BORDETELLA

Bordetella

Bordetella is a genus of gram-negative coccobacilli bacteria, with Bordetella pertussis being the most significant species responsible for whooping cough (pertussis). This highly contagious respiratory disease primarily affects infants and young children, although it can occur in individuals of all ages. Pertussis is known for its severe, paroxysmal coughing fits followed by a characteristic "whooping" sound during inspiration.

Microscopic Appearance

• Gram-negative coccobacilli.

• Small, oval-shaped rods, often appearing singly or in pairs.

• Non-motile.

• Does not form spores.

• Humans are the only known reservoir for B.pertussis.

Virulence Factors

• Pertussis Toxin (PT):

  • An A-B exotoxin that interferes with cellular signaling by ADP-ribosylating the G proteins involved in signal transduction, leading to lymphocytosis and increased sensitivity to histamine.

• Filamentous Hemagglutinin (FHA):

  • An adhesin that helps the bacteria adhere to the ciliated epithelial cells of the respiratory tract, facilitating colonization.

• Adenylate Cyclase Toxin:

  • Increases cyclic AMP levels within host cells, impairing phagocytic function and promoting immune evasion.

• Tracheal Cytotoxin:

  • A peptidoglycan fragment that damages ciliated epithelial cells in the respiratory tract, leading to the characteristic cough.

• Fimbriae:

  • Surface structures that enhance adherence to the respiratory epithelium and play a role in immune evasion.

Pathogenesis

• Attachment:

  • Bordetella pertussis attaches to the ciliated epithelial cells of the upper respiratory tract using adhesins like FHA and fimbriae.

• Toxin Production:

  • Pertussis toxin disrupts immune responses and local cellular functions, leading to systemic effects such as lymphocytosis.

  • Tracheal cytotoxin specifically targets ciliated cells, causing their destruction and impairing the clearance of mucus, resulting in the persistent cough.

• Immune Evasion:

  • The bacteria evade the immune system through the action of adenylate cyclase toxin and by modulating host immune responses.

Clinical Manifestations

• Catarrhal Stage:

  • The initial phase, lasting 1-2 weeks, characterized by mild, nonspecific symptoms such as runny nose, sneezing, mild cough, and low-grade fever. This stage is the most contagious.

• Paroxysmal Stage:

  • Lasting 1-6 weeks, this stage is marked by intense, paroxysmal coughing fits, followed by a high-pitched "whoop" during the inspiratory phase.

  • Coughing episodes may lead to vomiting, exhaustion, and cyanosis, particularly in infants.

  • Apnea, or periods of respiratory pause, may occur in young infants, which can be life-threatening.

• Convalescent Stage:

  • Gradual recovery over weeks to months, with a decrease in the frequency and severity of coughing episodes. However, secondary infections like pneumonia can occur during this stage.

Lab Diagnosis

• Culture:

  • Nasopharyngeal swabs or aspirates are cultured on selective media like Bordet-Gengou agar or Regan-Lowe agar. Bordetella pertussis colonies are small, shiny, and resemble mercury droplets, and incubated for 48 – 72 hours

  • They grow best at 35 - 36ºC

  • They are surrounded with hazy haemolysis.

• Polymerase Chain Reaction (PCR):

  • PCR is a rapid and sensitive method for detecting B. pertussis DNA from nasopharyngeal specimens, making it the preferred diagnostic tool during outbreaks.

• Serology:

  • Measurement of anti-pertussis toxin antibodies can be used for diagnosis in later stages of the disease or in unvaccinated individuals.

• Direct Fluorescent Antibody (DFA) Testing:

  • DFA staining of nasopharyngeal secretions can be used, but it is less sensitive and specific than PCR.

Treatment

• Antibiotics:

  • Macrolides (e.g., azithromycin, erythromycin, clarithromycin) are the treatment of choice, particularly when given in the early catarrhal stage. Antibiotic therapy can reduce the spread of the bacteria and shorten the duration of symptoms.

  • For patients allergic to macrolides, trimethoprim-sulfamethoxazole can be used as an alternative.

• Supportive Care:

  • Hospitalization may be necessary for infants and those with severe disease. Supportive care includes monitoring respiratory function, providing oxygen, and ensuring adequate hydration and nutrition.

  • Cough suppressants are generally not effective and are usually avoided.

• Prevention:

  • Vaccination is the primary preventive measure. The acellular pertussis vaccine (part of the DTaP and Tdap vaccines) provides protection against Bordetella pertussis.

  • Booster vaccinations are recommended for adolescents, adults, and pregnant women to reduce the risk of transmission to infants.

  • Post-exposure prophylaxis with antibiotics is recommended for close contacts of confirmed cases, especially in high-risk groups like infants, pregnant women, and immunocompromised individuals.