Histoplasmosis can be fatal. Thankfully, it usually can be diagnosed and treated successfully, although diagnosis can be rather difficult.
A veterinarian presented with a dog that is showing respiratory and/or gastrointestinal signs and has a history of recent travel to the Mississippi, Missouri or Ohio River basins, to areas around the Great Lakes or to the Appalachian Mountains, probably will have a heightened suspicion of fungal infection by the Histoplasma capsulatum organism. Sick dogs that recently traveled in Tennessee, Texas, California or any of the southeastern states may also be considered to be possibly infected by H. capsulatum.
The normal initial data base will include a thorough history and physical examination, followed by routine blood work. The complete blood count in a dog suffering from histoplasmosis often reflects nonregenerative anemia, although anemia can be caused by a number of unrelated disorders. A blood smear examined on a slide under a microscope may – but rarely does – reveal the fungal organism inside circulating mononuclear cells. The results of a serum chemistry panel will vary based upon which organ systems are affected, but they typically will reveal low blood albumin levels, high blood calcium levels and elevated liver enzymes. Another routine diagnostic tool is taking thoracic radiographs (chest X-rays).
More advanced testing is necessary to confirm a diagnosis of histoplasmosis. The veterinarian may take a sample of bone marrow, spleen, liver, lung and/or lymph node tissues by a process called fine-needle aspiration, or FNA. Rectal scrapings and colon biopsies can also be taken. These samples can be examined microscopically on a slide (this is called cytology). They can be examined in greater detail by a veterinary pathologist using a process called histology. Finally, it also is possible to culture (grow) the H. capsulatum organism in the laboratory from a tissue or fluid sample.
Because H. capsulatum can be infectious to people, pathologists and other participating laboratory personnel must be extremely careful when working with and culturing tissue samples from a dog suspected to have histoplasmosis.