Blastomycosis is a fungal infection that can cause severe illness in dogs.
After spores are inhaled, the fungus Blastomyces dermatitidis infects the lungs and then migrates to other organs. Potentially, it can attack any part of the body including bone, skin, nerves, lymph nodes, mammary glands, the prostate, eyes, and the nervous system.
Both people and dogs can catch blastomycosis, but dogs are 10 times more likely to be infected than humans. Most of the cases are young, male, large-breed dogs. However, blastomycosis is not widespread and is limited to certain parts of the United States.
Of those dogs infected with blastomycosis, around 85% have respiratory signs. This includes a cough, rapid breathing even at rest, shallow breathing and breathlessness when exercising. The dog seems generally unwell; he may run a fever and lose his appetite, his energy levels dwindle and he is no longer interested in playing.
Other symptoms depend on which part of the body the fungus invades. Most common are skin lesions and enlarged lymph nodes, followed by lameness because of bone involvement.
The eye is often affected, with symptoms ranging from conjunctivitis to inflammation deep within the eye that causes the retina to detach, resulting in blindness.
In severe cases, the fungus invades the brain. This is very serious and leads to loss of coordination, seizures, possibly coma and death.
The causative organism is the fungus Blastomyces dermatitidis, found mainly in Missouri, Mississippi, Tennessee, Ohio and the St. Lawrence River Valley.
Blastomyces is a yeast-like organism that loves to grow in moist, humid conditions, which helps it spread. The fungus colonizes rotting vegetation and animal waste, and most dogs pick up infection when sniffing around such material.
People can catch blastomycosis, but only when the fungus is reproducing, and there is little danger of aerosol infection from the dog. Lab workers, however, have been infected by blastomyces reproducing in nutrient plates. Special precautions, such as employing an isolation cabinet, must be taken when handling samples.
Any young, male, large-breed dog who lives in a blastomyces-endemic area and suddenly develops breathing difficulty should raise suspicion of blastomycosis. Chest radiographs can increase the suspicion if certain shadows are seen in the lungs.
Routine blood tests typically show raised globulins and calcium with low protein levels. A specific agar gel immunodiffusion (AGID) test can give a definitive diagnosis with 90% accuracy. Sometimes samples of tissue are taken from enlarged lymph nodes, and with special stains, the fungus can appear.
Blastomyces is sensitive to modern antifungal drugs, but the drawback is the cost of treatment in a large dog. The most effective — and costly — treatment, itraconazole, needs to be given for at least 2 months.
Less expensive antifungals are available, but they tend to be less effective. Severely ill animals may need intravenous treatment with amphotericin, but this drug has a low safety margin and kidney health needs close monitoring.
Dogs with mild illness need treatment for 60–90 days, and there is a relapse rate of 20% within one year of the treatment finishing. Unfortunately, those with brain disease respond poorly, although the outlook for the majority of mild to moderately sick animals is good with adequate treatment.
There is no known means of preventing this infection.
- “Blastomycosis in dogs — 115 cases.” Arceneaux, Taboada & Hosgood. JAVMA, 213(5): 658–664.
- “Evaluation of risk factors for blastomycosis in dogs.” Rudmann et al. JAVMA, 201: 1754.
This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Oct. 13, 2018.