Blastomycosis in Dogs

What is blastomycosis?

Blastomycosis is a fungal disease caused by Blastomyces dermatitidis. This fungus most commonly infects humans and animals through the respiratory tract. When the fungal spores are inhaled, they settle in the small airways and begin to reproduce. 

The organism then spreads throughout the body and may infect other organs. Rarely, infection occurs through contamination of an open wound.

Researchers in human medicine have been largely unsuccessful in isolating the organism from the environment, but it appears that both humans and animals become infected in particular geographical locations. 

In the United States, the disease is most prevalent in the warm, moist environments found along the Ohio, Missouri, Tennessee, and St. Lawrence river valleys. It is common in the southeastern United States. Blastomycosis has also been diagnosed in the middle Atlantic states, Quebec, Ontario, Manitoba, and the Great Lakes region.

Dogs appear to be more susceptible to blastomycosis than many other species, with the incidence of blastomycosis in cats and humans being much lower than in dogs.

What can I do to rid the environment of the fungal organism?

Unfortunately, there is nothing you can do to eliminate the fungus from the environment. The organism is ubiquitous, meaning it lives everywhere.

What are the signs of blastomycosis?

The blastomycosis fungus seems to target the respiratory tract, although it may spread throughout the entire body. Lung disease is the most common occurrence. Fever, depression, weight loss, and loss of appetite are also common clinical signs. Draining skin lesions are seen in many cases. Infection of the eyes may cause sudden blindness. Lameness, orchitis (testicular inflammation), seizures, coughing, enlarged lymph nodes, and a variety of other signs may occur.

How is blastomycosis diagnosed?

Cytology and/or histopathology are required to diagnose blastomycosis conclusively.

• Cytology, the microscopic examination of cells, may be done using fluid draining from an open wound or aspirated from a nodule or lymph node. Since the organism is usually shed in large numbers in the draining lesions, blastomycosis may be diagnosed in the clinic with cytology; however, your veterinarian may recommend having a pathologist review the cytology to confirm the diagnosis.
• Histopathology, the microscopic examination tissue cells, may be done using a tissue sample (biopsy) and sent to a veterinary pathologist.

There is an antigen test (enzyme immunoassay or EIA) for blastomycosis that may allow veterinarians to diagnose this condition more quickly and accurately. This test can be run on urine, serum, plasma, fluid aspirated from the lungs, and cerebrospinal fluid (the fluid around the brain and spinal cord). This test can be positive for other fungal infections, so cytology results are still important for definitive diagnosis. Ask your veterinarian about the availability of antigen urine and serum tests if your dog is suspected of having blastomycosis.

"Ask your veterinarian about the availability of antigen urine and serum tests if your dog is suspected of having blastomycosis."

A screening blood test, called an agar-gel immunodiffusion (AGID) test, can be used to determine potential exposure by measuring antibodies. A positive result on this test does not necessarily mean your dog is infected with blastomycosis; it only means that your dog has been exposed to the fungal organism.

False positives can occur with previous exposure and do not mean current infection, while false negatives can occur in the early stage, if the infection is contained in one area of the body or if the dog has a poor immune response.

Can blastomycosis be treated?

Yes, blastomycosis can be treated, although not all pets will survive. Fortunately, newer anti-fungal agents such as fluconazole and itraconazole are well tolerated by most dogs and have relatively few side effects compared to the drugs used several years ago.

Itraconazole is the preferred drug of treatment for most dogs. Dogs may require several months of therapy (at least four to six months in most cases) to successfully treat this disease. Amphotericin B (Abelcet®), another type of antifungal medication, is also sometimes used. Depending on the extent of the lesions caused by the organism, other treatments such as corticosteroid may also be used.

What is the prognosis for my dog?

Prognosis is good for many cases of blastomycosis infection, with recovery rates between 50% and 75%. There is no way to determine the outcome before starting treatment, although a dog in poor condition or with advanced disease is less likely to survive.

For many, the critical period comes in the first 24–72 hours when the drug first takes effect and the fungi begin to die. Because the lungs usually harbor many of these fungal organisms, a severe inflammatory response may occur, causing respiratory distress or failure. As well, involvement of three or more body systems carries a poor prognosis for recovery. Your veterinarian will radiograph (X-ray) your dog's chest prior to therapy to assess the condition of the lungs, although chest X-rays cannot always predict the outcome of treatment.

Relapse of blastomycosis is more common when the fungus invades the nervous system, the testicles, or the eyes. Many drugs have difficulty penetrating the natural defensive barriers of these body systems, making it much harder to eliminate organisms in these sites. Castration of male dogs may be required to remove this potential source of organisms.

For similar reasons, one or both eyes may be removed, especially if the disease has already blinded the pet. Even when treatment is successful, the risk of relapse is very real with this disease. Follow-up testing involves regular physical exams, X-rays, and laboratory tests. Assessment with the MiraVista urine antigen test may be recommended to evaluate treatment success and to determine when medication may be discontinued.

Am I at risk of infection from my animal?

Studies on the fungus have shown that once an animal is infected, the organism enters a different form or phase that does not appear to be infectious to other animals or to humans.

However, common sense dictates that strict hygiene should be followed when handling any draining lesions. Handlers should wear protective gloves and wash their hands thoroughly after contact with an infected animal. Those individuals with a compromised immune system should limit their contact with an infected pet.

In most cases, the infected pet does not need to be segregated from other people or pets in the family. The true risk of infection to others comes from sharing the same environment where the original infection occurred (i.e., soil). Because the Blastomyces organism may be harbored near your home, you should advise your family physician of your pet's diagnosis.