There’s a fungus among us! According to the Centers for Disease Control, each year 500,000 people in the U.S. contract histoplasmosis, a potentially fatal fungal infection. Histoplasmosis also infects dogs and cats. In fact, Oklahoma probably sees the most cases of histoplasmosis in pets in the U.S.
Histoplasma, the fungus that causes histoplasmosis, exists primarily in dust and soil most commonly contaminated by bird or bat feces. Inhaling fungus spores can cause lung disease or potentially cause disease in any organ in the body. In humans, any activity that leads to the aerosolization of dust such as excavation, construction and demolition work, among others, can make infection more likely. Behavioral risk factors in dogs are unknown, but exposure to poultry and outbuildings are risk factors for outdoor cats.
Interestingly, about a third of cats diagnosed with histoplasmosis are indoor only, meaning they never leave the home. This suggests that Histoplasma is found in the home. Recent research from Oklahoma State University’s Center for Veterinary Health Sciences shows that the genetic make-up of Histoplasma that infects pet dogs and cats is identical to the Histoplasma that infects humans.
It is unknown how often humans are infected in their homes. Additional research on this potential health hazard is needed.
Clinical signs of histoplasmosis in pet dogs and cats vary widely and depend upon the organs infected. The most common clinical signs in cats include fever (not responsive to antibiotics), decreased energy, weight loss, decreased appetite and increased respiratory rate. The most common clinical signs in dogs include similar fever, diarrhea, weight loss, decreased energy and decreased appetite.
If not diagnosed early and properly treated, histoplasmosis often proves fatal. Sadly, a misdiagnosis of a more common bacterial infection or other diseases with similar clinical signs in dogs and cats often comes first, delaying the correct treatment.
The diagnosis of histoplasmosis has generally required finding the fungus in tissue or bodily fluid samples, but a more recent test that requires only a urine sample has been developed and is a reliable and non-invasive means of diagnosis in dogs and cats.
Once diagnosed, long-term treatment (at least six months) with an antifungal medication is required. Treatment with an antibiotic is not effective. Treatment continues until clinical signs and other indicators of disease are resolved. The infection can be difficult to clear, and relapses after discontinuing the antifungal drug is reported in up to 40 percent of dogs and cats. When relapse occurs, long term treatment with an antifungal medication is again required. Unfortunately, even when treated appropriately, only about 70 percent of dogs and cats survive to six months after diagnosis — even otherwise healthy and young to middle-aged ones. This highlights the need for a better recognition of histoplasmosis, hopefully leading to earlier diagnoses and better treatment outcomes.
No vaccines can prevent histoplasmosis in pet dogs and cats. Minimizing exposure to dust/soil contaminated with bird or bat feces makes sense. Preventing outdoor cat exposure to poultry and outbuildings might help minimize the chance of infection. When a dog or cat is diagnosed with histoplasmosis, other pets in the household should be tested. Due to common environmental exposures, it is not uncommon to diagnosis multiple dogs or cats in the same household with histoplasmosis. Consult with your veterinarian early, when your pet dog or cat is not feeling well, to ensure an accurate diagnosis and timely treatment of histoplasmosis.
Dr. Andrew S. Hanzlicek, DVM, MS, DACVIM, holds the Joan Kirkpatrick Chair in Small Animal Internal Medicine at Oklahoma State University’s Center for Veterinary Health Sciences.