So how does Haemophilus influenzae cause disease and what
diseases does it cause?
There are several different manifestations
depending on whether the Haemophilus is encapsulated or
The unencapsulated are the most common.
These are the nasal mucosal colonizers which pretty much
every human being
has at some point or frequent points in their life.
They colonize the upper respiratory tract and when they
break down normal mucosal barriers,
they can cause infections such as otitis media,
conjunctivitis, bronchitis, pneumonia.
The encapsulated Haemophilus influenzae in general,
are more often associated with more severe infections.
Type B Haemophilus influenza, also known as Hib.
Haemophilus influenza type B is historically the most
and it does so by being quite good at invading mucosa
through an Ig A protease.
Because it is so invasive, it can enter bloodstream quite
readily causing systemic infections,
sepsis, pneumonia, meningitis, you name it.
In addition to being able to invade its mucosal barriers so
it also produces endotoxins which further trigger an
and contribute to the symptoms.
Because historically, Haemophilus influenza type B was so
so evil at causing invasive disease,
it became a target for one of the most successful
in the history of the medical world.
The Hib vaccine was so successful that it largely eradicated
caused by Haemophilus influenza type B.
Unfortunately, nature abhors a vacuum,
and so other encapsulated types of Haemophilus; type A,
type C that have emerged or grown into the ecological niche
left by Haemophilus influenza type B.
So let's now look a little bit more closely at the types of
caused by Haemophilus influenza type B
even though we are able to vaccine prevent it,
it's a good indicator of other types of disease
from the other encapsulated bacteria.
So meningitis. The feared word which everybody everywhere
Infants especially before their second or third birthday are
most at risk for disease
caused by Haemophilus influenza type B.
Even though it is vaccine preventable and even though the
infants are growing their own humoral
antibody immune response, they are still at high risk
because their overall immune system
is still largely infantile.
It's still learning how to address encapsulated antigens or
Therefore infants in that age group are most likely to
suffer from invasive disease.
The clinical manifestations: typically a brief, mild upper
which may be some runny nose, some low-grade fever, some
But then, invasion occurs followed by bacteremia,
followed by delivery to the central nervous system
and then the classic signs and symptoms of meningitis:
severe headache, extreme fuzziness and nuchal rigidity or
if the patient is able to manifest that.
Otitis media, the most common disease associated with
Haemophilus influenza of any type;
encapsulated, unencapsulated, you name it.
All ages are at risk for this although again, children are
most at risk.
As anybody who has suffered from otitis media as an adult
the ear pain can be quite extreme
and it can be associated with nausea, malaise and sometimes
In this case, most patients will have, again that mild upper
followed by localization to one or sometimes both of the
ears with severe pain, higher fever,
and the irritability due to the pain. Epiglottitis.
This is the classic illness associated with Haemophilus
influenza type B.
And in fact, it was relatively quite common prior to the
days of the Hib vaccine.
Children at risk for epiglottitis were under age - four
years of age, again prior to immunologic maturity.
And they would typically develop that same upper respiratory
followed by a progressive pain in their pharynx;
very much like a pharyngitis.
However, the epiglottis itself was specifically targeted by
both the bacteria and immunolidre -
excuse me, immunologic reaction to the bacteria.
And as that reaction occurred, further engorged and further
swelling occurred at the -
of the epiglottis until it would actually occlude or block
the complete airway.
You can see an x-ray picture of that projected on the screen
with the green arrow showing what's called the thumb sign.
That dense structure projecting from the left side of the
across the airway is the edematous swollen epiglottitis
very much like a thumb which has been smudged across the
That is simply is showing that there's no way airway is open
and no air can pass through.
That constitutes a surgical emergency and the patients have
to be managed very carefully.
Fortunately, many of them will present prior to complete
occlusion of the airway
with inspiratory stridor, that high-pitched sound of air
whistling as it tries to get past that swollen epiglottis.
And so those patients could be anesthetized, intubated with
a breathing tube quite easily
and then allowed to be treated for their underlying
The next most common after otitis media, is typically seen
in all ages.
Although now, we're looking at older patients who now - may
have waning immunity.
Also those who are asplenic. The spleen; a part of the
which is largely responsible for responding to encapsulated
such as some pneumococci and Haemophilus species.
Patients who have compliment deficiencies, patients who have
other immuno deficiencies
all are at risk for getting invasive progressive disease due
to Haemophilus influenza type B and all the others.
Typical manifestation; may or may not be preceded by that
mild upper respiratory infection
depending on whether the patient aspirated infected
or developed a mild upper respiratory tract infection first.
Regardless, the onset of the pneumonia is associated with
abrupt onset of fever,
high fever with rigor, shaking chills, a productive cough
which can become blood-tinged
and also chest pain overlying the focal or the locus of the
Chest x-ray or chest radiograph will show you white out
or a complete occlusion of that particular lobe and it's
quite striking when seen.
Cellulitis. Although one - doesn't commonly associate
as a cause of skin infection it indeed can cause cellulitis
especially in young children.
And when it typically occurs, it is over the buccal mucosa,
right overlying the cheek
and it can spread downwards to the neck or across the entire
The cellulitis itself is striking because it looks not just
bright red, but almost purplish.
In fact, many describe a blue or blue purple hue or color to
That appearance alone and the location over the cheeks is
to strongly suggest Haemophilus influenza as the source.
And then arthritis. Again, not commonly associated with
but Haemophilus influenza can be a source of septic
and when so, it usually affects a single large joint,
especially the hip.
So, treatment. Another important thing to remember about
all types, is that many express or carry a beta-lactamase.
This would mean that the typical infection such as an otitis
which you might initially treat with simply amoxicillin
may not be an adequate effective treatment for Haemophilus
Instead, one would start with a beta-lactamase stabilized
beta-lactam, like amoxicillin.
And especially amoxicillin clavulanate.
which would be very effective against Haemophilus
For patients with more invasive diseases especially
meningitis or pneumonia, epiglottitis,
mastoiditis, a use of a cephalosporin such as ceftriaxone is
to get excellent tissue penetration and to avoid any chance
that the penicillin focused beta-lactamase might if -
inactivate the antibiotic.
And then those patients who have a severe invasive disease,
especially including meningitis are very contagious,
and the thoughts are that they may be contagious with the
strain of Haemophilus
which itself might be quite invasive or virulent.
If so, people who are exposed to such patients require
prophylaxis with a drug like rifampin.
Prevention is always the better treatment of course.
And in this case, the Haemophilus influenza type B vaccine
as noted has been highly effective.
It is part of the primary vaccine series for all infants and
typically conveys a lasting protection
without need for boosters after the age of 15 months or so.