Update on Diagnosis and Treatment -2013

By Shelagh Grant

Since last reported about canine blastomycosis which has affected several of our dogs over the years, new reports have added more details about the nature of the disease, even though diagnosis and treatment has changed little. Fortunately, the hot dry summers of the past two years have lessened the incidence of the disease in the Pointe au Baril area, but this could change with a warm wet spring or fall. At the request of numerous cottagers, I have updated an earlier account and suggest removing or copying this section for future reference.

General Description

Blastomycosis is a relatively rare disease found in many parts of the United States, especially in the Mississippi, Missouri and Ohio valleys. In Canada, it is found in southern Saskatchewan, Manitoba, Ontario and Quebec. While the disease is more prevalent in the Northwestern Ontario, the shoreline of Georgian Bay – from Manitoulin Island south to and San Souci – is also considered an endemic area.

Blastomycosis affects both dogs and humans. Fortunately, unless one has a compromised immune system, a person usually exhibits a few flu-like symptoms and fully recovers without treatment. The good news is that there have been no recent human cases reported in our area.

Dogs, however, are far more susceptible to the disease that is fatal without treatment. With the exception of the summer of 2005, which experienced an unusual number of cases, incidents in the Pointe au Baril area rarely exceeds two or three annually. Last summer none were reported.

Causes

Blastomycosis is a systemic infection caused by the dimorphic fungus Blastomyces dermatitidis. The infection is spread by dispersal of fungal spores that are most often found in moist sandy, acidic soil near freshwater lakes and rivers. High organic content from decaying wood or animal waste, promotes growth of the fungus, especially in moist warm weather. As a consequence, the extensive shorelines of the Georgian Bay Islands have made this area particularly vulnerable.

If conditions are right, spores will be released in the air upon disturbance of the soil, whether by construction, removal of old logs, gardening, or even dismantling an old beaver lodge. Although a number of documented cases suggested a possible connection with the disease and beavers, only one report referred to incidents where individuals contracted the disease after attempting to remove old beaver dams.¹ Other studies indicated that it was the feces used to construct the dams and lodges that created nutrient rich conditions for growth of the fungus, rather than the animal itself.

Nature of the Disease

Most often, a dog becomes infected by inhaling spores released into the air by the fungus. Upon inhalation, the spores transform into a yeast which invades the lung tissues and multiplies – hence the term “dimorphic.”

Eventually, the yeast may be transferred to other organs through the blood or lymphatic systems, including lymph nodes, eyes, skin, bone, the brain, and in rare cases the central nervous system. A less common form of the disease is a primary skin lesion, usually on a paw or leg, where the spores have entered through a small cut or open sore.

The incubation period may be several weeks or less, but in some cases it can be over a month before symptoms occur. This may mean that your dog will be without any symptoms until its return home, where veterinarians may have had little experience with the disease. As a result, this may lead to misdiagnosis and/or delay in proper treatment.

At present, the only available treatment is costly and extends over a period of months. Some pet owners consider purchasing veterinary insurance to offset potential costs, but read the terms carefully to ensure that it covers infections contracted in endemic areas. Since this is a painful and debilitating disease, a compassionate vet may wisely suggest euthanasia if the infection is so far advanced that prognosis is bleak.

Spread of the Disease

Since contagion can only take place by inhalation of the spores into the lungs or an existing cut or lesion, the only means of the disease passing from a dog to a human is when a person with an open cut on their hand comes in contact with fresh drainage from an infected pet’s skin lesion. Otherwise, it is cannot be transferred directly from dog to humans, from humans to dogs, or from one dog to another.

On the other hand, once the spores are released from disturbed soil, they attach themselves to dust particles and spread by the wind over a wide area. This may partly explain why one’s pet may contract the disease without any prior incidence over a long history of cottage ownership. The fungus may also remain dormant in the soil for years without ever producing spores. To date, there is no successful means of testing the soil for presence of the fungus.

Dogs at Risk

Most studies suggest that larger hunting breeds such as retrievers are most susceptible, with intact males being four to five times more vulnerable than females. One study, however, indicated that in endemic areas like ours both males and females are equally affected, as are working breeds such as terriers, daschunds, spaniels and even poodles. The tendency for the disease to occur with younger dogs may be related to their eagerness to dig and rout, even in the worst kinds of weather.

Other animals can also contract the disease. Cats, for instance, have been known to contract blastomycosis in endemic areas, but only rarely. To my knowledge, there have been no studies on the prevalence of the disease among wild animals in North America.

Symptoms

Since early diagnosis is critical to survival, all cottagers should be aware of the symptoms and their order of priority. The first symptoms to appear in the vast majority of cases are related to infected lungs: loss of appetite, shortness of breath, lethargy and/or cough. Fever occurs in only 40-60% of cases. Skin lesions may also occur, generally on the limbs, nail beds or around the nose. Advanced cases may result in blindness and difficulty walking if bones are affected. Tissues less commonly affected are kidneys, prostrate, testes, joints, nasal passages and the brain, making diagnosis of an advanced illness even more difficult.

Since chances of survival are dependent on early diagnosis and commencement of treatment, all dog owners should take notice if their pet is suddenly behaving out of character, especially if exhibiting lack of appetite, shortness of breath or lethargy, with or without cough and fever. If still at their cottage, they should seek immediate help at one of the Parry Sound veterinary hospitals, which are familiar with diagnostic procedures and can initiate treatment immediately.

Unfortunately the incubation period varies, thus changes in behaviour may not be evident until return home. In this event, it is critical to inform your vet that the dog had spent time in an area endemic for blastomycosis and request immediate chest x-rays and treatment. Suspicious skin lesions will require other forms of testing.

Diagnosis

Thoracic x-rays are the first step in diagnosis, even if your dog is only lethargic and off his or her food. If blastomycosis is present, the x-rays will show diffuse millary or nodular pulmonary changes consistent with the infection. In this event, medication should begin immediately while waiting for confirmation from further tests.

Even then, veterinarians unfamiliar with disease often misdiagnose the disease as pneumonia or lung cancer. Thus one cannot emphasize the importance of an accurate diagnosis – since the fungal disease can be exacerbated if treated with antibiotics. Confirmation of a preliminary diagnosis based on x-rays is normally carried out by cytologic tests from fine-needle aspirates from lymph nodes. Fine-needle biopsies from specific locations of infection identified on x-rays are equally reliable but more difficult to obtain. Both are more reliable than aspirates taken from transtracheal washings or bronchoalveolar lavage.

Serological tests for antibodies may be used to confirm diagnosis, but are often negative in early stages of the disease. Similarly, urinary antigen testing is used but has been known to show false positives if other fungal organisms are present. Polymerase Chain Reaction (PCR) testing based on DNA may help in early diagnosis, but is costly and not readily available.

Forms of Treatment

The most common treatment for less severe cases of canine blastomycosis is an azole antifungal medication. In Canada, this is usually Itraconazole, which can be administered orally but requires several months of treatment.

More severe cases will require hospitalization for additional intravenous medication of Amphotericin B in addition to an anti-fungal drug. Although initially causing toxic side effects and kidney damage, a new lipid-complexed formulation is now available, which is less toxic and can be administered in higher doses. After treatment with Amphotericin B has been competed, you will be able to take your dog home and administer the anti-fungal drug orally, usually for at least a month after clinical evidence has disappeared, perhaps longer depending on the severity of the lung involvement.

A relapse can occur, within six or more months, and is usually related to the severity of the initial infection. A further 60-90 days treatment with Itraconazole has approximately an 80% chance of full cure.

Prognosis

Even with new forms of medication, the survival rate will depend on the stage of the disease at the time of diagnosis. As noted above, advanced stages have poor survival rate and often a vet will advise euthanasia to avoid further suffering. Otherwise, the earlier the diagnosis, the better are the chances of survival. For this reason, all area cottagers should be aware of early symptoms, whether they own dogs themselves or have visitors bringing their pets. For those whose dogs have recovered from the disease, it is wise to keep them on a “short leash” during periods of moist, warm weather. The sudden appearance of mushrooms on your property should be fair warning of a potential release of fungal spores. Further Information While there are many articles on canine blastomycosis available on the internet, the most reliable are prepared by veterinary colleges. One of the best – detailed but easily understood – is available on line from the United States Department of Veterinary Medicine (DVM360). Another is found on the DogHeirs website. American dog owners may also wish to contact, or have their vet contact, the Veterinary College at the University of Wisconsin – Madison for updated diagnostic procedures and treatment. Unfortunately, no similar studies or reports are available from Canadian sources.

A Final Word

Please remember that there is a far less chance of your dog contracting blastomycosis than being hit by a car. Being alert to the symptoms and understanding the need for immediate treatment should allay undue concerns and ensure worry-free enjoyment of your cottage for years and generations to come.

¹ Center for Human-Wildlife Conflict Resolution: Beaver, see under section on disease: http://www.humanwildlife.org/Species/beaver.htm