Blastomycosis is a fungal infection caused by the fungal organism Blastomyces dermatitidis.
What Is Blastomycosis?
Blastomycosis is a fungal infection caused by the fungal organism Blastomyces dermatitidis (B. dermatitidis). The infection occurs in several regions of the world, including Europe, India and Central America, but in North America, most cases occur in the Ohio, Mississippi and Missouri River valleys. Cases have also been reported in other areas such as southern Canada and parts of the southern United States.
Blastomyces dermatitidis lives in soil. Although dogs can get infected through a skin wound that becomes contaminated by soil, most dogs get blastomycosis from inhaling B. dermatitidis fungal spores. Once inhaled, at body temperature, the spores become yeast and cause a fungal infection in the lungs that progresses to pneumonia. In some cases, the infection remains in the lungs, but in many other cases,B. dermatitidis uses the bloodstream to spread to other organs, such as the eyes, skin and bones. Less commonly, the infection can spread to the brain, heart, testicles and other parts of the body.
Blastomyces dermatitidis likes moisture, so the organism is more common near lakes, swamps and river banks. The organism does well in sandy, acidic soil; organic material such as decaying wood and animal feces also promote growth of B. dermatitidis, as does heavy rain. Recent construction or other activities that disturb the soil can increase the likelihood of spores getting into the air and being inhaled.
Blastomycosis is more common in dogs than in humans, and it's rare in cats. Most affected dogs live near water, because that’s where the spores are likely to be found. The disease tends to affect young dogs, hunting dogs or outdoor working dogs, most likely from increased exposure to B. dermatitidis spores rather than from any genetic tendencies.
What Are the Clinical Signs of Blastomycosis?
Because blastomycosis can affect so many regions of the body, clinical signs can vary depending on where the infection is. Signs of blastomycosis may include the following:
- Weight loss
- Decreased appetite
- Coughing
- Abnormal breathing (including rapid or labored breathing)
- Eye infection/eye discharge
- Open, draining skin wounds (primarily on the face, nose and feet, but skin anywhere on the body can become infected)
- Limping (if the bones become involved)
- Fever
- Enlarged lymph nodes
How Is Blastomycosis Diagnosed?
Results of routine blood tests, such as chemistry panel and complete blood cell count (CBC), are likely to be inconclusive, but they can help rule out other conditions. For dogs with respiratory problems, radiographs (X-rays) can be helpful. However, the lung changes caused by blastomycosis can resemble those of other types of pneumonia or even cancer. Similarly, radiographs of bones in affected dogs may show changes resembling bone cancer or bacterial bone infection. In these cases, your veterinarian may recommend submitting samples of lung or bone tissue to a diagnostic laboratory to confirm a suspected diagnosis of blastomycosis.
Currently, cytology is considered the most reliable way to diagnose blastomycosis. This may involve your veterinarian using a needle to take a small sample of cells from a lymph node, skin wound or other area of the body and submitting the sample to a diagnostic laboratory for examination. Although this is the preferred method of diagnosis, several samples may be required for confirmation.
Fungal culture testing can identify B. dermatitidis, so this can be helpful. Specific blood testing to look for antibodies to B. dermatitidis can also be performed, and there is also a urine test that can diagnose the condition in approximately 90 percent of infected dogs. Your veterinarian will discuss the best diagnostic options for your pet.
Treatment and Prognosis (Outcome)
Dogs with severe pneumonia may require hospitalization for oxygen therapy and intensive nursing care. Dogs who are not severely ill can usually be treated at home. Anti-fungal medication successfully treats the infection in over 75 percent of dogs. However, most dogs require treatment for several months.
Because recovery takes a long time, many veterinarians recommend monitoring pets during treatment to assess response to medication. Some veterinarians use the urine test to monitor response to treatment. For dogs with pneumonia, your veterinarian may recommend periodically repeating chest radiographs to see if the lung abnormalities are improving.
Over 75 percent of dogs respond to treatment for blastomycosis, so the likelihood of recovery is good. Dogs with brain disease or severe pneumonia are less likely to survive. Also, some dogs with pneumonia die during the first week or so of treatment as the result of an inflammatory reaction initiated by the death of the B. dermatitidis organisms in the lungs. Some dogs who recover develop permanent scar tissue in their lungs.
For dogs who survive the initial infection, approximately 20 percent relapse within the first several months to years following infection. In 80 percent of those cases, a repeat course of treatment is successful.
Research suggests that some dogs contract blastomycosis but don’t develop clinical signs and get better without treatment. However, dogs who do develop signs of illness should receive aggressive treatment as recommended by a veterinarian.
Can Humans Get Blastomycosis?
Blastomycosis is not contagious, so humans can’t catch it from a sick dog, and it can’t be spread from one dog to another. However, humans can be exposed through inhalation of spores just like dogs can. So walking a dog in an area where spores may be present increases the risk for human exposure. Pet owners are advised to notify their physicians if they develop skin or respiratory abnormalities and their dog has been diagnosed with blastomycosis.
Studies have shown that humans can also become infected through skin wounds if exposure to contaminated soil occurs. Humans are also advised to avoid being bitten by infected dogs.
Can Blastomycosis Be Prevented?
There is currently no vaccine to protect dogs from blastomycosis, but research to develop one is ongoing. Preventing exposure to areas where the organism lives can reduce risk of blastomycosis, but this may be impractical, especially for dogs who live near these areas.
