1. The MVista® Histoplasma and Blastomyces Antigen EIA tests are highly sensitive
2. There is high cross-reactivity between Histoplasma and Blastomyces antigen
3. Urine and serum antigen testing are neg in approximately 5% of cases
4. MVista® Histoplasma and Blastomyces IgG Antibody EIA tests are indicated if antigen testing is negative and clinical suspicion remains
5. The MVista® Histoplasma and Blastomyces IgG Antibody EIA tests can help differentiate between histoplasmosis and blastomycosis
Casey, a 2-year-old, male Labrador retriever was presented for a dry cough and lethargy of a couple weeks duration. Casey lives in southern Kansas on acreage. He has not travelled outside of the state. Physical examination revealed a fever 104.0F and adventitial lung sounds- wheezes. Casey was tachypneic at rest (45 breaths / minute). Routine lab work was unremarkable. Chest radiographs revealed a diffused mostly unstructured interstitial pattern and large tracheobronchial lymph nodes. Based on the full clinical picture, fungal pneumonia was suspected.
Histoplasmosis was considered most likely due to it being the most common systemic mycosis in Kansas. Urine and serum were negative for antigen by the MVista® Histoplasma Antigen Quantitative EIA (test code 310). Due to continued high suspicion for fungal pneumonia, serum was submitted for Histoplasma antibody testing by the MVista® Histoplasma Canine IgG Antibody EIA (test code 327) and Coccidioides antibody testing by MVista® Coccidioides Canine IgG Antibody EIA (test code 329). Anti-Coccidioides IgG antibody concentrations were low (14 EU; Reference Interval <8) while anti-Histoplasma IgG antibody concentrations were at high (>80 EU; Reference interval <8). A small amount of cross-reactivity is expected between the 2 organisms and the significantly higher concentration of anti-Histoplasma antibodies is indicative of histoplasmosis.
Figure 1: Lateral thoracic radiograph showing a diffuse mostly unstructured interstitial pattern and tracheobronchial lymphadenopathy.
Casey’s case is a good example of when to test for anti-Histoplasma antibodies. When urine is tested for antigen with the MVista® Histo Antigen EIA it is highly sensitive (89% dog and 94% cat) for histoplasmosis [1-4]. Likewise the MVista® Blasto Antigen EIA is highly sensitive (94% dog) for blastomycosis . Urine and serum antigen testing are both negative in approximately 5±2% of cases. This is more common in localized histoplasmosis or blastomycosis, which might include disease localized to the eye, GI tract, skin, bone, and joint. Possibly due to lower fungal burden, some animals with pulmonary or disseminated infection can have negative antigen testing. In Casey, the infection was apparently localized to the lung. In humans, the addition of antibody testing to antigen testing for pulmonary histoplasmosis increases the sensitivity from 87.5% to 96.3% . Thus, testing for both is recommended in cases of suspected pulmonary histoplasmosis. Like Casey, clinical experience suggests the same is true for dogs and cats.
MiraVista Diagnostics offers antibody testing by immunodiffusion (ID) and by enzyme immunoassay (EIA). The antibody EIA tests were developed by, and are only available from, MiraVista Diagnostics. The MVista® Blastomyces IgG Antibody EIA has been shown to be far superior to Blastomyces antibody ID testing in dogs . Sensitivity for the MVista® Blastomyces IgG Antibody EIA is 95% as compared with 65% for the antibody ID test . Rigorous investigation comparing the diagnostic performance of the MVista® Histoplasma IgG Antibody EIA with the antibody ID test is ongoing.
Like Casey, the antibody EIA tests can help differentiate between different endemic mycoses that potentially crossreact with antigen testing. This would be most common between Histoplasma and Blastomyces, but lesser crossreactivity does occur between Coccidioides and Histoplasma. As in Casey’s case, with minimal cross-reactivity, the antibody concentration is highest for the infecting organismHistoplasma. In contrast to antibody EIA testing the Histoplasma and Blastomyces Antigen EIA tests are highly cross-reactive, precluding the need to test for both antigens in the same patient.
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