Whooping cough, medically known as pertussis, is a highly contagious respiratory infection. Characterized by severe coughing fits, this illness remains a public health concern, especially for infants. Understanding its cause is essential to comprehending its progression and impact on the respiratory system.
The Specific Germ Responsible
The bacterium exclusively responsible for whooping cough is Bordetella pertussis. This small, rod-shaped bacterium is Gram-negative and aerobic, meaning it requires oxygen to survive. Bordetella pertussis is the sole etiologic agent of pertussis, though Bordetella parapertussis can cause a milder, similar illness.
Humans serve as the only known natural reservoir for Bordetella pertussis. The bacterium is encapsulated and does not produce spores. Its unique characteristics enable it to colonize the human respiratory tract and produce virulence factors that lead to the distinct symptoms of whooping cough.
How the Infection Takes Hold
Whooping cough spreads easily from person to person, primarily through airborne droplets. When an infected individual coughs or sneezes, tiny, germ-filled particles are released into the air. Others in close proximity can then inhale these contaminated droplets, initiating a new infection. This direct person-to-person transmission is the most common route, as the bacteria do not survive long outside the body.
Once inhaled, Bordetella pertussis bacteria establish themselves in the upper respiratory tract. They specifically attach to the cilia, which are tiny, hair-like structures lining the airways. This attachment is facilitated by various surface proteins, such as filamentous hemagglutinin and pertactin. The bacteria then multiply on the surface of the respiratory epithelium, extending into the bronchi and bronchioles. This initial colonization phase is crucial for the infection to progress.
The Body’s Response to the Infection
Following colonization, Bordetella pertussis produces several toxins that contribute to whooping cough symptoms. Two primary toxins are pertussis toxin (PT) and tracheal cytotoxin (TCT). Pertussis toxin inhibits signaling pathways in host cells, suppressing early immune responses and promoting bacterial growth. It also contributes to systemic effects, including an increase in lymphocyte count.
Tracheal cytotoxin directly damages the ciliated cells lining the respiratory tract. This toxin causes ciliated cells to lose function and die, leading to their extrusion from the airway lining. When cilia are damaged, they cannot effectively clear mucus, debris, and bacteria from the airways, allowing these substances to accumulate. This buildup of mucus and inflammation triggers the severe coughing fits characteristic of whooping cough, often with a distinctive “whooping” sound as the person inhales.