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Nocardiosis is
an infection of the lung that may spread to the brain and skin and
less commonly to the thyroid, liver, or other organs.
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Causative organism:
The causative organism is Nocardia asteroides,
a gram-positive, aerobic, branching, filamentous bacillus that is weakly acid-fast and belongs to the order Actinomycetales. In about 85% of cases, infection is caused by Nocardia asteroides,
and the remaining 15% are caused by N. brasiliensis and N.
otitidiscaviarum (caviae).
Distribution:
Nocardia species inhabit
the soil and have worldwide distribution.
Mode of infection:
Unlike actinomycosis,
nocardiosis is an exogenous disease and infections are usually
contracted by inhalation of nocardiae that live as saprophytes in
nature. The disease is not contagious.
Predisposing factors:
Debility and immunosuppression predispose to infection. Well
recognized conditions that predispose patients to nocardial infection
include lymphoma, Hodgkin's disease, chronic granulomatous disease of
childhood, and pulmonary alveolar proteinosis.
Clinical presentation:
Fever, weight loss, night sweats, and cough are usual. Invasion of the
lung causes a bronchopneumonia, which may extend to become lobar.
Direct extension to the pleura, trachea and heart and extension to the
brain or skin through the circulation are recognized
complications and carry grave prognosis.
Pathological features:
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Lungs:
The gross
appearance of lung depends on the duration and extent of infection.
The lesions may be focal or diffuse and usually have necrotic centers
surrounded by a perimeter of organizing pneumonia. Large cavitating
abscesses may be present or there may be diffuse fibrinopurulent
pneumonia similar to that caused by certain nonfilamentous bacteria.
Fibrosis is usually minimal
Microscopically,
the necrotic centers are rich in fibrin and degeneration neutrophils.
The nocardia infiltrate the pulmonary tissue freely and are most
easily seen after silver impregnation. A modified acid-fast stain
demontrates weak and segmental staining.
Central Nervous
System:
About 20%
of patients with pulmonary nocardiosis have central nervous
system involvement, usually in the form of cerebral abscesses.
Meningitis , a rare complication, results from rupture of an
intracerebral abscess or direct extension of nocardial osteomyelitis.
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Skin and
subcutaneous lesion:
Nocardiosis may
also present as solitary or multiple subcutaneous lesions that are
attributable either to traumatic implantation or to systemic
infection. These localized lesions, with chains of nodules leading
from a primary skin ulcer, can mimic those of cutaneous sporotrichosis.
This entity is known as the sporotrichoid form of nocardiosis.
Nocardia
species:
In systemic infections, the Nocardia spp. almost never form granules.
Rather, these organisms occur as individual, gram-positive, beaded
filaments, about 1micrometer in width, that branch at approximately
right angles.
Special stains:
The
delicate filaments are not stained by hematoxylin and eosin, periodic
acid-Schiff, or Gridley stains. However they are readily demontrated
with Gomori methenamine-silver and tissue Gram stains. All three
Nocardia spp. are often partially acid-fast in tissue sections when
stained with modified acid-fast procedures using a weak decolorizing
agent. Usually the agents of actinomycosis are not acid fast.
N. brasiliensis
and N.
otitidiscaviarum (caviae)
may cause pulmonary nocardiosis resembling that produced by N.
asteroides but
more characteristically they may present as Mycetomas. They are
inoculated into the skin and subcutaneous tissue at the time of
penetrating trauma, grow slowly, and form nocardial grains. A
pyogranulomatous reaction erupts through the skin to form draining
sinuses. Nocardial mycetomas must be distinguished from the mycetomas
caused by fungi and nonfilamentous bacteria.
Sulphonamides are
useful for treating all forms of the disease. Because of a strong
tendency for relapse, prolonged therapy may be required.
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