Coccidioidomycosis laboratory tests

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: ; Vidit Bhargava, M.B.B.S [2]

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Overview

The fungal infection can be demonstrated by microscopic detection of diagnostic cells in body fluids, exudates, sputum and biopsy-tissue. With specific nucleotide primers C.immitis DNA can be amplified by PCR. It can also be detected in culture by morphological identification or by using molecular probes that hybridize with C.immitis RNA.

Staining & Culture

Usual KOH staining doesn't help in diagnosis of this fungal infection but organisms can be stained by using special stains such as papanicolaou stain or gomori methamine silver stain on sputum or other respiratory fluids. Even biopsy specimens can be stained by methamine silver stain to demonstrate spherules surrounded by inflammation. Coccidioides is easy to culture, grows in about 3-7 days at 37°C on simple media such as blood agar, however precautions needs to be exercised as it poses a significant health hazard to lab personnel.

Laboratory culture of Coccidioides immitis

Serology

• These form the mainstay of commercial testing for the diagnosis of coccidioidomycosis.
• Several techniques are available such as the older Tube-precipitin method as well as the newer CF(complement fixation), Immunodiffusion and EIA(enzyme immuno assay) assays.
• EIA assay picks up IgM and IgG antibodies against the coccidioidal antigens. IgM antibodies are found early in the disease progression and are not useful for assessing prognosis, while IgG antibodies appear late and persist longer.
• CF titres can be used as a measure of disease progression and to diagnose meningitits by measuring CF antibodies in CSF.
• {| class="wikitable" !Type of test !Specificity !Sensitivity !Comment |- |EIA | ++++ | ++++ |IgM and IgG |- |CF | ++++ | +++ |Used to confirm EIA IgG |- |IDCF | ++++ | +++ |Used to confirm EIA IgG |- |IDTP | +++ | +++ |Not employed now |} EIA based rapid diagnostic commercial kits are available but are not as sensitive or specific as lab based tests and have a risk of false positives.

PCR based methods

PCR assays are useful to detect a target gene one DNA has been extracted from the target tissue. It is highly sensitive and much safer compared to lab cultures. The C.immitans DNA can be amplified by PCR and then hybridised to molecular probes to aid in identification.

Gallery

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  Smear of an exudate from an infected mouse, reveals spherules of Coccidioides immitis. From Public Health Image Library (PHIL).

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  Spherule of Coccidioides immitis with endospores. From Public Health Image Library (PHIL).

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  Spherule with endospores of Coccidioides immitis. From Public Health Image Library (PHIL).

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  Large spherule containing endospores in a case of coccidioidomycosis. From Public Health Image Library (PHIL).

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  Methenamine silver stain reveals spherules of Coccidioides immitis fungus. From Public Health Image Library (PHIL).

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  Methenamine silver stain reveals reveals spherules of Coccidioides immitis fungus. From Public Health Image Library (PHIL).

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  Slant culture growing Coccidioides immitis fungal organisms on a medium of glucose PhytoneTM, yeast extract. From Public Health Image Library (PHIL).

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  Photomicrograph reveals presence of two round, thick-walled, spherule-staged Coccidioides immitis fungal organisms. From Public Health Image Library (PHIL).

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  Photomicrograph reveals presence of numerous thick-walled Coccidioides immitis arthroconidia and arthrospores (500x mag). From Public Health Image Library (PHIL).

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  Photomicrograph reveals presence of numerous thick-walled Coccidioides immitis arthroconidia and arthrospores (500x mag). From Public Health Image Library (PHIL).