How to prevent and treat blastomycosis in dogs

Canine blastomycosis, also known as “blasto,” is a systemic fungal infection caused by the fungus Blastomyces dermatitidis. The fungus lives in moist soil and decaying organic matter, particularly in areas near rivers, lakes, and wetlands. It’s most commonly found in North America, especially in regions that include the Great Lakes, areas of the southeastern United States, and the Ohio, Mississippi, and Missouri river valleys. Cases are rare outside these regions, but isolated reports have occurred elsewhere.

Blastomycosis can become a life-threatening condition as it spreads throughout the body and affects the lungs. Treatment involves antifungal therapy for multiple months in addition to supportive care for symptoms, secondary infections, and damaged lungs or other organs. If you’re concerned about your dog’s risk of contracting blastomycosis, read on to learn about:

• Signs of blastomycosis in dogs
• How canine blastomycosis can be prevented
• The treatment process for blastomycosis in dogs
• What to do if you think your dog has a fungal infection

Clinical signs of blastomycosis in dogs are often vague at first and may appear similar to many other health issues. Common early symptoms include coughing, weight loss, swollen lymph nodes, and lethargy. As the infection progresses, it can affect multiple organs, including the lungs, eyes, skin, bones, and nervous system, leading to more severe symptoms. Treatment with antifungal medication typically lasts for several months and may affect liver function or cause other side effects, so close veterinary monitoring is important throughout the treatment process. Prognosis is best when the infection is treated early, so it’s always best to seek veterinary care when your dog shows signs of illness, even if those signs seem minor.

Blastomycosis, also known as “blasto,” is a serious fungal disease that can affect dogs, people, and other mammals. The infection is caused by Blastomyces dermatitidis, a fungus that lives in moist soil and decaying organic matter. It’s most commonly found in North America in regions that include the Great Lakes, parts of the southeastern US, and the Mississippi, Ohio, and Missouri river valleys.

The risk of blastomycosis is higher in certain geographic areas, so the best preventative strategy is to minimize a dog’s exposure to moist soil and decaying organic material in regions where the fungus is known to be present. While completely avoiding these environments may not always be practical, being aware of the risk and limiting activities like digging or sniffing in high-risk areas can help reduce the chance of infection.

Infection typically occurs when blasto mold spores are inhaled. Once inside the dog, the mold transforms into yeast, which can more easily spread and infect multiple parts of the body. Because blasto spores are typically inhaled, infection usually begins in the lungs before spreading to other areas of the body. Though rare, dogs can also become infected if spores contaminate an open wound. Blastomycosis may also affect:

• Eyes
• Lymph nodes
• Skin
• Bones
• Nervous system (brain, spinal cord)

Other areas can be affected in dogs with blastomycosis, but this is less common. Systemic blastomycosis occurs when the fungal infection spreads to multiple body systems.

Symptoms of blastomycosis in dogs

Because blastomycosis can affect so many parts of the body, clinical signs can vary widely. Respiratory signs are common, as the respiratory tract is typically the first site of infection. Respiratory symptoms may include:

• Coughing
• Increased breathing effort
• Exercise intolerance
• Bloody nasal discharge

Canine blastomycosis may cause skin or musculoskeletal symptoms, such as:

• Skin lesions or ulcers
• Swollen lymph nodes
• Limping
• Enlarged testicles (intact males only)
• Swollen mammary glands

Neurologic signs may develop, such as:

• Head pressing
• Seizures
• Incoordination (ataxia)

Ocular blastomycosis, or infection in the eyes, can cause:

• Eye inflammation
• Vision loss or blindness
• Changes in the appearance of the eyes

Finally, blastomycosis can cause various generalized signs of illness, including:

• Lethargy
• Loss of appetite
• Weight loss
• Bloody urine (rare)

Early signs of canine blastomycosis are often generalized and nonspecific, with signs like lethargy and loss of appetite appearing before more severe or organ-specific symptoms develop. Symptoms change and become worse over time as the infection spreads and becomes more severe.

How common is blastomycosis in dogs?

The exact number of cases of canine blastomycosis that occur in the United States each year is not well documented. This is largely due to a lack of nationwide reporting requirements for the disease and the fact that the disease often goes undiagnosed. Studies in specific regions help to provide insight into the disease’s prevalence in certain parts of North America. Every region is different, so it’s important to talk to a veterinarian in your area to learn more about the risk of blastomycosis for your dog.

While it’s not always possible to completely prevent blastomycosis, you can take steps to reduce your dog’s risk, especially in areas where the fungus Blastomyces is endemic, or known to be present.

If you live in or are visiting a blasto-endemic region, you can lower your dog’s risk by:

• Avoiding unnecessary exposure to areas with disturbed or moist soil
• Limiting activities like digging, sniffing, or swimming in high-risk environments
• Keeping dogs on leash during hikes or outdoor recreation in affected areas

For people and dogs who do not live in blasto-endemic regions, the risk of blastomycosis is extremely low. Infection prevention in these cases simply involves awareness when traveling or relocating to areas where the fungus is found.

Geographic distribution of blastomycosis

Blastomycosis can be found worldwide but is most prevalent near bodies of water in specific regions of the US and Canada. At-risk areas in the United States include:

• Ohio River valley (Ohio, Indiana, Kentucky, southwest Pennsylvania, northwest West Virginia)
• Mississippi River valley (Minnesota, Iowa, Wisconsin, Illinois, Missouri, Arkansas, Louisiana, Mississippi)
• Missouri River valley (North and South Dakota, Kansas, Nebraska)

At-risk areas in Canada include:

• Manitoba
• Southern Saskatchewan
• Southern Quebec
• Ontario

Dogs most at risk for blastomycosis

Many dogs diagnosed with blastomycosis live in or have visited areas near bodies of water such as lakes, rivers, or wetlands. Proximity to waterways is a recognized environmental risk factor, especially in regions where the fungus is known to be present. While any dog can contract blastomycosis, rates of infection are highest in dogs who are:

• Living in or visiting blasto-endemic areas
• Between the ages of one and five years
• Intact males
• Large breeds

Hunting and sport dogs are at a higher risk of infection due to increased exposure from roaming, sniffing, and digging in soil that may contain spores. Though humans and dogs can both contract blastomycosis, the disease has not been shown to spread between people and their pets. Most cases of blastomycosis in dogs are diagnosed in late summer or early fall.

Steps for preventing blastomycosis in at-risk dogs

If you live in or visit high-risk areas, especially if your dog is in a higher risk group, try to prevent them from digging or sniffing near waterways or where decaying vegetation is abundant. It’s also best to avoid areas where dirt has recently been dug up, such as near construction sites.

There is currently no vaccine to prevent a blastomycosis infection. If you’re concerned about your dog’s overall risk for contracting blastomycosis and have questions about the best way to prevent it, you can talk to an online vet for expert advice tailored to your unique situation.

The diagnostic process usually starts with a veterinarian taking a full history and assessing the patient, including getting a full understanding of any symptoms. Early symptoms of blastomycosis may be vague, nondescript, and common to many health issues. After a physical exam, patient history, and discussion of presenting symptoms, a vet may recommend diagnostic testing to determine the underlying cause of the symptoms. Initial testing may involve:

• Routine blood tests
• Diagnostic imaging, such as X-rays or ultrasounds
• Examination of skin samples under a microscope (cytology)
• Urine tests
• Biopsy of skin lesions or other affected areas of the body
• Fungal culture

Blastomycosis cannot be identified in routine blood tests alone, and diagnosis often isn’t straightforward. “The symptoms associated with systemic fungal diseases occur more commonly with other diseases, such as bacterial infections, viruses, tumors, foreign bodies, or parasites. This is especially true for respiratory symptoms,” explains Dr. Jo Myers, a veterinarian on the Vetster platform. “In some cases, a vet may opt to try treating a dog with respiratory symptoms for a more common viral or bacterial infection to see if that works first before taking a more aggressive diagnostic approach." A definitive diagnosis of blastomycosis is made by identifying it on samples from skin lesions, lymph nodes, or respiratory secretions. Additionally, urine or blood serum antigen testing is commonly used and offers quick, non-invasive results.

Once a diagnosis of blastomycosis is made, treatment involves long-term antifungal therapy, often for several months. Treating blastomycosis with antifungal medication is very different from treating common bacterial infections, which often respond to a short course of inexpensive antibiotics. Antifungal drugs must often be used for several months, and they are typically more expensive than antibiotics, especially if you have a larger breed dog requiring higher doses.

Antifungal medications can cause liver toxicity, so regular blood work is necessary before and during treatment. Symptoms may grow worse shortly after starting treatment, especially in cases of severe infection. Supportive care, such as hospitalization, supplemental oxygen, pain medication, and anti-inflammatory drugs, may be needed.

Prognosis varies widely depending on how severe the infection is, how the dog responds to treatment, and what part(s) of the body is affected. Dogs with mild respiratory disease often have a good outcome. Dogs with severe lung disease or neurologic symptoms often have a poor prognosis. Mortality rates are also high if multiple body systems are affected. The risk of relapse is high, with roughly one in five affected dogs developing symptoms again after seemingly successful treatment.

It’s important to talk to a veterinarian any time your dog is showing changes to their physical health, even if those changes seem minor. Blastomycosis and other severe diseases can have vague and seemingly minor symptoms at first.

Symptoms alone are not enough to accurately diagnose a dog with blastomycosis. Talking to a vet is the first step toward getting a definitive diagnosis, which is necessary in determining what kind of treatment is needed.

If you live in or have traveled to an area where blastomycosis is common and your dog is showing symptoms, let your veterinarian know. Systemic fungal infections can be life threatening, but they typically progress slowly, with symptoms worsening over time rather than appearing all at once. Emergency care is always necessary if your dog has:

• Difficulty breathing
• Neurologic symptoms, such as seizures, ataxia, and head pressing
• Sudden weight loss or loss of appetite

If you’re unsure if your dog’s symptoms are considered an emergency, online veterinary professionals can help by virtually examining your dog, discussing their symptoms, and providing professional advice on what to do next.

Can blastomycosis be prevented in dogs?

Blastomycosis can be found worldwide where there’s damp, acidic soil with decaying organic matter, but it’s especially common in the Ohio, Mississippi, and Missouri river valleys as well as the Great Lakes region and parts of the southeastern US. Because blastomycosis is more common in certain geographic regions, the best way to reduce your dog’s risk is to be cautious in areas where the fungus is known to live, especially around moist soil, wooded areas, and water sources. While completely avoiding these environments may not be practical, limiting activities like digging or sniffing in high-risk areas can help lower the chance of infection. If you live in these areas or visit with your dog, prevent your dog from digging, sniffing, or rolling in the soil, and avoid areas where the soil has been dug up, such as around construction sites. There is currently no vaccine to prevent blastomycosis in dogs.

How do you treat blastomycosis in dogs?

The treatment of choice for canine blastomycosis is antifungal medications for an extended period of time. Treating blastomycosis in dogs takes a long time, with the duration of treatment often lasting several months. Extensive use of antifungal drugs can lead to liver failure, so monitoring symptoms and blood work is crucial during treatment.

Can dogs recover from blastomycosis?

Many dogs can recover from blastomycosis with prompt diagnosis and appropriate treatment. However, blastomycosis is a serious systemic fungal infection that can affect multiple organs, most commonly the lungs, eyes, skin, bones, and lymph nodes. Dogs with mild or localized disease limited to the respiratory tract often respond well to antifungal treatment and have a good prognosis. However, the prognosis is often poor for dogs with severe lung involvement, neurologic symptoms, or multiple affected body systems. Treatment for blastomycosis can be challenging because it may appear to be resolved, even when it isn’t, leading to about one in five dogs suffering relapse. Close monitoring during and after treatment is important to prevent and treat recurrence or treatment-related complications early.

Disclaimer: This article, its content, and its related references do not constitute veterinary advice and should not be considered a substitute for veterinary care. If your pet is showing any symptoms that could indicate a medical emergency, please seek immediate emergency care.