What is blastomycosis, and how does an animal get this disease?
Blastomycosis is a fungal disease caused by Blastomyces dermatitidis. This fungus most commonly infects humans and animals through the respiratory tract. After 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.
Although researchers in human medicine have been largely unsuccessful in isolating the organism from the environment, 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 environment found along the Ohio and Mississippi River valleys. It is very common in the Southeastern United States.
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 certain body systems, although it frequently spreads throughout the entire body. Fever, depression, weight loss, and loss of appetite are common clinical signs. Draining skin lesions are seen in many cases. Some degree of respiratory distress is present in advanced cases. Infection of the eyes may cause sudden blindness. Lameness, orchitis (testicular inflammation), seizures, coughing, enlarged lymph nodes, and a variety of other signs are reported.
How is blastomycosis diagnosed?
The only tests which conclusively diagnose blastomycosis are cytology and histopathology. Cytology, the microscopic study of cells, may be performed in the veterinarian’s office on some of the fluid draining from an open wound or aspirated from a nodule or lymph node. Histopathology is the study of cells and tissue architecture. A tissue sample is obtained and sent to a veterinary pathologist for review and diagnosis. Because the organism is usually shed in large numbers in the draining lesions, blastomycosis may be diagnosed in the office with cytology.
There is a screening blood test called an agar-gel immunodiffusion or AGID test, that can be used to determine potential exposure. 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. Many humans and animals have positive blastomycosis screening tests, but this does not mean that they have (or had) blastomycosis. For that reason, the results of an AGID test must be evaluated with other clinical findings and tests before diagnosing blastomycosis.
Can the disease be treated?
Yes, although not all pets will survive. Fortunately, the newest anti-fungal agent being used, itraconazole, is well-tolerated by most dogs and has relatively few side effects when compared to the agents being used several years ago. Dogs may require several months of therapy to successfully treat this disease.
How do I know if my dog will survive?
There is no way to determine this before treatment is begun, 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 a large number of these fungal organisms, a severe inflammatory response may occur, causing respiratory distress or failure. Your veterinarian will radiograph 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 tends to be more common when the organism involves the nervous system, the testicles, or the eyes. Many drugs have difficulty penetrating the natural barriers of these body systems, making it much harder to eliminate organisms in these sites. Male dogs may need to be neutered to remove this potential source of organism. For similar reasons, one or both eyes may be removed, especially if the pet has already been blinded by the disease. The risk of relapse is very real with this disease, even though treatment appears successful.
Am I at risk of infection from my animal?
Studies on the fungus have found 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, strict hygiene should be followed in handling any draining lesions. Protective gloves must be worn, and thorough hand washing should follow contact with these animals.
The infected pet does not need to be segregated from the owner or other household pets. The true risk of infection to others probably comes from sharing the same environment where the original infection occurred (i.e. soil, etc.). Because the Blastomyces organism may be harbored near your home, we would recommend that you advise your family physician of your pet’s diagnosis. Also, if anyone in your family falls into one of the following categories, we would recommend that you consult with your physician:
1. Infants or small children
2. Transplant patients
3. Chemotherapy patients
4. HIV/AIDS
5. Elderly family members
6. Anyone with a known immunosuppressed state
Based on material written by Ernest Ward, DVM. and modified with permission from Lifelearn Inc.
© Copyright 2005 Lifelearn Inc.
