Dog Blastomycosis Fungal Disease

Blastomycosis of Blasto is a systemic fungal disease that primarily affects dogs and humans, but has also been known to infect cats, sea lions and horses. It is caused by the fungal organism Blastomyces dermatitidis, which is found as a mold in the soil or at room temperatures and as yeast in tissues or at body temperatures. Infection occurs primarily through inhalation. In the lung, alveolar macrophages phagocytize spores and the organism transforms to the yeast phase. Pulmonary macrophages transport the organism to the pulmonary interstitium. Other routes of infection include skin lesions or penetrating injuries that introduce the organism into the body.

Blastomycosis grows in forms: fungal form and yeast form. In fungal form, the bacteria occurs in the environment and the organism creates microscopically tiny spores that, once airborne, are able to pass far into the depths of the lungs. These spores are released from the fungus when the soil is disturbed by the dog digging for gophers or simply by the dog probing the soils following the odor trails that they love so much. Once the spores have taken hold, they grow as single celled yeast forms rather than the fungal form. This is why the Blasto organism is called a biphasic organism... it can grow in the environment as a fungus and within a mammal as a yeast. 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.

After inhalation of organisms the incubation period for Blasto can be from a few days to many weeks before any signs of disease show up. Fever of 104 to 105 degrees, poor appetite, low grade deep cough, loss of exercise tolerance, and listlessness are cardinal signs of Blastomycosis. Similar to the other systemic fungal infections, Blastomycosis can spread throughout the body from the lungs and invade lymph nodes, joints, eye structures and skin. Often the first evidence a veterinarian has of Blastomycosis is a small draining ulcer that looks like a small abscess. Sudden blindness, lameness, and blood in the urine may be the first signs of disease... sometimes showing up before any coughing is noticed.

In one retrospective study, clinical signs of Blastomycosis includes respiratory tract problems, depression, anorexia, ocular problems, weight loss, dermatologic abnormalities, lethargy, fever of unknown origin, lameness, exercise intolerance, gastrointestinal tract problems, polyuria or polydipsia, mammary gland mass and urogenital tract problems. Cats show similar clinical signs as dogs, with respiratory difficulty characterized by dyspnea or chronic cough being the most common sign. Depression, dehydration, ocular disease, and CNS signs are also seen. Physical findings reflect clinical history and vary greatly. These include lymphadenopathy, fever, harsh lung sounds, draining skin lesions, chorioretinitis, anterior uveitis, cough, emaciation, retinal detachment, soft–tissue mass and more. Blastomycosis may be diagnosed by identification of the organisms in cytologic samples. Blastomyces dermatitidis usually is seen in the form of round yeast ranging from approximately 5-20 µm in diameter.

There are a few treatments and most of them are going to be drug related depending on how far the disease has progressed. If you do not seek treatment the disease can be life threatening. When the dog has been treated for the disease you should see an improvement over a three to five day period. The first drug usually provided for the disease is amphotericin B. This is an intravenous injection that goes directly to the source of the infection. Since the drug can have adverse effects the vet usually keeps the dog for a couple of days to make sure there is no kidney failure. Ketoconazole is also given. This drug is provided orally. Most often this drug takes ten to fourteen days to see any effects. It has been known to cure the issue, but sometimes both drugs may have to be given. Itraconazole is the newest treatment on the market; however some vets have yet to take up this medication because of the expense to the owner. It can cost about $1000 for treatment since they need to have several doses over a period of days for the effects to work.

Blastomycosis is rare fungal disease with a prevalence of 205/100,000 cases presenting to tertiary facilities.3 While it is a rare disease, it should be considered as a differential diagnosis in any case exhibiting the clinical signs discussed above. If other more common conditions have been excluded, blastomycosis should be investigated. It is an insidious disease that often is recognized only through extensive search for the organism or through a combination of clinical signs, history and signalment.