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Tularemia is an acute,
febrile,granulomatous, zoonotic disease caused by Francisella
tularensis.
Rabbits and rodents are the most
important reservoirs, but many other wild and domestic animals are
infected.
Humans become infected through broken
skin or intact mucosa by handling infected animals and carcasses ; by
ingesting contaminated food and water ; by inhaling bacteria ; or by
being bitten by infected insects, including ticks, deer flies, and
mosquitoes. Human-to-human transmission has not been reported.
Bioterror attacks
and other world events have focused the medical community's attention
on agents that might be used in biological warfare. One of these
potential biological weapons is Francisella tularensis, that is one of
the most infectious bacteria known. F. tularensis can cause severe,
even fatal, systemic tularemia. As a weapon of terrorism, the
bacterium would likely be disseminated as an aerosol and
contracted by
inhalation.
F. tularensis is a small (0.2 by 0.5
micrometer), aerobic, gram-negative bacillus that is not motile,
encapsulated, or spore-forming.
It does not grow in the standard
bacteriologic media, but will grow slowly on media enriched with cystine or cysteine.
Two types of F. tularensis are
identified.
Type A
: (in the continental United States only where it causes 90% of
tularemia).
It is highly virulent and is usually tick-borne and rabbit
associated. The mortality from untreated Type A tularemia ranges from
5% for ulceroglandular tularemia to
30% for the typhoidal and pneumonic forms.
Type B :
It
is less virulent and
has a worldwide distributation.
It occurs in goats and may be
transmitted to humans by mosquitos or ticks, or by water contaminated
by infected rodents, such as beavers and muskrats. The mortality
caused by Type B is less than 1%, even if the disease is left
untreated.
In humans,
tularemia is classified into six major syndromes:
ulceroglandular
(the most common form), glandular, typhoidal, oculoglandular,
oropharyngeal, and pneumonic.
Pneumonia may complicate any one of the
types.
The incubation period ranges from 1 to 20 days, depending on
the dose and route of transmission, with a mean of 3 to 4 days.
The
duration of illness is 1 week to 3 months, but may be shortened by
prompt treatment.
The most common form of tularemia,
ulceroglandular, begins as a tender erythematous papule
at the site of inoculation, usually on a limb.
This develops into a pustule, which ulcerates.
The regional lymph nodes become large and
tender and may suppurate and drain through sinus tracts.
In some
patients an erythematous, maculopapular rash, thought to represent
delayed hypersensitivity, may develop.
The initial bacteremia is
followed in a week by generalized lymphadenopathy and splenomegaly.
Symptoms include fever, headache, myalgia, and occasionally
prostration. Some patients develop meningitis, endocarditis,
pericarditis, or osteomyelitis.
The most serious infections are
complicated by a secondary pneumonia and endotoxic shock, in which
case the prognosis is grave.
In oculoglandular
tularemia the primary lesion is a papule in the conjunctiva, which
becomes a pustule and ulcerates. This is accompanied by lymphadenopathy of the head and neck.
Severe ulceration may penetrate
the sclera entering the eye and infect the optic nerve, causing
blindness.
In glandular tularemia generalized lymphadenopathy is the
first manifeatation.
A diagnosis of the
typhoidal form is made when fever, hepato-splenomagaly, and toxemia,
resembling salmonella sepsis, are presenting features.
Diagnostic
antibody titers usually appear within 2 weeks of infection.
The initial skin lesion in tularemia is
a pyogenic ulcer with purulent exudates.
Later, disseminated lesions
undergo central necrosis and are surrounded by a perimeter of granulomatous reaction resembling lesions of tuberculosis.
The lymphnodes become large and firm from hyperemia and large numbers of
histiocytes in the subcapsular and peritrabecular sinusoids. Later
they soften as area of stellate necrosis and suppuration develop.
By
contrast, the enlarged spleen shows only congestion, nonspecific
inflammatory cell infiltrates, and lymphoreticular hyperplasia.
The lesions in the lung resemble those
of primary tuberculosis.
On rare
occasions bacilli may be identified in histiocytes.
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