Health Topics

Canine Blastomycosis

BASIC INFORMATION

Description:

Blastomycosis is a fungal infection that usually starts in the lungs and then spreads to other parts of the body. Dogs and cats are both susceptible, but the disease occurs most often in dogs.

Cause:

Blastomycosis is caused by the fungus Blastomyces dermatitidis. It is an organism that grows in and receives its nourishment from dead or decaying organic matter in the soil. It can survive in a wide range of moistures and temperatures. Infection occurs most commonly via inhalation or ingestion of the fungus from the environment. Infection may be limited to the lungs or may spread to multiple sites in the body.

Clinical Signs:

Signs may initially be nonspecific and include lethargy, decreased appetite, and weight loss. Increased respiratory rate, difficulty breathing, and coughing may occur. Inflammation of the eyes, vision changes, enlarged lymph nodes (glands), and lameness from bony involvement are other possible signs.

Skin lesions occur in about half of the cases and are usually crusty bumps that drain a thick liquid. Organisms present in this liquid may spread the infection if it contaminates an open wound. Feet and nail beds are sometimes affected.

Altered mental attitude and difficulty walking are signs of central nervous system (primarily brain) disease. Other organs that may be affected include the testicles, prostate, and mammary glands (breast tissue). Gastrointestinal signs are uncommon.

Diagnostic Tests:

Routine laboratory tests and chest x-rays are commonly recommended to investigate the clinical signs. X-rays of the legs may be done if lameness and bone pain are present. Blood tests are available for blastomycosis at certain outside laboratories. Although these tests are sensitive, false-negative and false-positive results can occur.

Finding the organism in affected tissues is the best diagnostic test. Needle aspiration or biopsy of an enlarged lymph node, skin lesion, blind eye, or other affected tissue may be done. A tracheal wash (collecting fluid from the airway through a catheter inserted into the windpipe) or bronchoscopy (examination of the airway through a fiberoptic viewing scope) may be considered if the lungs are involved. In cases of central nervous system involvement, a spinal tap may be considered to collect cerebrospinal fluid. The fluid is then submitted for analysis and culture. In some cases, the fungal agent can be cultured from infected tissues.

TREATMENT AND FOLLOW-UP

Treatment Options:

The oral antifungal drugs, itraconazole and fluconazole, are generally considered effective but must be given for a prolonged period (months). Ketoconazole may be less expensive, but it is also less effective and has more side effects. Intravenous amphotericin B may be needed in animals with severe disease, especially if they do not respond to or do not tolerate oral medications. Amphotericin B is used cautiously, because it is toxic to the kidneys. Some forms are less toxic than others.

Seriously ill animals often require hospitalization, with intensive treatment and monitoring for the first several days of treatment. Dogs with severe lung disease may need supplemental oxygen. Death of the fungal organisms that occurs within the first few days of therapy may make the lung changes temporarily worse. Eyes that are blind and have persistent inflammation or glaucoma may be surgically removed.

Follow-up Care:

Periodic recheck visits and repeated laboratory testing are required for several months to monitor the response to treatment and for side effects of the medications. Treatment is continued for at least 2-3 months, and for 1 month beyond the disappearance of all clinical signs.

After therapy is completed, the animal is commonly re-evaluated for up to 6 months to assess for relapses, which can occur in as many as 20-25% of treated animals. Strict adherence to treatment protocols is needed for the best response.

Prognosis:

Prognosis is variable and depends on the extent and severity of the disease, as well as the initial response to treatment. Prognosis is better in animals with mild clinical signs or signs limited to one organ system. Prognosis is guarded (uncertain) in animals with widespread disease, central nervous system involvement, or severe involvement of any organ system.