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Epidemic typhus
(louse-borne typhus) is caused by Rickettsia prowazekii.
The
disease is widely distributed in some regions of Africa, Asia, Europe
and the Western Hemisphere.
Its devastating epidemics were associated with cold climates, poor
sanitation, and crowding during natural disasters, famine, or war as
for instance, the epidemics in Russia
and easter Europe during
1918-22.
Epidemic typhus may kill 60% or more of the untreated aged,
but kills only about 10% of untreated children.
R. prowazeki is a small
gram-negative bacillus (rickettsia) that has a man-louse-man life
cycle.
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 These rickettsiae infect and multiply in human endothelial
cells.
Infected endothelial cells detach and rupture, releasing
organisms into the circulation (rickettsemia).
A person is usually
infectious for lice only during the febrile stage of the disease.
A
louse taking a blood meal becomes infected with rickettsiae, after
which the organisms enter the epithelial cells of the midgut,
multiply, and rupture the cells within 3 to 5 days.
Large numbers of rickettsiae are released into the lumen of the louse intestine.
The
louse deposits its contaminated faeces on the skin or clothing of a
second host, where the faeces may remain infectious for more than three
months.
A person becomes infected when the contaminated louse faeces
penetrate an abrasion or scratch in the skin or when the person
inhales airborne rickettsiae from clothing contaminating louse faeces.
After penetrating the skin or nasal mucous membrane, the rickettsiae
enter the endothelial cells, multiply, and rupture the cells, thus
completing the life cycle.
An incubation
period of 10 to 14 days is followed by the sudden onset of severe
headaches, generalized aching and high fever, which may continue 10 to
14 days before subsiding.
About 4 to 6 days after the onset of
symptoms, the patient develops a maculopapular rash on the back,
chest, and abdomen.
The rash is composed of 1mm to 4mm “spots”, but
in fatal cases commonly becomes confluent and purpuric.
Mild rickettsial pneumonia is followed by a severe superimposed bacterial
pneumonia.
Although the brain, heart, and kidneys may be involved
during the acute phase, patients who recover have no sequelae.
Dying
patients may exhibit encephalitis, myocarditis, interstitial
rickettsial pneumonia, interstitial nephritis, and shock.
Although rickettsiae do not produce local lesions on entering the skin or
respiratory tract, they do cause a generalized vasculitis of minute
blood vessels as they multiply within endothelial cells, and fibrin
thrombi often form in capillaries, especially those of the brain,
skin, and heart.
Fibrin thrombi also occlude blood vessels, and cutaneous necrosis (gangrene of the skin) develops in a few patients.
At autopsy,
there are few gross findings except for splenomegaly and occasional
areas of necrosis.
However, histologic sections commonly reveal
collections of mononuclear cell in various organs ; for example, the
skin, brain, and heart.
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The mononuclear cell infiltrate includes mast
cells, lymphocytes, plasma cells and macrophages and are frequently
arranged as “typhus nodules”
around arterioles and capillaries.
The Brown-Hopps tissue Gram stain
(or the Giemsa stain) demonstrates the rickettsiae within endothelial
cells.
The rickettsiae divide by biniary fission.
When there are many rickettsiae in an endothelial cell, they are usually lined up in a
“flotilla” pattern several columns wide, parallel to each other and to
the longitudinal axis of the endothelial cells.
Brill-Zinsser
disease is a recrudescence of a latent infection by R. prowazekii in
people who have previously had epidemic typhus.
In the past, the disease primarily
affected older immigrants from Eastern Europe, especially those who
had a weakened immune system.
Years after the initial infection,
sporadic illness reappears, characterized by headache, fever and
macular rash.
Histologic features are similar to but
milder than those for the initial epidemic typhus.
Lice fed with
blood from these patients become infected with R. prowazekii.
Epidemic typhus can be controlled by large-scale delousing of the
population by, for example, steam sterilization of clothing or use of
insecticides.
Tetracycline antibiotics are the preferred treatment for rickettsial diseases.
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