Definition

A systemic, mycotic infection caused by the dimorphic soil organism Blastomyces dermatitidis.

Pathophysiology

• A small spore (conidia) is shed from the mycelial phase (Ajellomyces dermatitidis) of the organism growing in the soil and inhaled, entering the terminal airway.
• At body temperature, the spore transforms to its yeast form, which initiates the infection in the lungs.
• From this focus of mycotic pneumonia, the yeast disseminates hematogenously throughout the body.
• The immune response to the invading organism produces a pyogranulomatous infiltrate to control the organism.

Systems Affected

• Respiratory-85% of affected dogs have lung disease.
• Eyes, skin, subcutaneous tissues, lymphatic system, testes, CNS, and bones-commonly affected.
• Prostate, mammary gland, nasal cavity, nasopharynx, gums, heart, and vulva-less commonly affected.
• Subclinical infection is uncommon.

Incidence/Prevalence

Depends on environmental and soil conditions that favor growth of Blastomyces. Growth of the organism requires sandy, acid soil, and a proximity to water.

Geographic Distribution

Most common along the Mississippi, Ohio, and Tennessee river basins. Also reported in the area of the Great Lakes and St. Lawrence River, southern Canada, mid-Atlantic states, and has been found outside the endemic area in Colorado.

Signalment

Signs

Risk Factors

• Wet environment-banks of rivers, streams, and lakes or in swamps; most affected dogs live within 400meters of water.
• Exposure to recently excavated areas.
• Blastomycosis has been reported in indoor-only cats.

Differential Diagnosis

• Respiratory signs-bacterial pneumonia, neoplasia, heart failure, pleural effusion, or other fungal infection.
• Lymph node enlargement-similar to lymphoma.
• The combination of respiratory disease with ocular, bony, or skin/subcutaneous involvement in a young dog suggests the diagnosis.

CBC/Biochemistry/Urinalysis

• CBC changes reflect mild to moderate inflammation; may note mild anemia in chronic cases.
• High serum globulins with borderline low albumin concentrations with chronic infection.
• Hypercalcemia (generally mild) in some dogs secondary to the granulomatous changes.
• Blastomyces yeasts may be found in the urine of dogs with prostatic involvement; mild proteinuria consistent with systemic inflammation/disease.

Other Laboratory Tests

• Urine or serum antigen testing-useful for making a diagnosis if organisms cannot be found on cytology or histopathology; positive test strongly supports diagnosis with a sensitivity of >90%; greater sensitivity reported with urine antigen test.
• Urine antigen test cross-reacts with some other fungal infections such as histoplasmosis.
• AGID-is not sensitive early in the disease but very specific for fungal infection.

Imaging

Diagnostic Procedures

• Cytology of lymph node aspirates, lung aspirates, tracheal wash fluid, or impression smears of draining skin lesions-best method for diagnosis.
• Histopathology of bone biopsies or enucleated blind eyes-identify the organism.
• Organisms-usually plentiful in the tissues; may be scarce in tracheal washes if there is no productive cough.

Pathologic Findings

• Lesions-pyogranulomatous with budding yeast 5–20 µm diameter with a thick, refractile, double-contoured cell wall; occasionally very fibrous with few organisms found.
• Lungs with large amounts of inflammatory infiltrate do not collapse when the chest is opened.
• Special fungal stains-facilitate finding the organisms.

Nursing Care

Severely dyspneic dogs-require an oxygen cage/support for a protracted period ( days to more than a week); about 25% of dogs have worsening of lung disease during the first few days of treatment, attributed to an inflammatory response after the Blastomyces organisms die and release their contents.

Activity

Patients with respiratory compromise must be restricted.

Diet

Palatable and high quality to stimulate the appetite.

Client Education

• Inform owner that treatment is costly and requires a minimum of 60–90 days.
• Reassure owners that an infected dog is not contagious to other animals or people (although other dogs in the house may have also been environmentally exposed).

Surgical Considerations

• Removal of an abscessed lung lobe may be required when medical treatment cannot resolve the infection.
• Blind eyes should be enucleated to remove potential sites of residual infection.

Drug(s) Of Choice

Contraindications

Corticosteroids-usually contraindicated because the anti-inflammatory effects allow uninhibited proliferation of the organisms; patients with previous steroid therapy require a longer duration of treatment; for dogs with life-threatening dyspnea, dexamethasone (0.1–0.2 mg/kg daily) for 2–3 days may be lifesaving when given in conjunction with itraconazole treatment; taper and discontinue steroids as soon as possible.

Precautions

Patient Monitoring

Serum chemistry-monthly to monitor for hepatic toxicity or if anorexia develops.

Prevention/Avoidance

• Location of environmental growth of Blastomyces organisms unknown, thus difficult to avoid exposure; exposure to lakes and streams could be restricted but is not very practical.
• Dogs that recover from the infection may be immune to reinfection.

Expected Course and Prognosis

• Death-25% of dogs die during the first week of treatment; early diagnosis improves chance of survival.
• More severe pulmonary disease and invasion into the CNS decrease prognosis.
• Recurrence-about 20% of dogs; usually within 3–6months after completion of treatment; may occur >1 year after treatment; a second course of azole treatment cures most patients; drug resistance has not been observed.
• With early detection of blastomycosis, the prognosis in cats appears similar to dogs.

Zoonotic Potential

• Yeast form is not spread from animals to humans, except through bite wounds; inoculation of organisms from dog bites has occurred.
• Avoid cuts during necropsy of infected dogs and avoid needle sticks when aspirating lesions.
• Warn clients that blastomycosis is acquired from an environmental source and that they may have been exposed at the same time as the patient; the incidence in dogs is 10times that in humans.
• Encourage clients with respiratory and skin lesions to inform their physicians that they may have been exposed to blastomycosis.

Pregnancy/Fertility/Breeding

Azole drugs can have teratogenic effects (embryotoxicity found at high doses) and should ideally be avoided during pregnancy (but the risk of not treating the mother needs to be balanced with the theoretical risk of azole therapy to the fetuses).

Abbreviations

• AGID = agar gel immunodiffusion
• ALT = alanine transaminase
• AMB = amphotericin B
• CBC = complete blood count
• CNS = central nervous system

Internet Resources

Information on antigen testing: www.miravistalabs.com.

Client Education Handout Available Online