Overview of Canine Blastomycosis

Blastomycosis, commonly referred to as “Blasto”, is a systemic disease that can occur in dogs caused by a fungus present in the soil of certain regions, such as Mississippi, Missouri and Ohio River Valley. The organism is present in the soil and infection occurs by inhalation of the fungus. Once infection is established in the lung, the fungus changes its characteristics by converting from the mycelial form into the yeast form and spreads to other organs causing a disseminated infection.

This fungus exists in two different forms:

Mycelial form. This form is present in the environment and is contagious.

Yeast. This form is found in the tissues and is not contagious.

Hunting dogs that spend a lot of time outdoors and live in endemic areas are at risk for inhaling this organism and developing the disease.

Some animals may be infected but not show clinical signs for a long time. These animals are not a risk for contagion of other animals and people because the stage of the organism present in the animal’s tissues is not that of an infectious stage.

If left untreated, these dogs can become seriously ill. Dogs may develop infection in the kidneys, eyes, brain and bones. According to the organ that is affected, the clinical signs may vary. They may have ocular problems or neurological signs like seizures and head tilt. In some dogs lameness may be the primary complaint due to infection in the bones.

What to Watch For

Symptoms of blastomycosis in dogs may include: 

Coughing

Difficulty breathing

Fever

Loss of appetite

Weight loss

 

Diagnosis of Blastomycosis in Dogs

Suspicion of blastomycosis comes from the history of living in an area at risk for this infection, especially in animals that hunt or spend a lot of time outside. Clinical signs may not be very specific.

There are some tests that can be run to see if the animal has been exposed to the organism and has produced antibodies against it. This type of test (serology) requires a blood sample and is not 100 percent reliable. It could be falsely negative in the early stage of the disease.

The definitive diagnosis comes from the identification of the fungus in the tissues. This is possible when skin lesions are present in the form of nodules that drain purulent material. In those cases a biopsy is taken and sent to the laboratory for microscopic examination and for culture.

Treatment of Blastomycosis in Dogs

Affected animals require many months of antifungal therapy.

Some drugs are given intravenously (amphotericin B) while others are given orally (e.g. ketoconazole). Depending on the severity of the disease a combination of drugs may be selected.

These drugs have the potential to cause kidney and liver damage, thus it is very important that your pet is closely monitored and that blood work is repeated frequently to check for signs of toxicity.

The prognosis (outcome) depends on the severity of the lung disease, and how extensive is the infection in the body. It is important to take chest radiographs and evaluate the extent of the lung involvement.

Home Care and Prevention

It is important that you administer the medications as instructed by your veterinarian and that you monitor the appetite and bowel movements of your pet. Some drugs may induce nausea and vomiting. If your pet stops eating, your veterinarian needs to be notified immediately.

There is no vaccine or effective way of disinfecting the contaminated soil.

In-depth Information on Blastomycosis in Dogs

Other medical problems can cause symptoms similar to those encountered in pets with blastomycosis. Your vet will exclude these conditions as necessary before establishing a diagnosis of blastomycosis:

Bacterial pneumonia

Brucellosis

Histoplasmosis

Coccidioidomycosis

Cryptococcosis

Nocardiosis

Actinomcyosis

Rocky Mountain Spotted Fever

Ehrlichiosis

Neoplasia (cancer)

Lymphosarcoma

Primary lung tumor

Tumor elsewhere in the body that has spread (metastasized) to the lungs

Heart failure

Heartworm disease

Systemic immune-mediated disease such as systemic lupus erythematosus

Nodular panniculitis

Lymphomatoid granulomatosis

Eosinophilic lung disease

Blastomycosis is a systemic disease caused by a fungus (Blastomyces dermatidis) present in the soil of certain geographic regions like Mississippi, Missouri and Ohio River Valley). Dogs and people are most commonly infected.

Infection occurs by inhalation of spores from the “mycelial” form of the organism found in the environment, especially moist soil. After the organism becomes established in the lung, dissemination throughout the body occurs. Blastomycosis is endemic in the Mississippi, Missouri and Ohio River valleys.

Young male dogs, especially hunting dogs, are at increased risk presumably because of increased contact with contaminated soil. The yeast form of the organism found in infected body tissues is not contagious, and thus the disease is not readily transmissible between animals or from animals to people.

The prognosis depends on the extent and severity of lung involvement. In dogs, blastomycosis affects the lungs (80 percent of cases), eyes (40 percent of cases), skin (20 to 40 percent of cases), and bones (30 percent of cases).

Most affected animals have systemic symptoms such as fever, lethargy, loss of appetite, and weight loss. Lung involvement leads to respiratory symptoms such as exercise intolerance, cough, and difficulty breathing. The animal’s peripheral lymph nodes often are enlarged. These are found under the neck, in the shoulder region and behind the knee. Bone involvement may occur and result in lameness.

Infection of the urogenital tract, for example the prostate gland in male dogs, occasionally may occur and cause clinical symptoms like blood in the urine or difficult urinations. Nervous system involvement may cause seizures, uncoordination, head tilt, and other symptoms.

Eye involvement can lead to squinting due to pain and light sensitivity. Involvement of the retina may lead to blindness. Involvement of the iris of the eye may be complicated by glaucoma, which is high pressure within the eye. Draining nodules may be found in the skin, and microscopic examination of this material often discloses the organism and yields a diagnosis.

In-depth Information on Diagnosis

Certain diagnostic tests must be performed to confirm the diagnosis of blastomycosis and exclude other diseases that may cause similar symptoms. Tests may include:

A complete medical history and physical examination, including auscultation (listening with a stethoscope) of the lungs, careful examination of the eyes and nervous system, and evaluation of the skin for draining nodules

A complete blood count (CBC or hemogram) to evaluate the severity and chronicity of inflammation, detect the presence of non-regenerative anemia, and check platelet count. The clinical presentation of animals with some diseases associated with low platelet count (Ehrlichiosis, Rocky Mountain spotted fever) can resemble blastomycosis.

Serum biochemistry tests to determine the effect of blastomycosis on other organ systems, and to evaluate the health of other organ systems, especially the liver and kidneys, before treatment with anti-fungal drugs that can be toxic for the liver and kidneys. Rarely, high blood calcium concentration (hypercalcemia) is found in animals with systemic fungal infection and hypercalcemia can occur in diseases that can be confused with systemic fungal infection like lymphosarcoma. Certain blood proteins may be increased in the blood of animals with systemic fungal infection and in those with other chronic infectious diseases.

Urinalysis to identify urogential involvement, evaluate kidney function and check for bacterial urinary tract infection.

X-rays of the chest to evaluate the severity of lung involvement and to check for enlarged lymph nodes in the chest. Bone involvement also may be identified on X-rays of the chest.

X-rays of the abdomen to evaluate vital organs, especially the liver and kidneys. Bone involvement also may be identified on X-rays of the abdomen.

Serologic tests for heartworm disease, brucellosis, and rickettsial infection as well as the agar gel immunodiffusion test to identify blastomycosis. The agar gel test is very reliable but may be negative early in the course of infection.

Finding the blastomyces organism during microscopic examination of material collected from draining skin nodules results in a definitive diagnosis.

Microscopic examination of a biopsy specimen from affected tissue by a veterinary pathologist can also lead to a definitive diagnosis, but this method is more invasive, and results take longer to return from the laboratory.

In-depth Information on Treatment 

Treatment of blastomycosis must be individualized based on the severity of the condition and other factors that must be evaluated by your veterinarian. Therapy is aimed at relief of specific symptoms (e.g. difficulty breathing, coughing, eye problems) and elimination of the fungus from the body. Treatment may include one or more of the following:

Antifungal drugs. Those effective against blastomyces include amphotericin B and the imidazole derivatives (e.g. ketoconazole, itraconazole, fluconazole).

Amphotericin B is often administered intravenously followed by oral administration of ketoconazole, one of the imidazole derivatives. Then it is administered three times per week until a sufficient cumulative dose has been achieved. Amphotericin must be given in relatively small amounts over time because it is very toxic to the kidneys. Kidney function tests must be monitored during the course of amphotericin B therapy. Amphotericin B is given diluted in a 5 percent dextrose solution, and the intravenous administration of the fluid also serves to protect the kidneys from toxicity.

Ketoconazole is an imidazole drug that can be administered orally (often after a course of amphotericin B). Ketoconazole is well absorbed from the gastrointestinal tract and has reasonable activity against blastomyces. Treated animals should be watched for loss of appetite, vomiting, or diarrhea because these symptoms may indicate drug toxicity. Ketoconazole is potentially toxic to the liver, and liver function tests should be monitored in treated animals. Ketoconazole has the potential to produce adverse reactions when used in combination with some other drugs, and other medications being administered to the animal should be reviewed before beginning therapy with ketoconazole. Unfortunately, treatment with ketoconazole usually does not completely eliminate the fungus from the animal’s body.

Itraconazole is another imidazole effective against blastomyces that has less potential for liver toxicity than does ketoconazole. It usually produces a more rapid response than does ketoconazole. Itraconazole must be administered for two to three months, and approximately 20 percent of treated dogs ultimately experience a recurrence of disease. Adverse effects include loss of appetite, vomiting, and diarrhea.

Fluconazole is an imidazole derivative active against blastomyces that has good penetration into the nervous system, eyes, and urinary tract. It is especially useful in animals with urogential infections because ketoconazole and itraconazole are not excreted into the urine in any appreciable amount. The dosage of fluconazole should be adjusted in animals with poor kidney function. In general, however, fluconazole is less toxic than ketoconazole. Also, it is not associated with the adverse drug interactions occasionally observed with ketoconazole use. Like the other imidazole derivatives, it must be administered for a minimum of 60 days and recurrence may occur in up to 20 percent of treated animals.

Follow-up Care for Dogs with Blastomycosis 

Optimal treatment for your dog requires a combination of home and professional veterinary care. Follow-up with your veterinarian is essential. Administer all medications as directed and call your veterinarian if you have questions or problems administering medications to your pet.

Follow-up with your veterinarian for physical examinations and blood tests.

The prognosis is guarded for dogs with severe lung involvement and for those with eye or nervous system involvement. Approximately half of dogs with severe lung involvement experience a worsening of the respiratory function during the first week of treatment. This complication is thought to be caused by rapid killing of the fungal organisms and can lead to death. It is very difficult to treat animals with nervous system involvement. Those with advanced eye involvement have a poor prognosis for return of vision.

The agar gel immunodiffusion test tends to remain positive after treatment and cannot be used to gauge response to treatment. Therapy should be continued for at least a month past resolution of all clinical signs. Most dogs with mild to moderate disease will require 60 days of therapy. If severe disease is present 90 days may be required. Recurrence within one year occurs in 20 percent of cases.

No vaccine is available. Even if areas are identified as infected, sterilization of the soil is not possible.