Introduction

Blastomycosis is a fungal infection caused by the organism Blastomyces dermatitidis.  It is exceedingly common in Ohio, Mississippi, St. Lawrence river valleys, the Great lakes region, and along the eastern seaboard.   This mold occurs in sandy, acid soils near river valleys or other waterways. It seems to be more common in the fall.

Blastomycosis infects dogs, people, and occasionally cats.

Any dog may contract blastomycosis under the right circumstances, but some are higher risk. Hunting and sporting breeds are often exposed to soil in wet areas.  Male dogs are more likely to contract blastomycosis than females. One study found that, while female dogs may have better survival rates with therapy, they are more likely to suffer relapses than males.

All dogs are susceptible to blastomycosis,  but those at greatest risk for infection are two-to-four year old intact males of hunting or sporting breeds that weigh 50-75 pounds and are exposed to river valleys or lakes during late summer or fall.

The disease

Dogs usually acquire blastomycosis by inhaling the spores from the soil into the lungs. The mold spores become a yeast once inside the body. In the lungs it causes a minor respiratory infection, that typically goes away on its own.   Puncture wounds can directly inoculate the spores into the skin and may cause localized skin infections. The most common form of blastomycosis seen by veterinarians is the generalized  form, which spreads by way of  the bloodstream or lymphatic system from the lungs into the eyes, brain, bone, lymph nodes, urogenital system, skin, and subcutaneous tissues.

The clinical signs of blastomycosis depends on which organs are affected and may include one or all of the following: anorexia, depression, weight loss, fever (103 degrees or higher) that doesn't respond to antibiotics, coughing, shortness of breath, exercise intolerance, enlarged lymph nodes, eye disease, or skin lesions that drain bloody or purulent material.

Diagnosis

Diagnosis of blastomycosis involves finding the yeast organism in samples from draining skin lesions or a lymph node. Chest x-rays and a complete blood count (CBC) are important tests but are not diagnostic by themselves.  Fungal titers confirm the diagnosis.

About 65 percent of dogs diagnosed with blastomycosis do survive. The treatment is long, complicated, and quite expensive.  There is also the risk of  serious side effects from the medication.  Due to this, some owners elect to euthanize affected pets.  Survival rates for dogs that are treated is approximately 85 percent, but up to 25 percent suffer relapses.  Dogs with poor liver or kidney function may not be able to tolerate the necessary medication. Dogs with brain or eye involvement have a worse prognosis. If an eye is involved, it usually must be removed since eyes don't respond well to treatment and become a source of infection.

Treatment

Standard therapy for blastomycosis has been amphotericin B. It is still the best choice for acute, life-threatening illness, and treated dogs show improvement in three to five days. Amphotericin must be given as an intravenous injection, either as a slow IV drip over several hours or as rapid IV bolus injections, one to three times weekly until a maximum cumulative dose is reached. Rapid injections increase the potential for acute drug reactions, and the drug has a toxic effect on kidney function, which requires close monitoring. The doctor may delay therapy to allow the kidneys recover from the injury.

Ketoconazole given orally twice a day is been effective against blastomycosis. but it usually takes 10-14 days to see any improvement with this drug.  For dogs with poor kidney function or a mild form of the disease it is often the best choice.  Ketoconazole has a lower cure rate than amphotericin, but if both drugs are  given together, they work synergistically.  This allows a lower dose of amphotericin and minimizes the risk of kidney failure. The combination also provides a more rapid and complete cure. The side effects of ketoconazole are related to liver toxicity and include anorexia, nausea, and vomiting. It can be harmful to pregnant dogs and may also affect the fertility of male dogs.

Itraconazole is the newest drug used to treat blastomycosis. It is given orally twice a day at first, then once daily for 60-90 days. Like amphotericin, it takes effect quickly, and has the same cure rate as the amphotericin-ketoconazole combination. The side effects are similar to ketoconazole, with the addition of ulcerative skin lesions and swelling of the legs.

The biggest drawback to itraconazole is the cost.  It is about twice the cost of ketoconazole. Because these drugs are dosed on body weight, larger dogs will have comparably larger drug costs. Blood tests and other veterinary services should also be considered in the cost of treatment. Total fees of $1000 or more are not unusual.

Blastomycosis is not contagious to people. However, if you have a pet with this infection, it indicates that you may be at risk for contracting the disease through a common environmental source. Since it is the mold form that releases infective spores through the air, you cannot get blastomycosis from the air around your dog who is infected with the yeast form of the fungus.

Recent studies indicate some risk of exposure through  wounds from contaminated objects such as a sharp stick or a dog bite. Good safety and hygiene precautions are highly recommended.  Needless to say, persons with deficient immune systems should not be handling infected dogs.