Canine blastomycosis: A review and update on diagnosis and treatment - Veterinary Medicine
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Canine blastomycosis: A review and update on diagnosis and treatment
This fungal disease, which readily infects dogs and people, typically starts out in the lungs but can go on to invade many tissues throughout the body. Identifying it quickly and implementing antifungal therapy can result in a good prognosis.


Serologic testing

When cytologic samples from infected tissues are not diagnostic yet you highly suspect blastomycosis, serologic testing may be performed. Dogs with blastomycosis produce antibodies directed against the Wisconsin-1 (WI-01) and A antigens of B. dermatitidis.4

The agar gel immunodiffusion test for antibodies against the Blastomyces A antigen is the most commonly used serologic test and has a reported sensitivity of 40% to 90% and specificity of 90% to 100%.1,2,4,27 The test results are often negative early in the course of infection,2,4,27,30 making it unlikely that the agar gel immunodiffusion test will be useful in dogs without overt systemic blastomycosis. Antibody titers may persist in cured animals, making it impossible to use agar gel immunodiffusion results to monitor response to therapy or disease recurrence.2,30 No published studies have evaluated the duration of persistent positive titers after treatment.

A radioimmunoassay test to detect serum antibodies against the WI-01 antigen has been reported to detect 92% of infected dogs while maintaining 100% specificity, but this test is not commercially available.1,4

Urinary antigen testing

Recently, an enzyme immunoassay test to detect B. dermatitidis antigen in urine has been described (MVista Blastomyces dermatitidis Antigen EIA—MiraVista Diagnostics).31-33 This test is reported to be highly sensitive, detecting antigen in urine from 93% of people and up to 100% of dogs with systemic or pulmonary blastomycosis. Cross-reactivity with other fungal agents (especially Histoplasma capsulatum) and a few nonspecific false positive results have been reported.31-33

PCR testing

A few veterinary laboratories offer PCR-based diagnostic tests for fungal diseases. The South Dakota Animal Disease Research & Diagnostic Laboratory and HealthGene offer veterinary-specific tests for Blastomyces species. PCR promises increased sensitivity and speed of diagnosis compared with conventional diagnostic tests, but the utility of PCR in early diagnosis of canine blastomycosis has not been evaluated.30 For now, PCR tests should be used as supportive evidence to complete a clinical picture rather than as a sole method of diagnosis.


Table 1. Drugs and Dosages for Treating Canine Blastomycosis
Drugs most often recommended to treat dogs with blastomycosis are amphotericin B and the azole antifungals, including itraconazole, ketoconazole, fluconazole, and voriconazole. Table 1 provides a summary of recommended dosages for commonly used antifungal agents. These drugs are not FDA-approved for veterinary use.

Amphotericin B

Amphotericin B is a polyene antibiotic that binds ergosterol, an essential component of the fungal cell wall, thereby disrupting the cell wall and causing the organism's death. Amphotericin B's principal adverse effect is cumulative nephrotoxicosis, so monitor renal function before each dose.4,11,34

The desoxycholate form of amphotericin B is most commonly administered as an intravenous infusion. Sodium loading before treatment (20 ml/kg of 0.9% sodium chloride solution administered intravenously over 60 minutes) and slow intravenous administration (over five or six hours) of the amphotericin B dose (0.5 mg/kg) diluted in 5% dextrose in water solution or 2.5% dextrose in 0.45% saline solution (500 ml for dogs < 20 kg; 1,000 ml for dogs > 20 kg) may decrease the nephrotoxicity of this formulation.2,34 Alternatively, the amphotericin B dose can be diluted in 2.5% dextrose in 0.45% saline solution (500 ml for dogs < 20 kg; 1,000 ml for dogs > 20 kg) and administered subcutaneously.35 Most protocols require that the amphotericin B be administered every other day until a cumulative dose of 8 to 10 mg/kg has been achieved or until renal function deteriorates.1,34 A cumulative dose of 4 to 6 mg/kg may be adequate when amphotericin B is administered together with an azole.

Amphotericin B may be administered during the initial treatment of dogs with severe or rapidly progressive blastomycosis to improve the rate of recovery. Combination therapy with amphotericin B and an azole antifungal allows for a decreased total dose of amphotericin B to be administered, potentially decreasing the potential for nephrotoxicity.

A lipid-complexed formulation of amphotericin B has recently become available. This formulation, Abelcet (The Liposome Company), is eight to 10 times less nephrotoxic than amphotericin B desoxycholate, allowing higher cumulative doses to be administered.2,12,34,36 The higher administered dose and increased uptake of lipid complexes by phagocytic cells of the reticuloendothelial system may improve efficacy with this formulation.36 Dilute Abelcet in 5% dextrose in water to a concentration of 1 mg/ml, and administer to dogs at a dose of 1 to 3 mg/kg as a one- to two-hour intravenous infusion every other day until a cumulative dose of 24 to 27 mg/kg has been achieved.


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