Haemophilus influenzae infects only humans. There are no animal reservoirs found.
This organism enters the body through inhalation of airborne respiratory droplets.
H. influenzae causes variety of invasive infections such as meningitis, epiglottitis, and septic arthritis.
Moreover, they cause localized colonization diseases such as bronchitis, sinusitis, conjunctivitis and otitis media.
There are six serotypes have been identified according to the serological typing done, based on the capsular polysaccharide.
Type b is the main causative organism for severe, invasive diseases such as meningitis and sepsis.
Unencapsulated strains also cause noninvasive diseases, specially related to the upper respiratory tract such as sinusitis and otitis media.
In developing countries H. influenzae are the major cause of premature deaths and ill health in infants and young children.
They usually cause bacteremia. Mainly they cause pyogenic meningitis in young children below 5 years old.
Mainly in adults they cause pneumonia and empyema. Furthermore, acute epiglottitis also caused by this organism and it may lead to fatal airway obstruction.
According to the site of infection sample collection should be done.
Collection of cerebrospinal fluid (CSF), nasopharyngeal specimens, pus, aspirates from joints, middle ears or sinuses and blood for culture. The viability of H. influenzae in clinical specimens declines with time.
Therefore, it is necessary to transport specimens immediately and specimens should be cultured as soon as possible and should not be refrigerated.
If necessary, enrichment should be done.
Microscopy
influenzae is a small, Gram-negative coccobacillus or short bacillus with occasional longer, filamentous forms. It is a non-motile organism.
In direct smears prepared by CSF, long thread-like pleomorphic forms can be seen among pus cells.
Culture
influenzae grows poorly in anaerobic conditions. Plates should be incubated in 5-10% moist carbon dioxide enriched aerobic atmosphere.
The growth temperature range is 20–40 ºC. Optimum growth temperature is 35–37 ºC.
The media used for the growth of H. influenzae must contain growth factor X (porphyrin requirement) and V (NAD or NADP requirement).
Heated blood (Chocolate agar) can supply both X and V factors for the growth of H. influenzae.
Blood agar
influenzae grows poorly on blood agar due to lack of X and V factors. The colonies are colorless and show small water bubble like appearance.
For the enhancement of the growth of H. influenzae by giving X and V factors, stabbing of the blood agar medium with Staphylococcus aureus can be done.
Chocolate agar
influenzae produce smooth, mucoid, grey color or colorless colonies on chocolate agar.
Colonies measure 1.5 mm in diameter. There is a characteristic seminal odour.
Other methods
Routinely biochemical tests are not done to identify H. influenzae.
Serological diagnosis is done to identify the capsular polysaccharide antigens. Capsular swelling (quelling) reaction is done for the identification.
A rapid latex agglutination test is used for the identification of polysaccharide antigen.
There are additional tests to identify encapsulated strains. Fluorescent antibody staining, counter immune-electrophoresis which can detect capsular polysaccharide.
PCR assays are also used for the identification and confirmation of H. influenzae from clinical specimens.
Treatment
Accurate detection of the antibiotic sensitivity of H. influenzae is necessary.
β-lactamase and chloramphenicol acetyltransferase production should be studied carefully.
influenzae sensitive to ampicillin (or amoxicillin), chloramphenicol and tetracyclines.
Moreover, sensitive to cephalosporins such as cefuroxime, cefotaxime and ceftriaxone. Co-amoxiclav, ciprofloxacin, azithromycin and clarithromycin are also useful in the treatment of H. influenzae.
The first choice of antibiotic that are being used for the treatment of meningitis and also acute epiglottitis is Ceftriaxone or cefotaxime.
For the treatment of H. influenzae upper respiratory tract infections, either amoxicillin-clavulanate or trimethoprim-sulfamethoxazole are used.
Prevention and control of the disease
There is a vaccine for the immunization purpose. It contains capsular polysaccharide of H. influenzae type b conjugated toxoid or other related carrier proteins. It is given to the children age between 2 and 15 months.
Rifampicin is used as a Prophylaxis. Rifampicin reduces the respiratory carriage of the organism and by that reduce the transmission.
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