among high-risk contacts. Post-exposure prophylaxis (PEP) with macrolide antibiotics—such as azithromycin, clarithromycin, or erythromycin—is recommended for all close contacts of a confirmed case, especially neonates, pregnant women, and healthcare workers, to prevent the development of disease [93]. This intervention is most effective when administered within 21 days of exposure and is considered a key component of outbreak management in households, schools, and healthcare settings [180].

Infected individuals remain highly contagious for up to three weeks after the onset of cough if untreated, but this period is reduced to just five days with appropriate antibiotic therapy [87]. Therefore, isolation of symptomatic patients and strict adherence to respiratory hygiene measures—such as mask-wearing and hand hygiene—are essential to limit spread [182]. In healthcare settings, airborne precautions are advised to protect vulnerable patients.

Antimicrobial Resistance and Therapeutic Implications

An emerging threat to pertussis control is the rise of macrolide-resistant B. pertussis strains, particularly those with the A2047G mutation in the 23S rRNA gene, which confers high-level resistance to erythromycin, azithromycin, and clarithromycin [98]. While most isolates remain susceptible, resistance has been documented in several countries, including China, where some regions report nearly 100% resistance [99]. In such cases, alternative antibiotics like trimethoprim-sulfamethoxazole (cotrimoxazole) are recommended for both treatment and prophylaxis [185].

The presence of resistant strains underscores the importance of molecular diagnostics, such as PCR with genotyping for resistance markers, to guide targeted therapy and prevent treatment failure [6]. Public health authorities, including the CDC and ECDC, emphasize the need for systematic antimicrobial susceptibility testing and enhanced surveillance to monitor the spread of resistant variants and inform clinical guidelines [100].

Challenges and Future Directions

Despite advances in prevention and control, pertussis continues to pose challenges due to the interplay of biological, immunological, and behavioral factors. The resurgence of cases in recent years—such as the over 60,000 cases reported in Europe between 2023 and April 2024—highlights the limitations of current strategies [188]. Contributing factors include waning vaccine immunity, reduced natural boosting due to lower circulation during the COVID-19 pandemic, and the emergence of antigenic variants, such as pertactin-deficient (Prn-negative) strains, which may partially evade vaccine-induced immunity [8].

Future efforts must focus on improving vaccine formulations to induce longer-lasting and broader immune responses, potentially through next-generation vaccines that stimulate stronger Th1/Th17 immunity, similar to wP vaccines [190]. In the interim, strengthening vaccination coverage—especially among pregnant women and adults—and enhancing surveillance and diagnostic capabilities are essential to protect public health and prevent severe outcomes in the most vulnerable populations.

References