H. Influenzae is a gram negative coccobacilli, and a human obligate aerobe transmitted via respiratory route. There are 6 different antigenic serotypes that could be encapsulated or non-encapsulated; HiB (i.e. type B) is the most common type associated with human illness.
The encapsulated H. parainfluenzae and non-encapsulated H. influenza colonize upper respiratory tract (after being inhaled in aerosol). They cause local infections such as otitis media, sinusitis, bronchitis, and pneumonia. On the other hand, encapsulated H. influenza is uncommon in the upper respiratory tract but is a common cause of disease in the absence of vaccination.
The most import virulence factor is the polysaccharide capsule in type HiB, while pili and other adhesions aid colonization. The components of the cell wall, IgA1 protease and lipooligosaccharide, impair ciliary function resulting in epithelial damage and allows them to translocate to blood.
The gram-negative rods coccobacilli can be seen on gram stain, they are also catalase and oxidase positive, further serotyping is needed to differentiate between encapsulated and non-encapsulated strains.
It can also be cultured in chocolate agar where colonies will appear pale, grey, or transparent. However, the latex particle agglutination test (LAT) is a more sensitive test since it relies on the antigen of the bacteria for detection. Haemophilus influenza type B is most often associated with human illness, although it's incidence has declined dramatically after implementation of the HiB vaccine. Nontypable H influenza serotypes are increasing in frequency
- Meningitis: H. influenzae was the most common cause of pediatric meningitis before the vaccine was developed.
- Acute Epiglottitis: Rapid swelling of the epiglottis that results in obstruction of the respiratory tract and esophagus, following sore throat, fever, and inability to swallow.
- Septic Arthritis: Most common cause of infection in infants. A single joint is infected, resulting in fever, pain, swelling and decreased motility of the junction.
- Sepsis: In children between 6 months and 3 years. It presents with fever, lethargy, loss of appetite and no presence of localized infections. Sepsis is the result of bacteria entering the bloodstream via prior infection of the upper respiratory tract.
Treatment/Therapy: There is an increasing resistance to amino-penicillins (ampicillin, amoxicillin and bacampicillin) and there is also resistance to macrolides, tetracyclines, chloramphenicol, and fluoroquinolones.
- Second or Third-generation cephalosporin (e.g., cefuoxime, ceftriaxone)
- Amoxacillin/clavulanate (ie. penicillin + beta lactamase inhibitor)
HiB vaccine: capsular polysaccharide of type B strain conjugated to diphtheria toxoid
Rifampin prophylaxis for close contacts
Quick, Useful Facts:
- Encapsulated organisms such as H. influenzae type B (Hib) frequently are invasive, whereas non-encapsulated organisms such as “non-typeable” H. influenzae tend to colonize locally.
- Hib poses a great risk to asplenic patients (e.g., sickle cell patients) because the spleen is the primary organ to eliminate opsonized bacteria.
- Hib causes meningitis most commonly in children after maternal Ab protection has declined and before they have developed their own Abs.
- “Non-typeable” H. influenzae pneumonia occurs most often in adults with COPD or recent influenza viral infection.