Pathogenesis of Haemophilus influenzae

• The non-capsulated variety of Haemophilus influnzae is a very common commensal in upper respiratory tract.
• 60-80% cases have been found related to non-capsulated strains in the different surveys conducted in urban communities.
• This was carried in the throat or naso-pharynx of young children and the rates of 3 to 4% was of capsulated type notably type b.
• Lower carrier rate (30-80%) was noticed in older children and adults of non-capsulated strains and the capsulated form was rarely found in these age groups.
• The higher carrier rates of capsulated haemophili were found in semi-isolated communities of young children without overt diseases.
• These findings were quite similar to that of Pneumococci which have also high throat carrier rates that are only potentially pathogenic and the virulent invasive types have low carrier rates.
• The diseases caused by haemophilus genus can be categorized into two groups. They are:

i)Acute primary (exogenous) infections caused by capsulated haemophili, usually type b.

ii)Acute and chronic secondary (endogenus) respiratory infections associated with non-capsulated haemophili.

A) Capsulated strains of H. influenzae ( particularly type b) infection

• These produce naso-pharyngitis (sore throat with fever), laryngo-epiglottitis (croup), acute bronchitis, pneumonia, otitis media, septicaemia, meningitis, septic arthritis, and pericarditis.
• Age plays an important factor in the incidence of clinical infection with this form of Haemophili.
• In the age range between 2 months to 3 years, acute purulent meningitis is noticed mostly.
• Similarly the age of 2 years to 7 years may show laryngo-epiglottitis which is a form of croup with greatly enlarged and inflamed epiglottitis.
• There is an associated bacteremia for the both types of infection.
• Any suspected cases are followed by blood cultures in both types of infections.
• The route of infection of the meninges is probably from the naso-pharynx via the blood.
• These infections are restricted to infants and young children.
• The bactericidal antibodies against the capsulated strain from meningitis infected children of the age 2 months to 3 years are not demonstrated in blood samples.
• But, the antibodies might be present constantly in the blood of the children of age group 3 years to 10 years.
• Baby of age 2-3 months after birth is mostly infected by type-b H. influenzae as he/she loses the protective maternal antibody.
• This causes naso-pharyngitis with fever and infection might spread elsewhere in the respiratory tract.
• In some cases, it might invade the blood and causes meningitis or other metastatic infection.
• Subsequently, during his/her later ages, they are protected by active production of specific type-b antibodies.

B) Non-capsulated strains of H. influenzae infection

• These strains are more common in the sputum of cases of chronic bronchitis, particularly during acute exacerbations.
• In cases of bronchiectasis also, these strains can be noticed.
• Homogenization of the specimen should be done by pre-treatment with the mucolytic enzyme such as pancreatin.
• This is done due to the difficulty in the demonstration of haemophili in sputum because of its irregular distribution.
• This can also be done by shaking the specimen with sterile water and glass beads for 15 to30 minutes.
• H. influenzae may not be regarded as the primary etiological factor in the causation of chronic bronchitis.
• In case of secondary infection, it has got more importance. This has been demonstrated by various studies.
• This can be emphasized in three different points as:

1. In case of chronic bronchitis, H. influenzae is more frequently associated with purulent than the mucoid sputum.
2. The suppression of this organism temporarily by antimicrobial therapy and the association of clinical relapse with its return.
3. Species-specific antibodies to H. influenzae are present in the blood of patients with chronic bronchitis in purulent sputum cases. Such antibodies are present in low level in cases of asthma patients and in healthy subjects.

• In cases of young children, non-capsulated haemophili are often found in pure culture in cases of para-nasal sinusitis, and to some extent in otitis media.
• The hemophilia appears as a secondary pathogen after a primary virus infection in above cases.

References: 

i) https://www.nhs.uk/conditions/hib/#:~:text=Haemophilus%20influenzae%20type%20b%20(Hib)%20is%20a%20bacterial%20infection%20that,these%20infections%20are%20now%20rare.

ii) https://www.microbiologyresearch.org/content/journal/jmm/10.1099/00222615-18-1-1

Pathogenesis of  Haemophilus influenzae