Issue Description Histoplasmosis is a disease that affects many
mammals including dogs, cats, and humans. It is caused by the
dimorphic fungus Histoplasma capsulatum, which is endemic in many
temperate and subtropical regions of the world. This organism has been
isolated from the soil in 31 of the continental United States;
however, most clinical cases of disease occur in the Mississippi,
Missouri, and Ohio River areas. H. capsulatum lives in the soil and
thrives in moist and humid environments. It is present in particularly
high concentrations where birds and bats congregate due to the high
nitrogen concentration in their feces.
not contagious from animal to animal. Exposure to the mycelial stage
of development in the soil is required for fungal infection to occur.
While most cases of histoplasmosis are subclinical, the fungus can
replicate and cause severe local or disseminated infection. There is
no sex predilection and it affects animals of all ages, however
animals under the age of four are most commonly affected with severe
disease. Immunodeficiency has been identified as a predisposing factor
for severe infection since T-cell immunity is critical to clear the
disease, yet in cats there has been no correlation to date between
feline leukemia virus associated immunosuppression and histoplasmosis.
There appears to be no breed predilection in cats, but one canine
study found Pointers, Weimaraners and Brittany spaniels to be at
increased risk. Exposure of young animals with immature immune systems
and exposure to large amounts of the fungus are the most common
Symptoms In dogs, respiratory signs of histoplasmosis
include dyspnea, coughing, and abnormal lung sounds. Disseminated
histoplasmosis may be accompanied by fever (which is unresponsive to
antibiotic treatment), weight loss, depression, and inappetence.
Fungal infection of the small intestine may result in the production
of large amounts of watery stool, which may be accompanied by
protein-losing enteropathy. Colonic involvement is associated
tenesmus, mucus, and fresh blood in the feces. Pale mucous membranes
are common in dogs with gastrointestinal blood loss or bone marrow
involvement (myelophthisic disease). Other common clinical findings
include splenomegaly, hepatomegaly, visceral lymphadenomegaly,
icterus, and ascites. Ocular and cutaneous lesions, lameness from
osteimyelitis, vomiting, peripheral lymphadenomegaly ,and neurologic
signs are rare.
Diagnosis A definitive diagnosis of histoplasmosis is
made by identifying the organism in tissue samples obtained for
cytologic or histologic evaluation. Histoplasmosis causes extensive
granulomatous inflammation. Macrophages frequently contain yeasts of
H. capsulatum that appear as "clusters of grapes" in the cytoplasm of
these cells. Organisms also may be scattered in the background of
cytologic preparations. The organism is most easily identified in
cytologic aspirates of lung, liver, lymph nodes, spleen, and bone
marrow that are stained with Romanowsky stains (Wright, Giemsa, or
Leishman stains). The yeasts are in a round body, basophilic center,
and a surrounding halo caused by shrinkage of the organism during
staining. Rectal scrapings and imprints of colonic biopsies also are
useful in diagnosing histoplasmosis in dogs. Infrequently, H.
capsulatum also may be identified in macrophages, monocytes, or
neutrophils in smears of blood, body cavity effusion fluid, and
transtracheal wash or bronchoalveolar lavage fluid.
Granulomatous inflammation and the proliferation of yeasts results in
organomegaly and organ failure. Tissue biopsies may be necessary to
demonstrate H. capsulatum if cytologic studies are equivocal.
Histologic sections can be stained by the Periodic Acid-Schiff,
Gomori's methenamine silver (GMS), or Gridley techniques to
demonstrate the organism more effectively, especially if low numbers
of yeasts are present.
Fungal isolation can be done using
fine-needle aspirates, tissue biopsies, blood, or body cavity fluids.
However, fungal isolation is not recommended in routine veterinary
practice because the organism is highly pathogenic when grown in
Treatment Various oral antifungal drugs have been used to
treat histoplasmosis. The pulmonary form of disease is often
self-limiting. Disseminated histoplasmosis can be difficult to treat,
requiring a long course of combination drug therapy. Prevention of
disease is based upon avoidance of areas with heavy bird and bat fecal
contamination. Small areas of fungal contamination can be disinfected
with 3% formalin solution.