Causes of Canine Histoplasmosis
Histoplasmosis is caused by exposure to the soil-dwelling fungus, Histoplasma capsulatum. This organism is found in moist, nitrogen-rich soils contaminated by the feces of bats, chickens, starlings and other birds. The fungi produce and release tiny infectious airborne spores, called microconidia and macroconidia. The spores typically are inhaled, but they also can be ingested orally. They germinate in the animal’s lower airways and/or gastrointestinal tract, where they transform into yeast. The yeast reproduce by a process called “budding.” Certain cells triggered by the host animal’s immune system ingest or take up the yeast organisms through a process referred to as “phagocytosis.” The yeast continue to reproduce inside of those cells and can be transported throughout the dog’s body through lymphatic and hematogenous (blood) circulation. Most commonly, the fungi eventually congregate in large numbers in the respiratory tract, gastrointestinal tract, lymph nodes, bone marrow, liver and/or spleen.
H. capsulatum is endemic in many temperate regions of the world. “Endemic” means that it is present in a predictable, continuous pattern in a community at all times. In the United States, H. capsulatum is most commonly found in the Mississippi, Missouri and Ohio River valleys, in areas near the Great Lakes and in the Appalachian Mountains, although it has been reported in Tennessee, Texas, California and some southeastern states, as well.
The best way to prevent histoplasmosis is to avoid contact with contaminated soil in endemic areas known to have large populations of birds and/or bats. There is no effective vaccine against H. capsulatum at this time.
Owners of dogs showing respiratory or gastrointestinal symptoms should tell their veterinarians about any recent trips taken with their dogs to areas where H. capsulatum is especially prevalent – particularly within the prior 2 or 3 months. People can also become infected with this fungi by inhaling or ingesting it. Although transmission of the actual infection between dogs and humans has not been established, people and dogs can become infected at the same time during outings in contaminated areas.