Haemophilus

Haemophilus is a genus of Gram-negative bacteria that includes several significant human pathogens, such as Haemophilus influenzae, Haemophilus ducreyi, and Haemophilus aegyptius. These bacteria are small, pleomorphic, aerobic or facultatively anaerobic organisms that can cause a variety of diseases ranging from mild ear infections to severe bacteremia and meningitis.

Characteristics

Haemophilus species are characterized by their requirement for certain growth factors present in blood, namely X factor (haemin) and V factor (NAD or NADP). They are unable to grow on agar plates unless these factors are present, a characteristic that is utilized in laboratory identification of the organism. The most well-known species, Haemophilus influenzae, was once thought to be the cause of the influenza pandemic, but later research identified the influenza virus as the true culprit.

Pathogenesis

The pathogenicity of Haemophilus species is largely due to their ability to evade the host's immune system. They possess a polysaccharide capsule that is antiphagocytic, allowing them to survive and multiply in the bloodstream. Different serotypes, particularly in H. influenzae (types a through f), have varying degrees of virulence. Type b (Hib) was once the most common cause of bacterial meningitis in children before the introduction of effective vaccination.

Clinical Manifestations

Haemophilus species can cause a range of diseases, including:

• Pneumonia: Particularly in the elderly and those with chronic lung diseases.
• Meningitis: Mostly caused by H. influenzae type b in unvaccinated children.
• Epiglottitis: A life-threatening infection of the throat that can cause severe swelling and block the airway.
• Otitis media: Middle ear infections, common in children.
• Sinusitis: Infection of the sinuses.
• Chancroid: A sexually transmitted infection caused by H. ducreyi, leading to genital ulcers.

Diagnosis

Diagnosis of Haemophilus infections involves culturing the bacteria from specimens taken from the infected site. Blood agar plates supplemented with X and V factors (chocolate agar) are used to grow Haemophilus species. Polymerase chain reaction (PCR) tests and antigen detection techniques can also be employed for rapid diagnosis, especially in the case of H. influenzae type b infections.

Treatment

Treatment typically involves the use of antibiotics. However, resistance to commonly used antibiotics, such as ampicillin, has been increasing, necessitating the use of alternative treatments like third-generation cephalosporins or macrolides. Prevention of H. influenzae type b infections through vaccination has significantly reduced the incidence of serious diseases caused by this pathogen.

Prevention

The H. influenzae type b (Hib) vaccine is a critical component of childhood vaccination programs worldwide and has dramatically reduced the incidence of invasive diseases caused by H. influenzae type b. Vaccination against H. influenzae type b does not confer protection against other non-b serotypes or other Haemophilus species.

See Also

• Bacteremia
• Vaccination
• Antibiotic resistance

External Links

• CDC - Haemophilus influenzae Disease
• WHO - Haemophilus influenzae type b (Hib)