Understanding Blastomycosis in Dogs

Few infections are considered geographically specific to the United States, but blastomycosis in dogs is one of these rarities. This fungal infection primarily affects dogs, cats and humans in several Southeastern and Midwestern areas, the Pacific Northwest and around the Great Lakes.

Caused by the Blastomyces dermatitidis species of fungus, blastomycosis primarily affects the eyes, lungs and skin. However, a variety of other organ systems — such as the bones, heart, central nervous system and lymphatic system — may also be affected. Read on to learn more about blastomycosis and how to spot blastomycosis signs in dogs.

Transmission of Blastomycosis

Blastomycosis in dogs isn't just limited to geographical regions, it's also limited to very specific habitats characterized by moist, acidic soil containing decaying vegetation. Ideal environments for this fungus include beaver dams and marshes. Therefore, birding dogs and canine hiking partners are considered especially at risk. However, don't be fooled into thinking these are the only dogs that can contract this disease. In high prevalence areas (such as Wisconsin and Northern Illinois) this fungus can be nearly ubiquitous in the soil. It can even be tracked into your house through soil on our shoes, risking exposure even to purely indoor animals.

Infection by blastomycosis in dogs is thought to occur most commonly when the organism's infective particles, called conidia, are aerosolized. Certain weather conditions (such as dew, rain and fog) are believed to activate these fungal particles, which are either inhaled or absorbed by the skin.

Signs of Blastomycosis in Dogs

Blastomycosis signs in dogs can include one or more of the following, depending on which organ system is infected:

• Fever
• Weight loss
• Poor appetite
• Cough
• Lymph node swelling
• Difficulty breathing
• Limping
• Skin lesions (pimples/pustules, sometimes with draining tracts, and rash-like lesions)

Many dogs show signs of multiple organ system involvement. According to DVM 360, cough and difficulty breathing occurs in 85% of canine patients. Skin lesions and lymph node involvement are each only observed in about 50% of patients. Limping happens when the bone is infected, occurring in roughly 25% of cases. Ocular signs are also common, affecting about 50% of infected dogs.

Ocular Blastomycosis Signs

Ocular blastomycosis in dogs tends to develop initially in the back of the eye. Tiny, nodular foci of infection called granulomas affect the retina, leading to retinal detachment and an inflammatory process called chorioretinitis (inflammation of the retina). This may ultimately result in partial or complete blindness, which may be permanent, and could ultimately result in the eye needing to be removed.

The front of the eye is subsequently affected, leading to more readily observed blastomycosis signs in dogs, such as clouding, redness, pain and swelling of the eye. These signs occur as a result of uveitis (inflammation in the eye) and glaucoma (increased pressure in the eye) among other possibilities.

Diagnosing Blastomycosis

This disease can be very difficult to diagnose because the signs are often non-specific. The skin lesions often can be mistaken for a simple skin infection and infections of the bone or lung may look identical to certain types of cancer on imaging tests. The diagnostics your vet will perform will be determined largely by which organ systems are affected. In most cases, your vet may start with tests like chest X-rays (or leg X-rays if your pet is limping) or looking at samples from the skin lesions under a microscope. Most of the time the fungal organisms should be visible under the microscope to make a diagnosis, but sometimes more advanced diagnostics such as biopsies of the skin (or bone) may be necessary to make a diagnosis. There is also an extremely reliable test that looks for evidence of the fungal organism in the urine your vet can send off to an outside lab.