Blastomycosis
Blastomycosis is 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 the growth of Blastomyces. Growth of the organism requires sandy, acid soil, and proximity to water.
SIGNALMENT
Species
Predominantly dog
Occasionally cat
Breed Predilections
Large-breed dogs weighing ≥ 25 kg, especially sporting breeds; may reflect increased exposure rather than susceptibility.
Mean Age and Range
Dogs—most common in 1–5 years of age; uncommon after 7 years of age.
Cats—no age predilection noted for cats.
Predominant Sex
Dogs—males in most studies.
Cats—none noted.
SIGNS
Historical Findings
Serous, mucoid, or mucopurulent ocular discharge.
Weight loss; depressed appetite.
Cough and dyspnea
Eye inflammation and discharge
Lameness
Draining skin lesions
Syncope if cardiac involvement
Physical Examination Findings
Dogs
Fever up to 104°F (40 °C)—approximately 50% of patients.
Harsh, dry lung sounds associated with increased respiratory effort—common.
Generalized or regional lymphadenopathy with or without skin lesions or subcutaneous swellings.
Uveitis with or without secondary glaucoma and conjunctivitis, ocular exudates, and corneal edema.
Lameness—bone involvement in up to 30% cases.
Testicular enlargement and prostatomegaly—occasionally seen.
Murmur and AV block—with endocarditis and myocarditis.
Cats
Increased respiratory effort
Granulomatous skin lesions
Visual impairment
RISK FACTORS
Wet environment—banks of rivers, streams, and lakes or in swamps; most affected dogs live within 400 meters of water.
Exposure to recently excavated areas.
Blastomycosis has been reported in indoor-only cats.
DIAGNOSIS
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 is 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 is very specific for fungal infection
IMAGING
Radiographs
Lungs—generalized interstitial to nodular infiltrate but can see a non-uniform distribution of lesions.
Tracheobronchial lymphadenopathy—common.
Changes—inconsistent with bacterial pneumonia; may resemble metastatic tumors, especially hemangiosarcoma.
Chylothorax secondary to blastomycosis has been reported in dogs.
Focal bone lesions—lytic and proliferative; can be mistaken for osteosarcoma
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.
TREATMENT
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.
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.
FOLLOW-UP
PATIENT MONITORING
Serum chemistry—monthly to monitor for hepatic toxicity or if anorexia develops.
Thoracic Radiographs
Determine the duration of treatment.
Considerable permanent pulmonary changes (fibrosis/scarring) may occur after the infection has resolved, making the determination of persistent active disease difficult.
At 90 days of treatment—if active lung disease is seen, continue treatment for an additional 30 days.
If lungs are normal, stop treatment and repeat radiographs again in 30 days.
At 120 days of treatment—if the lungs are the same as day 90, changes are residual (fibrosis).
If better than day 90, continue treatment for 30 more days, if lesions are significantly worse than at 90 days, change treatment to amphotericin B and then repeat radiographs.
Continue treatment as long as there is improvement in the lungs; if there is no further improvement and no indication of active disease, the lesions are likely the result of scarring.
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 3–6 months after completion of treatment; may occur > 1 year after treatment; the 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 10 times 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).