Blastomycosis in Dogs: Symptoms, Treatment, and Prognosis

Blastomycosis is a serious fungal infection that affects dogs across North America, particularly in regions with warm, humid climates near bodies of water. This disease is caused by the fungus Blastomyces dermatitidis, which is found in soil and water environments. When dogs inhale spores from contaminated soil, they can develop this potentially life-threatening infection. Understanding the symptoms, diagnostic procedures, and treatment options is essential for dog owners and veterinary professionals to ensure the best possible outcomes for affected pets.

What Is Blastomycosis?

Blastomycosis is a systemic fungal infection that begins in the lungs after a dog inhales fungal spores from the environment. Unlike some fungal infections that remain localized, blastomycosis can spread to multiple organ systems throughout the body. The disease is not contagious between dogs or from dogs to humans through casual contact, though the fungus itself exists in environmental sources. Dogs of any age, breed, or gender can contract blastomycosis, though certain geographic locations and outdoor lifestyles increase the risk of exposure.

Geographic Distribution and Risk Factors

Blastomycosis is endemic to certain regions, particularly those with warm, moist climates near rivers, lakes, and other water sources. Dogs that spend significant time outdoors, especially those who dig in soil or play near water, face increased risk of exposure. Hunting dogs, working dogs, and pets with outdoor access in endemic areas are particularly vulnerable. While any dog can contract the disease, those with compromised immune systems may develop more severe disease or have worse prognosis.

Symptoms of Blastomycosis in Dogs

The symptoms of blastomycosis vary depending on which organs are affected. Since the fungus is inhaled, it typically settles in the lungs of 65 to 85 percent of infected dogs. However, the disease can spread to other organs and tissues, causing diverse clinical presentations. Early recognition of symptoms is crucial for successful treatment outcomes.

Respiratory Symptoms

Since most cases involve lung involvement, respiratory symptoms are among the most common signs of blastomycosis. Dogs typically experience a persistent cough that may worsen with exercise or exertion. Difficulty breathing, exercise intolerance, and labored respiration are frequent complaints. In advanced cases, dogs may show signs of respiratory distress or tachypnea. Some dogs develop nasal discharge, which may be bloody in severe cases. A thoracic radiographic examination is recommended in all dogs suspected of having blastomycosis, even if lung lesions are clinically silent.

Systemic Symptoms

Beyond respiratory signs, dogs with blastomycosis often exhibit general signs of illness. Fever (temperature greater than 103°F or 39.4°C) is present in 40 to 60 percent of infected dogs. Weight loss is common, as is poor appetite and anorexia. Lethargy and depression are typical, with affected dogs showing reduced energy and enthusiasm for normal activities. Lymph node enlargement, particularly in the chest cavity, occurs frequently and can be detected through physical examination or imaging.

Organ-Specific Symptoms

When blastomycosis spreads beyond the lungs, additional symptoms develop based on affected organ systems. Skin involvement may present as nodules, pustules, draining lesions, or rash-like eruptions, sometimes with open drainage tracts. Eye problems can occur, including inflammation, swelling, or blindness when the disease affects ocular tissues. Urinary symptoms may include bloody urine, difficulty urinating, or in males, enlarged testicles. Neurological signs such as head tilts, seizures, or behavioral changes indicate central nervous system involvement. Lameness or limping suggests bone or joint involvement. In most cases, skin lesions and respiratory problems are the earliest detectable symptoms.

Diagnosis of Blastomycosis

Accurate diagnosis is essential for initiating appropriate treatment. Veterinarians consider blastomycosis when dogs present with the combination of lethargy, fever, weight loss, and lymph node enlargement, particularly if they have a history of exposure in endemic regions. Several diagnostic methods are available, often used in combination to confirm the diagnosis.

Radiographic examination of the chest frequently reveals distinctive patterns consistent with blastomycosis. Blood analysis may show abnormalities such as leukocytosis, monocytosis, and neutrophilia. Urine antigen testing, such as the MVista EIA urine antigen test, can confirm the presence of the fungal organism. In some cases, tissue samples or fluid analysis may be necessary to identify the characteristic yeast forms of Blastomyces dermatitidis. Antigen readings are used both for diagnosis and to monitor treatment progress.

Treatment of Blastomycosis

Treatment of blastomycosis requires a long-term commitment and combination approach in many cases. The goal is to eliminate the fungal infection while managing inflammation and supporting organ function. Treatment duration typically extends from four to six months or longer, depending on disease severity and response to therapy.

First-Line Antifungal Medications

Itraconazole is the preferred drug of treatment for most dogs with blastomycosis. It is as effective as older treatments but is associated with fewer adverse effects and can be administered orally, making it convenient for long-term home treatment. Itraconazole should be administered with food to ensure optimal absorption. The typical dosing is 5 mg/kg twice daily for three days, then once daily thereafter. About 70 to 80 percent of dogs respond completely to treatment with itraconazole, amphotericin B, or the two drugs in combination.

Fluconazole and ketoconazole are alternative azole antifungal agents that may be used, though itraconazole remains the most commonly recommended option. These medications are generally well-tolerated by most dogs with relatively few side effects compared to drugs used several years ago. Terbinafine may be considered in some cases but should not be combined with itraconazole.

Amphotericin B Administration

Amphotericin B (Abelcet®) is sometimes used, particularly in severe or rapidly progressive cases. This medication is extremely potent but also carries significant side effects, including nephrotoxicity. It must be administered intravenously with careful protocols, including sodium loading before treatment and slow intravenous administration over five to six hours. The typical dosing is 0.5 to 1.0 mg/kg, three times weekly or every other day, with cumulative doses up to 12 to 24 mg/kg depending on severity. Combination therapy with amphotericin B and an azole antifungal allows for decreased total doses of amphotericin B, potentially reducing nephrotoxicity risk.

Supportive Care and Adjunctive Treatments

Depending on the extent of lesions and severity of disease, corticosteroids may be administered to reduce inflammation. These are particularly important during the initial weeks of treatment when dying fungal organisms can trigger severe inflammatory reactions. Dogs experiencing respiratory distress may require oxygen therapy and hospitalization until improvement is demonstrated. Additional supportive medications such as mirtazapine for appetite stimulation, meloxicam for pain management, furosemide for fluid management, and tetracycline for bacterial control may be incorporated into comprehensive treatment plans.

Treatment Duration and Monitoring

Itraconazole or other azoles should be administered for at least 60 days in all infected dogs and continued for at least one month after all clinical and radiographic evidence of disease has resolved. Dogs with severe lung involvement require treatment for at least 90 days. Regular monitoring through physical examination, radiographic reassessment, and antigen testing is essential to track treatment progress. Blood level monitoring may be performed two weeks after starting treatment to ensure therapeutic levels are achieved.

Even with appropriate treatment duration, 20 to 25 percent of dogs will experience relapse after itraconazole therapy, necessitating continued vigilance and follow-up care. Some cases require extended hospitalization, intravenous medications, and intensive care management.

Recovery and Cure Criteria

A dog is considered cured of blastomycosis when specific criteria are met. These include the absence of clinical signs of disease, completion of at least six months of treatment, and two consecutive antigen readings of zero. Achieving these zero readings is often a gradual process, with some dogs showing initial improvement followed by elevated readings before finally achieving sustained zero levels. Once cure is achieved, regular follow-up appointments are typically scheduled every few months to ensure the disease remains in remission and antigen levels stay at zero.

Prognosis for Dogs with Blastomycosis

If appropriate treatment is started early, most dogs can be cured of blastomycosis. However, the prognosis depends on several factors, including disease severity, organs affected, and the dog’s overall health status. Long-term expensive treatment may be required, making early diagnosis crucial. Dogs whose eyes are severely affected may not respond well to treatment and may require topical medications or, if significantly affected, surgical removal of the eye.

The first week or two of treatment is especially critical. In life-threatening cases, aggressive intervention may be necessary. Dogs can die from the disease, particularly if severe lung involvement occurs or if the inflammation from dying fungal organisms causes additional damage. However, with dedicated treatment and monitoring, many dogs achieve complete recovery and return to normal quality of life.

Prevention and Management

While no vaccine currently exists for blastomycosis, reducing exposure risk is advisable for dogs living in endemic areas. Limiting digging in soil near water sources, avoiding areas known to harbor the fungus, and maintaining good overall health to support immune function are reasonable precautions. Dogs with known exposure should be monitored closely for early symptoms.

Frequently Asked Questions About Blastomycosis in Dogs

Q: How long does blastomycosis treatment typically take?

A: Treatment usually requires four to six months or longer. Dogs must receive medication for at least 60 days and continue for at least one month after all clinical and radiographic signs resolve. Some cases require extended treatment up to one year or more.

Q: Can blastomycosis be transmitted from dogs to humans?

A: Blastomycosis is not contagious between dogs or from dogs to humans through casual contact. Both humans and dogs contract the disease independently by inhaling fungal spores from contaminated soil and water.

Q: What are the side effects of blastomycosis medications?

A: Itraconazole is generally well-tolerated with relatively few side effects. Amphotericin B, while effective, can cause kidney damage and requires careful monitoring. Corticosteroids used during initial treatment may have various effects depending on duration and dosage.

Q: Is blastomycosis fatal in dogs?

A: While blastomycosis is serious and potentially life-threatening, it is often curable with appropriate early treatment. However, approximately 20 to 25 percent of dogs may relapse after treatment, and dogs with severe lung involvement or central nervous system involvement face higher risk.

Q: Can my dog recover from blastomycosis?

A: Yes, many dogs recover completely from blastomycosis when treatment is started early and followed consistently. However, treatment is lengthy and expensive, and requires commitment from pet owners to administer medications regularly and maintain veterinary follow-up appointments.

Q: How is blastomycosis diagnosed?

A: Diagnosis involves a combination of methods including radiographic examination, blood analysis, urine antigen testing, and sometimes tissue sampling. Veterinarians consider blastomycosis when dogs present with fever, lethargy, weight loss, and lymph node enlargement, particularly if they have exposure history in endemic regions.

Q: What should I do if I suspect my dog has blastomycosis?

A: Contact your veterinarian immediately, especially if your dog shows signs of respiratory disease, persistent cough, fever, weight loss, or lymph node enlargement. Early diagnosis and treatment significantly improve the chances of successful recovery.