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.
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.
Blastomycosis signs in dogs can include one or more of the following, depending on which organ system is infected:
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 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.
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.
Under ordinary circumstances, dogs cannot infect one another, nor can they directly infect humans or other animals. Accidental punctures after needle aspiration of lesions in dogs has, however, led to skin infections in vets. For that reason, topical treatment of skin lesions should be undertaken with attention to personal protection for those with open cuts or sores, especially for humans with compromised immune systems. Here's some information from the Centers for Disease Control and Prevention (CDC) on blastomycosis in humans.
Thankfully, this infection is considered relatively uncommon in people. However, it's important to recognize that pets often serve as sentinels, or indicators of environmental exposure, for this disease. Thus, if your dog contracts this disease is it's like that there's a common source of environmental exposure that may leave you and any other pets in the house at risk. If you're ever concerned that you may have been exposed to this disease you should consult with your physician about what steps to take.
Thankfully, there is a family of anti-fungal drugs that can be used to help treat this infection. However, treatment courses are often extensive (6-8 months minimum) and anti-fungal medications can have significant side effects (including high cost). In some cases, treatment can require well over a year to completely clear the infection, assuming it can be cleared at all. While treatable in most cases, blastomycosis can be a very expensive disease. Dogs may need to be hospitalized for extended periods (longer for those with severe respiratory signs) and multiple drugs may need to be employed, depending on any complications from the infection. Dog's may also end up requiring limb amputation in cases of severe bone infection. Dogs with serious lung infections generally have a 50-50 prognosis for survival while in hospital, but the outlook generally becomes more favorable once they come home.
Infections in the eye can be particularly problematic to treat and may require the input of a veterinary eye specialist. Topical eye medications can help reduce pain and discomfort associated with infection in the eyes, but generally don't help with clearing the infection. Blastomycosis organisms often become entrenched in the eyes are difficult clear. Therefore, in some cases the infected eye may need to be removed either due to permanent vision loss or to help finish clearing the infection from the body.
Dogs with blastomycosis are often discharged from the hospital with instructions to administer oral or ophthalmic medications for an expended period of time. Topical treatment of skin lesions and respiratory treatments (such as nebulization) may be recommended as well.
Unfortunately, there is no vaccine to help with the prevention of blastomycosis in dogs. Keeping dogs away from wooded, marshy landscapes when there is snow or rain is the best way to prevent this infection.
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