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Haemophilus influenza is a
gram-negative, pleomorphic, aerobic bacillus that is non-motile and
does not form spores.
It is a strict parasite of humans, and causes
pneumonia, meningitis, epiglottitis, pericarditis, bacteremia,
cellulitis, pyarthrosis, and “pink eye” - an acute purulent conjunctivitis.
The presence or absence of a polysaccharide capsule
determines the morphology of its colonies and its pathogenicity.
Encapsulated bacteria secrete a capsular polymer that makes the colony
appear umbilicated.
Of six dissimilar antigens in the capsule, type
b causes more than 90% of human infections and is the most common cause
of bacterial meningitis.
H. influenzae is
recovered from 80% of healthy adults and is encountered worldwide.
By
5 years of age, all children have H. influenzae in their nasopharynx,
and in 1 in 200 children will at some time have a systemic infection
caused by type b.
Some children who become infected before 18 months
fail to develop adequate antibodies and may contract a second systemic
infection with H. influenzae. Children under 2 years of age in day
care centers or who are in families with an ill patient are at greater
risk.
Meningitis caused by H. influenzae
usually follows otitis , sinusitis, pneumonia , or impaired immunity.
An
early symptom is pain when the child sits up or is diapered.
Upper
respiratory symptoms, fever, vomiting, irritability, and lethargy
accompany the meningitis, and 5% to 10% die within 48 hours.
Neurologic deficits are permanent in one-third of those who survive.
Bacteria, neutrophils, and fibrin form an exudate in the leptomeninges.
The exudate extends from the basal portion of the subarachnoid space
into the brain along the vessels.
The typical gram-negative coccobacilli are in neutrophils in the exudates around the meningeal
blood vessels.
Infection of lung by H. influenzae
produces fever, cough, purulent sputum, dyspnea, and either
bronchopneumonia or lobar pneumonia.
The pneumonia usually complicates
chronic lung disease and in half the patients follows a viral
infection of the respiratory tract.
The alveoli are filled with neutrophils, macrophages containing bacilli, and fibrin.
The bronchial
epithelium is necrotic and replaced by macrophages.
Bacilli are in
macrophages and short and long filamentous bacilli are packed together
in extracellular foci.
Empyema may form and display a similar
inflammatory reaction.
Part of the pathogenicity of H. influenzae may
be due to an extracellular toxin that arrests the action of cilia.
The onset of epiglottitis is sudden ;
fever, dysphagia, accumulation of oropharyngeal secretions, tachypnea
and retraction precede obstruction of airways.
The epiglottis is
swollen by edema, neutrophils, and macrophages.
The infection may descend past the
larynx into the trachea and bronchi, plugging the small airways with
thick exudates.
Bacteremia may seed large
weight-bearing joints, leading to pyarthrosis.
There is fever, heat, erythema, swelling, pain on movement, and decreased movement.
The
diagnosis is made by culturing H. influenzae from the joint fluid.
Needle aspirates of exudates from
empyema and lung, pericardium, joint and blood or cerebral spinal
fluid may be cultured for specific diagnosis.
Chloramphenicol remains
the drug of choice in meningitis.
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