Introduction and Causes of Histoplasmosis
Histoplasmosis is a systemic fungal infection that can affect companion animals and people. It is caused by exposure to a soil-dwelling fungus called Histoplasma capsulatum. This organism is found in soils organically rich in bird and bat manure and is endemic in many temperate regions of the world. In the United States, it is most commonly found in the Mississippi, Missouri, and Ohio River basins, although it has been reported in Tennessee, Texas, California, the southeastern states and the Great Lakes region. Dogs are exposed to the fungus when they inhale infectious fungal spores. These spores germinate in the dog’s lungs, develop into forms of yeasts and are distributed throughout the dog’s body through the circulatory system. Sometimes, dogs ingest the fungal spores, which are then directly deposited into the lining of the gastrointestinal (GI) tract. Histoplasmosis can be life threatening in dogs, and prompt medical treatment is necessary for a full and speedy recovery. Owners should tell their veterinarian about any recent trips with their dogs to areas where H. capsulatum is especially prevalent.
Symptoms of Histoplasmosis
Young adult, large-breed dogs of either gender seem predisposed to developing histoplasmosis, although the reason for this is not known. The risk of exposure increases for dogs engaged in outdoor activities in endemic areas, especially hunting dogs. Disease may be confined to the lungs or GI tract, or it may become systemic due to dissemination of the organism throughout the dog’s body. The most common initial signs of histoplasmosis in dogs are fairly nonspecific and include loss of appetite, weight loss, exercise intolerance, lethargy and depression. With pulmonary involvement, owners often see labored breathing (dyspnea) and a chronic cough. When the GI tract is particularly affected, signs include significant weight loss, diarrhea, straining to defecate (tenesmus), and blood or mucus in the stool. As the infection progresses, dogs become emaciated, develop a fever and elevated heart and respiratory rates, and they may develop lameness and lesions on the skin or around the eyes, but these are uncommon.
Ultimately, histoplasmosis in dogs can affect a number of body systems in addition to the GI and respiratory tract: the liver, spleen, and lymph nodes are often involved; bones, bone marrow, adrenal glands, kidneys, mouth/tongue eyes and testes are less frequently affected. Histoplasmosis can be fatal. Fortunately, it can be diagnosed through fungal cultures and treated successfully with anti-fungal medications.