Blastomycosis is an infection found in dogs, people, and occasionally cats. It is caused by the fungal organism Blastomyces dermatitidis, which is found as a mold in the soil or at room temperatures and as a yeast in tissues or at body temperatures. The mold occurs in sandy, acid soils near river valleys or other waterways. Endemic in Ohio, Mississippi, and St. Lawrence river valleys, the Great lakes region, and along the eastern seaboard, veterinarians report blastomycosis more frequently in the fall.
While any dog may contract blastomycosis under the right circumstances, certain populations are at greater risk. These include the hunting and sporting breeds since their use involves exposure to soil in wet areas, and young adult dogs, which are more likely to be used in hunting or field trials than old dogs or pups. For unknown reasons, 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.
So, while all dogs are susceptible to blastomycosis, 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 process
Dogs usually acquire blastomycosis by inhaling the spores from the soil into the lungs, where it induces a self-limiting pulmonary infection. Direct inoculation of the spores into the skin through puncture wounds may cause local cutaneous infection. By far, the most common form of blastomycosis seen by veterinarians is the generalized or disseminated form, which spreads via the bloodstream or lymphatic system from the lungs to involve the eyes, brain, bone, lymph nodes, urogenital system, skin, and subcutaneous tissues.
The clinical signs of blastomycosis may vary with the target organs 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 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. Serum chemistries are usually within normal limits. Fungal titers confirm the diagnosis and help differentiate blastomycosis from histoplasmosis, a related fungal condition with similar signs and geographic distribution.
About 65 percent of dogs diagnosed with blastomycosis do survive. Because the treatment is long, complicated, and expensive with the potential for serious side effects, some owners elect to euthanize affected pets. In treated dogs, survival rates are approximately 85 percent, with up to 25 percent suffering relapses. Dogs with brain or eye involvement have a worse prognosis, and dogs with poor liver or kidney function may not be able to tolerate the necessary medications that must be metabolized by these organs. If an eye is involved, it usually must be removed since eyes don't respond well to therapy and serve as a source of infection.
For many years, the 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 veterinarian may temporarily delay therapy while the kidneys recover from the injury.
Ketoconazole given orally twice a day has been effective against blastomycosis. Although it may take 10-14 days to see clinical improvement with this drug, it may be useful in a dog with poor kidney function and a mild form of the disease. Given alone, ketoconazole has a lower cure rate than amphotericin, but when given together, the two drugs work synergistically, allowing veterinarians to use lower doses of amphotericin and minimize the risk of kidney failure as well as promoting 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 related to liver toxicity, like ketoconazole, with the addition of ulcerative skin lesions and swelling of the legs at the higher dose.
The biggest drawback to itraconazole is the cost, about $10 per day for a 40-pound dog for the medication alone, 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 must also be considered in the cost of treatment. Total fees of $1000 or more would not be unusual.
Blastomycosis is not generally considered a zoonotic disease, meaning one that is potentially contagious to people. 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 such as contaminated soil near a waterway. 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 penetrating wounds with sharp contaminated objects such as a sharp stick or a dog bite, so good safety and hygiene precautions are indicated. Needless to say, persons with deficient immune systems should not be handling infected dogs.