Health Effects of the Fungi: Virulent Infections
By Dr. Harriet Burge,
EMLab™ Chief Aerobiologist and Director of Scientific Advisory Board
Although we live with fungi every day, and most do
us no harm, there are a few fungi that can invade the human body and cause diseases, which can range from itchy and annoying
to life threatening. These fungi can be classified into four categories: virulent agents that affect all who are not immune,
opportunistic agents that affect only those with some damage to their immune systems, colonizers that grow in the mucous often
generated by allergic disease, and the dermatophytes that grow in human skin. In this article, we will discuss the virulent
infectious agents.
Virulent agents cause infection in anyone who is appropriately
exposed and who does not already have natural or acquired immunity. Natural immunity occurs when you have the disease and
develop antibodies. Acquired immunity occurs when you are given vaccinations or antibodies against the organism.
Histoplasma
The fungus
Histoplasma capsulatum is the only species in the
genus. It belongs to the group of fungi that produce an ascospore stage, and when producing ascospores it is called Ajellomyces
capsulatus. It is a dimorphic fungus that is yeast-like at 37°C and filamentous at 25°C. The fungus grows in wet soil
that is contaminated (enriched) with bird or bat droppings. It is common in the tropics, and in the Midwestern U.S. (in the
Ohio and Mississippi river valleys). Spore dissemination occurs when contaminated soil is disturbed. Notable outbreaks have
occurred when people have attempted to clean up starling roosts. It only occurs indoors when contaminated soil is brought
in or blown in through windows.
Histoplasmosis
There are two varieties of Histoplasma capsulatum: H. capsulatum var. capsulatum
which primarily cause lung disease, and H. capsulatum var. duboisii which invades skin and bones. The latter is common
in Africa.
In the U.S., histoplasmosis is primarily a respiratory disease that generally goes unnoticed (i.e., is subclinical).
Approximately 250,000 people per year contract histoplasmosis. These people develop natural immunity (have Histoplasma
specific antibodies). Many people who have lived in the Midwest have natural antibodies to Histoplasma. In immunocompromised
or elderly people, the disease can spread throughout the body and is often fatal.
Blastomyces
The fungus
Blastomyces dermatitidis is the only species in
the genus. Like Histoplasma, Blastomyces is an ascomycete; the sexual stage is called Ajellomyces dermatitis.
Also like Histoplasma, Blastomyces grows in dirt contaminated with bird droppings, and on wood buried in
soil. It is especially common in the Mississippi river valley. It has not been recovered from any indoor environment. Spore
dissemination probably occurs when contaminated soil is disturbed.
Blastomycosis
In the U.S. blastomycosis is rare. Only 12 human outbreaks have been reported. There may be unrecognized
cases that cannot be reported since the disease is unrecognizable. It may occur as a self-limiting lung infection, or may
spread to organs (including skin) and bones. The disseminated form, if untreated, is usually fatal. Blastomycosis also occurs
in dogs.
Coccidioides
The fungus
The genus Coccidioides includes two species: C.
immitis, and C. posadasii. C. immitis is endemic in the California San Joaquin Valley while C.
posadasii occurs throughout the southwestern desert. The sexual stage is unknown. Coccidioides grows in hot
dry alkaline soil and is often found in animal burrows. Spores are spread during dust storms, or mechanical disturbance of
the soil.
Coccidioidomycosis
Both species of Coccidioides produce the same disease. Most cases of coccidioidomycosis
are subclinical. When symptoms occur, the disease is called Valley fever and is an acute fungal pneumonia. Disseminated disease
may occur in the immunocompromised.
Other systemic infectious fungi
None of the other
virulent infectious fungi are common in the U.S., and, in fact, when cases are seen the patient has been in an endemic area.
Paracoccidioides brasiliensis occurs in South America. Infection with Penicillium marneffei occurs primarily
in Southeast Asia.
Reference:
1. NIOSH: Histoplasmosis
2. MedlinePlus Medical Encyclopedia: Coccidioidomycosis
3. MedlinePlus Medical Encyclopedia: Blastomycosis 
