the ultimate guide to Systemic Fungal Disease in dogs

There is no one-size-fits-all approach to tackling systemic fungal disease (SFD) in dogs, as the condition can vary widely from dog to dog. However, there are a few key steps that all pet owners should take to help ensure their dog is as healthy as possible and avoids development of SFD. In this article, we outline these key steps and provide a comprehensive guide to systemic fungal disease in dogs.

The veterinarian looks at the black and white patterns on the viewer in the dim light of the x-ray room.  She thinks how unusual this is . . . a four-year old Golden Retriever in the prime of its life, stricken with lung cancer.  The radiograph doesn’t lie though.  The light patches taking up space in what should be dark areas of the dog’s lungs literally demand attention, stark and unnatural, like  potholes on a busy expressway.  How unusual, the doctor muses, to see cancer like this in such a young dog; cancer in the lungs of any dog is almost never seen unless it has metastasized from somewhere else in the body.  And to come on so quickly!  According to the owners it was happy and energetic as it swam, played fetch and ran with the family just two weeks ago on their vacation.

Now the dog has a fever, is losing weight and coughs frequently… the patient is failing fast!  Something just doesn’t fit.  The veterinarian senses an unusual discomfort with her original diagnosis of metastatic lung cancer, a veritable death sentence for this patient.  She brings the two x-ray films of the dog’s chest, one taken side-to-side and one front-to-back, into the exam room where four anxious people await the doctor’s diagnosis.  The depressed Golden Retriever’s eyes focus upward on the doctor, ears tuned in to the soft voice. . .

“We need to do a little more detective work.  See those whitish areas in the lungs?  At first my thought was a possible cancerous condition, but given your dog’s age and the sudden onset of her sickness, I just don’t believe that’s our problem.  You said she was just fine on the family vacation?  Where was that?”

The astute doctor was now on the right track and refused to be led down the road to euthanasia by a disease in disguise.  Unfortunately, there are canine patients that have not been quite so lucky as the Golden in this case; this patient was accurately diagnosed with Blastomycosis.  Vigorous and immediate treatment was begun for this fungal disease that was acquired 350 miles from the dog’s home while vacationing with its human family.  It was the first case of “Blasto” this doctor had ever seen because soil and other conditions simply did not permit the presence of the organism in the local environment.  This case of Blastomycosis came disguised as lung cancer.

Systemic Fungal Disease in dogs

Valley Fever

This exact scenario has been played out many times – only with different actors in different disguises.  “Valley Fever” is a bad  actor and is caused by a fungal organism called Coccidioides immitis.  Unlike “Blasto”, which is prevalent along river systems of Northern Wisconsin, central Kentucky, Tennessee and Ohio, Valley Fever thrives in arid regions of the Southwest United States.  Disguised as other fungal diseases, pneumonia, cancer,  Lyme Disease, or bacterial dermatitis, Valley Fever can severely and permanently disable a dog if the proper detective work isn’t done to establish a diagnosis and to institute treatment.  Hiding in dusty, arid soils, the microscopically small spores of this organism are inhaled into the lungs and in most cases only a mild respiratory inflammation occurs as the dog’s defense mechanisms wall off the organisms.  In more severe cases the disease can spread throughout the lungs and invade other organs.  This is called systemic dissemination of the disease.  The lungs and long bones of the limbs are a favorite target of Coccidioides immitis.  Click on the image of Tanner on the right, a victim of Valley Fever, to see just how troublesome this “disease in disguise” can be to properly diagnose and treat.

Fungal Diseases

We’ve all heard of ringworm.  This is actually a localized surface dwelling skin fungus. This type of fungal infection is referred to as a CUTANEOUS FUNGAL DISEASE, or a dermatophyte. Non-invasive and rarely dangerous, it creeps along the surface of the skin.   On the other hand (no pun intended!) there are SYSTEMIC FUNGAL DISEASES that have the propensity to invade any organ system of the body. Humans can acquire these systemic fungal diseases from the environment, just like dogs do.

Systemic Fungal Diseases in Dogs Blastomycosis Valley Fever (Coccidioidomycosis) Histoplasmosis Cryptococcosis Aspergillosis CandidiasisCutaneous Fungal Diseases in Dogs Dermatophytosis (Ringworm) . . Cutaneous Yeast Infections Yeast Dermatitis Malassezia Dermatitis

A key factor in recognizing the actor behind the disguise is to garner a thorough medical history that includes noting any travel outside the dog’s home territory.  Every veterinarian in Northern Wisconsin is wise to the masquerading fungal disease Blastomycosis and many have treated dozens of cases.  However, a veterinarian on Marco Island, Florida, might never see a case!  Likewise, if a patient becomes infected with Valley Fever in Tucson, Arizona and is seen weeks later by a veterinarian in the dog’s hometown of Port Washington, N.Y. and no mention is made in the patient history of travel to an area endemic for Valley Fever, prompt diagnosis and treatment may be delayed! 


much like Valley Fever, is a commonly misdiagnosed systemic fungal disease of dogs.  It is a great masquerader and many dogs have been euthanized or had treatment delayed because of a diagnosis of cancer mistakenly being made.  It is acquired most often by inhalation of infective spores present in organic soils such as are present along streams, lakes, ponds and even within the dried mud mortar of beaver lodges.  Landscaping soil and even potting soil can harbor Blastomycosis  organisms and any cat or dog digging up these soils may be exposed to Blastomycosis.  Especially in dry environments where the soil may be more dusty and easily become airborne the potential for infection with Blastomycosis is greater.  The organism is present mostly in the Mississippi, Wisconsin, and Ohio River systems.

Blasto grows in two ways.  One form, called the fungal form, occurs in the environment and the organism creates microscopic spores that, once airborne, are able to pass far into the depths of the lungs.  These spores are released from the fungus when the soil is disturbed by the dog digging for gophers or simply by the dog probing the soils following the odor trails that dogs love so much.  Much less common in cats than dogs, Blasto is easily inhaled into the dog’s lungs.

Once there, the body’s normal defense mechanisms can simply eliminate these spores and no disease results.  However, if the load (numbers) of spores inhaled is very great or the dog is immune suppressed or stressed by other disease or poor diet the organisms may begin to reproduce rapidly and signs of disease occur.  Once the spores have taken hold, they grow as single celled yeast forms rather than the fungal form.  This is way the Blasto organism is called a biphasic organism… it can grow in the environment as a fungus and within a mammal as a yeast.

After inhalation of organisms the incubation period for Blasto can be from a few days to many weeks before any signs of disease show up.  Fever, poor appetite, low grade deep cough, loss of exercise tolerance, and listlessness are cardinal signs of Blastomycosis.  Similar to the other systemic fungal infections, Blastomycosis can spread throughout the body from the lungs and invade lymph nodes, joints, eye structures and skin.  Often the first evidence a veterinarian has of Blastomycosis is a small draining ulcer that looks like a small abscess.  Sudden blindness, lameness, and blood in the urine may be the first signs of disease… even showing up before any coughing is noticed.

Systemic Fungal Disease in Dogs-Prevention

What is the best way to insure that a dog does not fall under the spell of Valley Fever or another diseases in disguise?  Dr. Sheila McCullough, a veterinarian at the University of Illinois Veterinary Teaching Hospital in Urbana, Illinois, has a suggestion based upon her experiences with systemic fungal infections.  “Providing a thorough history is very important to obtain a full picture of what led up to the animal becoming ill,” says Dr. McCullough.  “The client should inform the veterinarian of the patient’s travel history within the past 6 months and what the daily environment is for the pet (i.e. camping, swimming, hunting, living near new construction or landscaping).  A thorough history is the key first step toward figuring out the puzzle.  It is just as important to keep an ongoing dialogue with your veterinarian and to create a plan of action if the initial tests for an expected disease are negative.”

Dr. McCullough’s point should not be underestimated.  Knowledge of a seemingly irrelevant environmental factor can be the key information the doctor needs to proceed toward a proper diagnosis.  Something as innocent as stating “My dog loves to dig into gopher holes, Doctor” or “Two months ago we had soil carted in for landscaping” can turn the doctor’s attention toward a fungal infection such as Valley Fever.


Dr. McCullough states, “Fungal diseases often masquerade as other diseases.  The affected pets present with lethargy, lameness, poor appetite, ‘not doing right’ and may have a fever. Treatment may also be delayed because it is difficult to get a sample of the  organism from a lymph node, skin cytology or trans-tracheal wash.” 

It is crucial that the organisms be identified under the microscope for establishing a positive diagnosis of a fungal disease such as Valley Fever or Blasto. Culturing infected material may take weeks and the patient simply cannot afford to wait even days for a diagnosis!  Blood tests are equivocal.  False positives and negatives are common.

The best and most direct method of establishing a definitive diagnosis is to gather tissue or fluid samples from infected areas such as a swollen lymph node, draining skin lesion or material coughed up by the patient.  A needle biopsy of a lymph node is commonly done and can be performed without anesthesia. During the office call the veterinarian will stain the specimen cells on a microscope slide and look for the infective organisms.  If organisms are seen, BINGO!  Start treatment right now.  If they aren’t seen, special stains at a diagnostic lab are required.  The important thing to do is to BE PERSISTENT in striving to get a diagnosis for the elusive disease in disguise.

Systemic Fungal Disease in Dogs-Treatment

In the past, Amphotericin B was the only known medication useful against Valley Fever and the other systemic fungal organisms.  It had to be given intravenously and with care to keep the dose from harming the kidneys.  This medication has saved thousands of canine (and human) lives.  Recently, though, researchers have provided us with oral medications just as effective in treating fungal infections.  The most popular today are Itraconazole (Sporanox) and Fluconazole (Diflucan).  These tablets are administered for three to six months (sometimes even longer) and your pharmacy bill will be substantial… but your formerly infected dog out in the yard playing fetch with the children wouldn’t be alive without it.

Whenever your dog is sick be sure to provide your veterinarian with a detailed patient history.  And you should be persistent in seeking a definitive diagnosis.  Persistent detective work is your best weapon for unmasking theses diseases in disguise.

In this blog, we explored the basics of systemic fungal disease in dogs and provided a comprehensive overview of the disease, its symptoms, and how to diagnose it. We also outlined a few effective steps that you can take to help your dog avoid or overcome this health issue. We hope that this blog was helpful and that you will find it useful as you work to keep your furry friend healthy and free from systemic fungal disease.

Frequently Asked Questions

How do you treat systemic fungal Disease in dogs?

Antifungal ointments and shampoos can be highly effective in treating the illness, but your veterinarian may also suggest that your pet undergo an oral medicine course.

What is the most common fungal disease in dogs?

Fungal diseases that affect the entire body. Systematic fungal infections damage your dog’s internal organs, most commonly the lungs, nasal passages, bones, and eyes. Three of the most prevalent systemic fungal infections encountered in dogs in the United States are Aspergillosis, Cryptococcosis, and Blastomycosis.

What are the symptoms of a fungal infection in a dogs?

Irritation, dry or crusty skin (especially around your dog’s nails, skin folds, armpits, and anal area), and ear infections are all indications of fungal dermatitis in dogs. Antibiotics may be used in combination with topical antifungal medications such as shampoos or ear drops to treat this illness.

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