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Overview
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Histoplasmosis is a non-contagious fungal infection of dogs, cats, other domestic and non-domestic animals, and humans.
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It is caused by inhalation of a fungus called Histoplasma capsulatum carried on dust with a primary infection in the lung.
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In dogs, evidence suggests that prolonged exposure to high levels of Histoplasma organisms increase the risk of contracting the disease.
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Young age is a risk factor, as well as breed and sex: a large study found an increased risk in pointers, weimaraners, and Brittanys.
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It is also seen more frequently in males, with the risk being 1.2 times that seen in females.
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The impact of the disease on a particular dog is significant; treatment is expensive and prolonged, and the prognosis is poor.
Symptoms
Diagnosis
Treatment
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Antifungal drugs
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Supportive care
Prevention
Information In-depth
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Histoplasmosis is a non-contagious infection caused by the fungus Histoplasma capsulatum.
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The organism responsible for the disorder is a soil fungus that is widely distributed in the Midwestern and southern United States, especially in river valleys and plains.
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Within these endemic areas, some locales are more heavily contaminated with Histoplasma organisms than others.
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In many instances, these variations have been correlated with proximity to bird habitats, which are often heavily contaminated because bird droppings serve as a good medium for the organism. Birds are not infected by the Histoplasma organism, although they can carry the organism on their feathers and spread the disease in this manner.
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Bat habitats can also be heavily contaminated with the organism, as bat guano is an excellent medium for Histoplasma growth. Unlike birds, bats can become infected with the organism.
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Infection occurs when spores in the air are inhaled. The spores go to the lungs and cause infection of cells deep in the lungs. Most dogs show a clinically unapparent pulmonary infection; the organism has infected the lungs, yet the dog shows no signs of being ill.
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Occasionally, dogs with pulmonary histoplasmosis shows signs of lung infection, such as fever, labored breathing, and coughing.
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The lung infection usually turns out to be benign and self limiting. If the unapparent respiratory infection extends and persists beyond the lungs and becomes systemic, a clinically apparent, often devastating form of the disease may develop. This has been referred to as "disseminated histoplasmosis," and it carries a guarded or even grave prognosis.
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It is uncommon for a clinically unapparent Histoplasma infection to disseminate from the lungs.
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When this happens, a large number of organs and body systems may be affected.
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In dogs, the organs most often involved are the lungs, intestine, lymph nodes, liver, spleen and bone marrow.
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Systemic signs of illness such as depression, fever, and anorexia are common.
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The disseminated infection in dogs most often results in chronic diarrhea, intestinal blood loss, anemia, and weight loss. In a few dogs, infection of the bones, eyes, skin, and central nervous system may occur.
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The factors that predispose particular dogs to histoplasmosis are not fully understood.
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In people, exposure to a large amount of the organism, a suppressed immune system, as in chemotherapy or HIV infection, or exposure at a young age or old age are believed to predispose individuals to disseminated infection.
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There is evidence suggesting that prolonged exposure to high levels of the organism may increase the risk of disease.
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Administration of immunosuppressive drugs has been correlated with a worsening of disease in dogs.
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Exposure to the organism at a young age is a predisposing factor; most affected dogs are less than three years old.
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Breed and sex are considered as possible predisposing factors in dogs, with pointers, Weimaraners, and Brittanys reported to be more susceptible, and male dogs being 1.2 times as likely to become infected as females.
Veterinary Care In-depth
Diagnosis
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Complete blood count. There may be several findings on a complete blood count that support a diagnosis of histoplasmosis.
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Anemia is a common finding in dogs with histoplasmosis. The anemia is mostly due to suppression of red blood cell production by the infected bone marrow.
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In dogs, blood loss in the stool, due to severe Histoplasma infiltration of the intestinal tract, contributes to the anemia.
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Thrombocytopenia (a low platelet count) is commonly seen. There is no consistent pattern to the white blood cell count in affected dogs; the white count may be high, low, or normal.
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Chemistry panel and urinalysis. Serum biochemical profiles often reveal a low albumin in dogs with the disease.
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Much of the low albumin is attributed to loss of albumin in the stool. Urinalysis shows no specific abnormalities.
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Radiographs. X-rays of the chest often reveal a nodular pattern that is characteristic for fungal infections.
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Pulmonary lymph nodes are often visibly enlarged.
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Abdominal films may show an enlarged liver and spleen. X-rays of the bones in animals presenting with lameness may show various degrees of bone destruction and new bone formation.
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Complete ophthalmic exam. An examination of the eye, especially the retina, may show abnormalities characteristic of fungal disease, lending further support to a diagnosis of histoplasmosis.
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Rectal scrapings. A rectal scraping, obtained using a small metal spatula and stained and examined under a microscope often reveals Histoplasma organisms inside many of the cells. This allows a definitive diagnosis of the disease.
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Bone marrow examination. Examination of stained bone marrow specimens often reveals cells containing the Histoplasma organism, allowing for a definitive diagnosis.
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Cytology of fine-needle aspirates. Obtaining a tissue sample by aspirating it through a fine needle and spreading it onto a slide, staining, and examining under a microscope may reveal fungal organism and allow a definitive diagnosis.
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The liver, lung, spleen, and lymph nodes are good tissues to sample, and may be better than rectal scrapings in dogs that do not have signs of gastrointestinal involvement.
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Biopsy. If examination of aspirated tissue cells is not diagnostic, examination of a biopsy specimen may be required.
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A specific type of inflammation is often seen in fungal infections and may be highly suggestive of the disorder, but finding actual fungal organisms may be difficult.
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Special fungal stains are often used to increase the chances of identifying the organism.
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Serologic tests. Blood tests that look for antibodies against the organism may be performed.
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A positive test means that the animal was exposed to the organism.
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It does not necessarily mean that the animal is infected. Results of serologic tests are inconsistent and are not very useful in making a diagnosis.
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Culture. Because of the large numbers of organism present in affected tissues, the Histoplasma organism can often be cultured from fine needle aspirates; however, the organism grows slowly and it may take 7 to 10 days before results are available.
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The organism, when grown in culture, also poses a significant hazard to human health, and laboratories involved need to be notified before submitting samples.
Therapy
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Treatment of disseminated histoplasmosis is difficult.
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It requires the use of antifungal agents and supportive therapy like adequate nutrition, hydration, and control of secondary bacterial infections.
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Antifungal drugs. A number of antifungal drugs such as ketaconazole, itraconazole, amphotericin B have shown some efficacy against Histoplasma infection, either alone or in combination.
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These drugs have to be given for prolonged periods of time, and are often fairly expensive.
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Supportive care. Intravenous fluids, good nutrition, and possible antibiotic therapy to control or prevent secondary bacterial infections may be necessary as part of the overall therapy for histoplasmosis.
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