Haemophilus Influenza

It is a gram-negative rod (coccobacillus). - A facultative anaerobe which grows best in media enriched with CO2.

Two major categories of H. influenzae were defined: the unencapsulated strains and the encapsulated strains.

• Encapsulated strains were classified on the basis of their distinct capsular antigens.

  • There are six generally recognized types of encapsulated H. influenzae: a, b, c, d, e, and f.
  • Serotype b is the most virulent type.
  • Organism found only in humans.

• Unencapsulated strains are termed nontypable (NTHi) because they lack capsular serotypes.

  • The unencapsulated strains are almost always less invasive.

The Incubation Period

It is not certain but could be as short as a few days.

Transmission

• Haemophilus influenzae bacteria, including Hib, are spread person-to-person by direct contact or through respiratory droplets coughing and sneezing.

• Usually, the bacteria remain in the nose and throat, causing no harm. Sometimes the bacteria can enter the blood and spread, causing serious infection in the individual.

Pathogenicity

• Enters the body through the respiratory tract;
  • Two types of behaviors:
    1. Asymptomatic colonization
    2. Infections such as sinusitis, otitis media, or pneumonia.
• Organism produces IgA protease which neutralizes respiratory mucosal IgA, this helps in its attachment to respiratory mucosa.
• After attachment to respiratory mucosa, it can enter the bloodstream and cause: Bacteremia and meningitis
• 95% of encapsulated forms (type b) are responsible for these diseases.
• Non-capsulated forms are responsible for otitis media, sinusitis, and pneumonia.
• In children, the age group 6 months - 6 years is most prone to infection by the organism.
• Peak incidence is from 6 months - 1 year.
• Virulence factors are polysaccharide capsule and endotoxin.

Risk Factors for Invasive Disease

Exposure factors:

• Household crowding
• Large household size
• Child care attendance
• Low socioeconomic status
• Low parental education
• School-aged siblings

Host factors:

• Race/ethnicity
• Chronic disease

Clinical Features

1. Meningitis:
   • Accounted for approximately 50%-65% of cases in the prevaccine era.
   • Hearing impairment or neurologic sequelae in 15%-30%.
   • Case-fatality rate 2%-5% despite effective antimicrobial therapy.
2. Otitis media and sinusitis cause pain in affected areas and redness and bulging of the tympanic membrane.
3. Septic arthritis, cellulitis, and sepsis (especially in splenectomized patients).
4. Rarely epiglottitis in young children.
5. Pneumonia in the elderly, especially those with chronic respiratory diseases.

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Diagnosis

• Diagnosis is considered confirmed when the organism is isolated from a sterile body site (cerebrospinal fluid or blood) which indicates H. influenzae infection.

• In this respect, H. influenzae cultured from the nasopharyngeal cavity or sputum would not indicate H. influenzae disease (because these sites are colonized in disease-free individuals).

• Gram staining.

• Organism is grown on chocolate agar.

• Definitive diagnosis can be made with Quellung test.

• Additional means of identifying encapsulated strains include fluorescent-antibody staining of the organism and latex agglutination tests, which detect the capsular polysaccharide.

Treatment

• Ceftriaxone is the drug of choice in meningitis and other serious infections.
• Otitis media and sinusitis are treated with Augmentin.

Prevention

It is by vaccination. The vaccine given is called Hib. It is in conjugated form, conjugated with a carrier protein. Given between 2-15 months. Conjugated is more effective than the unconjugated one. Incidence has fallen 99% since the prevaccine era. Most recent cases reported are unvaccinated or incompletely vaccinated children.

Polysaccharide Conjugate Vaccines:

• Stimulates T-dependent immunity.
• Enhanced antibody production, especially in young children.
• Repeat doses elicit booster response.

Contraindications and Precautions:

• Severe allergic reaction to vaccine component or following a prior dose.
• Moderate or severe acute illness.
• Age less than 6 weeks.