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Classification

A radiology text is not the proper place to classify the fungi!

Descriptions and classification of fungi can be controversial and experts admit that the terminology is sometimes ambiguous and changes frequently. If a detailed classification is required, reference should be made to one (or more) of the standard texts on clinical mycology. Sometimes reference to more than one text causes more confusion!

Laboratory Tests

Fungi can be recognized in cultures from biopsy specimens, exudates, or body fluids. Identification of the genus is often a challenge; deciding on the species is often even more difficult. Histopathology can provide the most rapid means of identification, but it is customary to set aside half of any tissue or fluid for culture to obtain in vitro morphological identification. Growth may be slow and may take several weeks before there are characteristic morphological forms. A recurring problem is in deciding whether the presence of an organism is of clinical significance. For example, Actinomyces may be present in the oral cavity in increased numbers when there are carious teeth, and Candida albicans may be found in the oral and respiratory mucosa without being invasive or pathogenic. Close correlation with clinical, imaging, and histopathological findings is required to determine the correct diagnosis and its relevance for any patient. Analysis of small aspiration biopsy specimens or sputum, pleural effusions, or bronchial washings is less reliable than excisional biopsy, because histopathology allows recognition of the tissue inflammatory response to the fungus. Unfortunately, negative tissue examination does not entirely exclude a fungal cause for infection, and culture may then be necessary.

The light microscope enables the identification of about 20 pathogenic fungi and the rapidly expanding fields of immunohistochemistry and electron microscopy have made histopathological diagnosis more accurate. However, the reaction to a fungus varies with the patient's immune status. When cellmediated immunity is defective (in cryptococcosis and histoplasmosis, for instance), tissue response to the fungi is minimal and the organisms proliferate and disseminate, but when the immune status is good, there are fewer fungi and the reaction tends to be vigorous. The primary site of infection for airborne fungi is usually the lungs. If there is an acute inflammatory response, the alveoli fill with fibrin and neutrophils, showing on imaging as consolidation. Many primary lesions, including those caused by Histoplasma capsulatum, Cryptococcus neoformans, Blastomyces dermatitidis, and Coccidioides immitis, may heal in a few weeks without dissemination. When the patient is immunodeficient, however, the infection spreads widely by lymph and blood to multiple organs. Some fungi, such as B. dermatitidis, P. brasiliensis, and C. immitis, cause a mixed pyogranulomatous reaction.

Other pathogenic genera, e. g., Aspergillus, include almost 100 well-described species: they rarely can be speciated in tissue sections, but some species can be identified when the tissue is well fixed and special stains are expertly used.

Skin tests are generally useful in epidemiological studies, but less useful in the diagnosis in individual patients, unless conversion from negative to reactivity is demonstrated. Serological tests are useful for diagnosis and prognosis in coccidioidomycoses, cryptococcosis, histoplasmosis, and paracoccidioidomycosis. More specific descriptions of the tissue response will be summarized in each section.

 

 

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Copyright: Palmer and Reeder