Histoplasmosis is a systemic fungal infection caused by Histoplasma capsulatum.
The mycelial form grows in soil enriched with bird manure or other organic materials therefore areas of old chicken coops
and bird roosts present a potential hazard. The spores (microconidia) produced by the mycelium are small and can be inhaled
into the lungs where at body temperatures they transform into the yeast form. The yeast are phagocytized by macrophages and
distributed throughout the body.
A subclinical infection can develop and the organisms can persist for a long time without significant clinical signs. In endemic
areas, Histoplasma organisms can be recovered by culture from mesenteric and tracheobronchial lymph nodes of asymptomatic dogs.
In cats, the respiratory tract is a main site of infection but bone, bone marrow, liver, spleen, skin, and lymph nodes also
affected. The intestinal tract, eyes, kidneys, adrenals, and brain less frequently involved
In dogs, the intestinal tract is frequently involved with liver, lung, spleen, and lymph nodes often involved. The bones,
bone marrow, kidneys, adrenals, oral cavity, tongue, eyes, and testes may affected.
Traditionally the Ohio, Missouri, Mississippi, Tennessee, and St. Lawrence River basins have areas where histoplasmosis is
common but Texas, the southeastern U.S., and the Great Lakes region have also seen histoplasmosis.
All ages can be infected but infections in young cats are common. Most infected dogs are middle aged.
In cats, anorexia, weight loss and dyspnea are associated with respiratory involvement. Coughing may be seen and lameness
is associated with bone involvement. The eyes and skin can be involved. Diarrhea with intestinal infection is less commonly
In dogs, diarrhea and weight loss is the most common sign. Dyspnea, coughing and exercise intolerance occurs with lung involvement.
Lymphadenopathy is occasional seen as is eye involvement.
Fever is common and harsh lung sounds occur especially if there is enlargement of the tracheobronchial lymph nodes with compression
of the tracheal bifurcation. Hilar lymphadenopathy is common and may persist after other signs of infection have resolved.
Emaciation occurs especially in animals with intestinal histoplasmosis because of a combination of anorexia and malabsorption.
The bowel loops may feel thickened. Signs of large bowel diarrhea with blood and mucous may be seen. Liver and splenic enlargement
is common in animals with intestinal histoplasmosis. Some animals may be icteric. Pale mucous membranes may be seen because
histoplasmosis may cause a hemolytic anemia. Lameness with bone pain is noted with boney involvement. Chorioretinitis and
anterior uveitis may been seen.
A diagnosis made by identification of the organism is most reliable. The clinician should be suspicious of histoplasmosis
when there is multiorgan disease that is unresponsive to antibiotic therapy. Lung radiographs may be characteristic of a fungal
pneumonia but other conditions such as lymphosarcoma may resemble the lungs of histoplasmosis.
In severe chronic diarrhea and weight loss, consider histoplasmosis as well as lymphocytic plasmacytic enteritis and lymphosarcoma.
Organisms are ofter found on cytology of rectal scrapings. Cytology may identify colonic lymphosarcoma. In animals with hepatomegaly
and spenomegaly, needle aspirates will often identify the organisms. Bone marrow aspirates are likely to yield Histoplasma organisms when there is systemic disease.
Complete blood counts may have a leukocytosis but this is non-specific. Moderate anemia is a common finding. The anemia may
be the anemia of chronic inflammation but may be a hemolytic anemia. At times the antibodies produced to the Histoplasma organisms cross react with the red blood cells thereby producing a Coombs positive anemia. Liver involvement may produce
increases in ALT and Alkaline Phosphatase while severe gut involvement may produce hypoproteinemia due to protein losing enteropathy.
The Agar Gel ImmunoDiffusion (AGID) for antibodies strongly supports a diagnosis of histoplasmosis but many infected dogs
will be negative early in the disease. Cross reactivity between Blastomyces and Histoplasma organisms is expected.