Haemophilus influenza: Morphology and Identification, Antigenic Structure, Pathogenesis, Pathology, Clinical Findings, Diagnostic Laboratory Tests, Immunity, Treatment,

Question 1

Haemophilus influenzae: morphology and identification

Answer 1

1. Haemophilus influenzae is a small, pleomorphic, Gram-negative coccobacillus.
2. It is non-motile and facultatively anaerobic.
3. It requires both X factor (hemin) and V factor (NAD) for growth, so it grows on chocolate agar but not on blood agar unless in satellite colonies near Staphylococcus aureus (which supplies V factor).
4. It grows best in 5–10% CO₂.
5. Colonies are smooth, convex, and moist.
6. Encapsulated strains are typeable (a–f), with type b (Hib) being the most virulent.

Question 2

Haemophilus influenzae: antigenic structure

Answer 2

1. H. influenzae strains are either encapsulated or non-encapsulated.
2. Encapsulated strains are divided into six serotypes (a–f) based on the antigenic structure of the polysaccharide capsule.
3. Type b (Hib) has a polyribosylribitol phosphate (PRP) capsule, which is antiphagocytic and a major virulence factor.
4. Non-encapsulated strains are termed nontypeable and are less invasive but more common in mucosal infections.

Question 3

Haemophilus influenzae: pathogenesis

Answer 3

1. H. influenzae enters via the respiratory tract and adheres to mucosal cells using pili and outer membrane proteins.
2. The capsule (especially PRP in type b) resists phagocytosis and complement-mediated lysis.
3. IgA protease facilitates colonization by destroying secretory IgA.
4. It can invade the bloodstream and spread to the meninges, epiglottis, or joints, especially in children.
5. Nontypeable strains cause localized infections like otitis media and sinusitis.

Question 4

Haemophilus influenzae: pathology

Answer 4

Encapsulated strains, especially Hib, cause invasive diseases such as meningitis, epiglottitis, and septic arthritis by spreading hematogenously. Non-encapsulated strains are associated with mucosal surface infections. The infection leads to inflammation, tissue necrosis, and in severe cases, bacteremia. Epiglottitis causes airway obstruction. In meningitis, purulent inflammation of the meninges is seen.

Question 5

Haemophilus influenzae: clinical findings

Answer 5

Hib causes meningitis (especially in children under 5), epiglottitis (sudden airway obstruction), cellulitis, bacteremia, and septic arthritis. Nontypeable strains are a major cause of otitis media, sinusitis, conjunctivitis, and exacerbations of chronic bronchitis in adults. Symptoms depend on the site: fever, stiff neck, difficulty breathing, sore throat, or localized pain.

Question 6

Haemophilus influenzae: diagnostic laboratory tests

Answer 6

Culture of sterile body fluids (e.g., CSF, blood) on chocolate agar in 5–10% CO₂ is diagnostic. Gram stain shows small Gram-negative coccobacilli. Latex agglutination tests detect PRP antigen in CSF or urine. PCR is also used for detection. Satellite growth near Staphylococcus aureus on blood agar confirms its need for V factor. Identification can be confirmed with biochemical tests or MALDI-TOF.

Question 7

Haemophilus influenzae: immunity

Answer 7

Antibodies to the PRP capsule (especially type b) provide protective immunity. Natural infection in older children and adults induces immunity. Infants are initially protected by maternal antibodies. Conjugate vaccines induce T-cell-dependent immunity even in infants, with IgG production and memory response. Hib vaccine greatly reduces invasive disease incidence.

Question 8

Haemophilus influenzae: treatment

Answer 8

Third-generation cephalosporins (e.g., cefotaxime or ceftriaxone) are used for serious infections. Ampicillin was once effective, but many strains now produce beta-lactamase. For otitis media and respiratory infections, amoxicillin-clavulanate or azithromycin may be used. Rifampin is used for chemoprophylaxis in household contacts of patients with Hib disease.

Question 9

Haemophilus influenzae: epidemiology

Answer 9

H. influenzae is found only in humans and colonizes the nasopharynx. Transmission is via respiratory droplets. Encapsulated strains mainly affect children under 5 years, especially in the unvaccinated. Nontypeable strains affect all ages and are more common. Since Hib vaccine introduction, invasive disease incidence has dramatically decreased.

Question 10

Haemophilus influenzae: prevention and control

Answer 10

The Hib conjugate vaccine (PRP linked to a protein carrier like diphtheria toxoid) is given in infancy and provides long-lasting protection. Vaccination has nearly eliminated Hib in developed countries. Rifampin prophylaxis is recommended for close contacts of Hib cases. Good hygiene and avoiding crowded settings reduce spread of nontypeable strains.