What
is histoplasmosis and what causes it?
Histoplasmosis is an infectious disease of the lungs caused by a
fungus called Histoplasma capsulatum. The infection sometimes can
spread to other parts of the body.
This Histoplasma organism thrives in moderate temperatures
and moist environments. Droppings from chickens, pigeons, starlings,
blackbirds, and bats support its growth. Birds are not infected with it
because of their high body temperatures, but they do carry it on their
feathers. Bats can be infected because they have a lower body temperature
than birds and can excrete the organism in their droppings.
To multiply, Histoplasma capsulatum produces small spores
called conidia. The conidia of Histoplasma capsulatum are only two
millionths of a meter (microns, µm) in diameter. When these conidia are
inhaled, they are small enough that they enter the lungs and start an
infection. Many of these infections are easily overlooked because they
either produce mild symptoms or none at all. However, histoplasmosis can be
severe and produce an illness similar to tuberculosis.
How does the infection develop?
When a person breathes in the conidia (spores) of Histoplasma
capsulatum, the lungs' defense mechanisms attempt to neutralize them. Not
all the conidia are neutralized. The ones that avoid the defense start an
infection. The symptoms of the infection appear within 5 to 18 days after
exposure, most commonly in 10 days.
There are five different forms of infection, as follows:
- Asymptomatic is recognized only
by performing medical laboratory tests as the victim does not show any
symptoms and is unaware of the infection.
- Acute disseminated
does not last long (i.e., acute) but it involves other organs outside
the lungs (i.e., disseminated). It is usually confined to infants and
young children and is marked by fever, cough, exhaustion and
enlargement of the liver and spleen.
- Acute benign respiratory
is produced by a heavy exposure to conidia. It is marked by weakness,
fever, chest pains, and cough. The severity of the symptoms depends
upon the magnitude of the exposure.
- Chronic disseminated
is of long duration (chronic) and it involves other organs outside of
the lungs. It occurs in people with a reduced capacity to fight
disease, for example, in patients with leukemia (cancer of the system
producing blood cells) and in persons being treated with drugs that
suppress the body's defense mechanisms against diseases. The chronic
disseminated form is marked by fever, anemia, hepatitis, pneumonia,
inflammation of the lining of the heart cavity, meningitis, and ulcers
of the mouth, tongue, nose and larynx.
- Chronic pulmonary occurs
in persons with pre-existing lung diseases such as emphysema. It
resembles tuberculosis and is more common in males over 40 years of
age.
How common is histoplasmosis in Canada?
The absolute number of cases of histoplasmosis in Canada at this
time is not known. In the St. Lawrence River Valley, where the infection
may occur frequently, 20-30 percent of the population test positive. It is
estimated that over 50 million people have been infected in North America
with about 500,000 persons testing positive each year.
How is histoplasmosis recognized?
Confirmation of a histoplasmosis diagnosis often requires laboratory
examinations which identify Histoplasma capsulatum in sputum or
lung biopsy. Blood and skin tests, and x-rays may be performed but they are
only of secondary value in diagnosis.
How is histoplasmosis treated?
Most patients who develop histoplasmosis do not require treatment. Some
may only require supportive treatment that relieves the symptoms of the
disease. Severe symptoms with a large involvement of the lungs require
treatment with specific antifungal drugs.
What occupations are at risk?
Persons whose occupations involve contact with the soil, in
particular soil enriched with bird and bat droppings, are at high risk of
acquiring infection. They include:
- farmers
and poultry keepers, especially when cleaning silos, chicken coops,
pigeon roosts and bat-infested lofts
- gardeners
and horticulturists using poultry manure as fertilizer
- construction
and other workers in earth-moving operations
- workers
in road construction, tree-clearing or landscaping
- workers
clearing or dismantling contaminated buildings
- workers
who monitor bird populations
- workers
who have contact with bats or bat caves
Others who may be at risk include archaeologists, geologists and
medical laboratory technicians who handle cultures of the organism.
How can we prevent histoplasmosis?
Prevention of histoplasmosis relies on avoiding exposure to dust in
a contaminated environment. Before anyone cleans chicken coops or other
contaminated soil, spraying with water is advisable to reduce dust. Decontamination
with 3% formaldehyde has been shown to be effective.
However, formaldehyde solutions should be used with caution since
this chemical may cause adverse health effects following inhalation,
ingestion, or skin or eye contact.
Persons working in contaminated areas should use protective clothing
such as gloves and coveralls. They should also use a respirator equipped
with a high efficiency particulate air (HEPA) filter that is capable of
filtering particles down to two microns in size. Such respirators are
suitable as long as the occupational exposure limit for formaldehyde is not
exceeded. For major clean up operations of prolonged exposure, a powered
air purifying or supplied air respirator may be necessary.
Document last updated on December 1, 1997
Copyright ©1997-2002 Canadian
Centre for Occupational Health & Safety
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