Haemophilus – Bacteria, Infections and Vaccination

What is Haemophilus?

Haemophilus is a genus of gram-negative, rod-shaped bacteria belonging to the family Pasteurellaceae. The name derives from the Greek, meaning roughly blood-loving, as these bacteria require specific blood components – particularly factor X (hemin) and factor V (NAD) – to grow. The most well-known and medically significant species is Haemophilus influenzae, which, despite its name, is not the cause of influenza (flu) but rather a wide range of bacterial infections.

Species and Classification

The genus Haemophilus includes several species that are pathogenic in humans:

• Haemophilus influenzae: The most common and clinically relevant species. It is divided into encapsulated types (a–f) and unencapsulated, non-typeable strains. Type b (Hib) was formerly the leading cause of bacterial meningitis in children.
• Haemophilus parainfluenzae: Part of the normal oral flora but can occasionally cause respiratory tract infections or endocarditis.
• Haemophilus ducreyi: The causative agent of chancroid (soft chancre), a sexually transmitted disease.
• Haemophilus aegyptius: Causes purulent conjunctivitis and is associated with Brazilian purpuric fever.

Transmission and Risk Factors

Haemophilus influenzae is a common component of the normal mucosal flora of the human nasopharynx. Transmission occurs mainly through:

• Droplet infection (coughing, sneezing, speaking)
• Direct contact with infectious nasal secretions

Risk groups for severe infections include:

• Infants and young children under 5 years of age (especially unvaccinated)
• Elderly individuals
• Immunocompromised persons (e.g., those with HIV, asplenia, or undergoing chemotherapy)
• Individuals with chronic respiratory diseases

Clinical Manifestations and Symptoms

Depending on the bacterial strain and the organ affected, Haemophilus can cause a wide variety of diseases:

Invasive Diseases (primarily caused by Hib)

• Meningitis: Inflammation of the meninges with fever, severe headache, neck stiffness, and light sensitivity – a medical emergency.
• Epiglottitis: Life-threatening inflammation of the epiglottis with difficulty swallowing, drooling, and respiratory distress.
• Septicemia: Bacterial bloodstream infection with chills, high fever, and circulatory failure.
• Septic arthritis: Purulent joint inflammation, especially in children.

Non-Invasive Diseases (often caused by non-typeable strains)

• Otitis media: Middle ear infection with ear pain and hearing loss.
• Sinusitis: Inflammation of the paranasal sinuses.
• Bronchitis and pneumonia: Particularly in patients with COPD or other underlying conditions.
• Conjunctivitis: Inflammation of the conjunctiva of the eye.

Diagnosis

Diagnosis of a Haemophilus infection is made through microbiological investigations:

• Gram stain: Detection of gram-negative rods in clinical specimens (e.g., sputum, cerebrospinal fluid, blood).
• Culture: Growth on chocolate agar or heated blood agar, which provides the necessary growth factors.
• PCR: Molecular detection for difficult-to-culture strains or rapid identification.
• Antigen detection: Rapid tests in cerebrospinal fluid or urine, particularly for meningitis.
• Blood count and inflammatory markers: CRP, white blood cell count, and procalcitonin to assess the degree of inflammation.

Treatment

Treatment of Haemophilus infections depends on the severity of the disease and the bacterial strain involved:

Antibiotic Therapy

• Ampicillin: First-line therapy for susceptible strains.
• Amoxicillin-clavulanate: Used for beta-lactamase-producing strains resistant to ampicillin.
• Third-generation cephalosporins: Such as ceftriaxone or cefotaxime – preferred for invasive infections like meningitis.
• Fluoroquinolones: Alternative options for adult patients with contraindications or resistance.
• Chloramphenicol: A historical option, rarely used today.

Supportive Measures

For severe infections such as meningitis or sepsis, intensive care measures are required, including fluid administration, vital sign monitoring, and mechanical ventilation if needed. In meningitis, adjunctive dexamethasone is commonly administered to reduce the inflammatory response and minimize long-term complications.

Vaccination and Prevention

The most important preventive measure against invasive Haemophilus influenzae type b disease is the Hib vaccine. In many countries, including the United States and the United Kingdom, it has been part of routine childhood immunization programs since the early 1990s. The vaccine is typically administered as part of a combination vaccine alongside other childhood immunizations. Since the introduction of the Hib vaccine, the incidence of invasive Hib disease has decreased dramatically worldwide.

References

1. World Health Organization (WHO): Haemophilus influenzae type b (Hib) vaccination position paper. Weekly Epidemiological Record, 2013. Available at: https://www.who.int/wer/2013/wer8839.pdf
2. Murray P.R., Rosenthal K.S., Pfaller M.A.: Medical Microbiology. 9th edition. Elsevier, 2020.
3. Centers for Disease Control and Prevention (CDC): Haemophilus influenzae Disease (Including Hib). Available at: https://www.cdc.gov/hi-disease/index.html