D413 - Histoplasma capsulatum

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. Histoplasma capsulatum lives in the soil and thrives in moist and humid environments but can also survive extreme temperature fluctuations. Histoplasmosis is not contagious from animal to animal. Exposure to the mycelial stage of development in the soil is required for fungal infection to occur. The incubation period after exposure is approximately 12 to 16 days. Histoplasmosis is probably acquired by inhalation of microconidia (produced during the mycelial stage), which are small enough to reach the lower respiratory tract. 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. Both cats and dogs are very susceptible hosts for Histoplasma capsulatum infection. Most infected cats have disseminated disease and exhibit a wide range of non-specific clinical signs including depression, weight loss, fever, anorexia, and pale mucous membranes. Coughing is uncommon, but dyspnea, tachypnea, and abnormal lung sounds are found in more than half of affected cats. Occasionally, Histoplasma infects the eye, causing conjunctivitis, granulomatous blepharitis, granulomatous chorioretinitis, retinal detachment, and optic neuritis. Common symptoms of Histoplasma capsulatum infection in canines include inappetence, weight loss, and fever (unresponsive to antibiotic therapy). In some dogs, the clinical signs may be limited to the respiratory tree and include coughing and abnormal lung sounds. However, in most dogs, clinical signs result from disseminated histoplasmosis with GI involvement. Signs of large-bowel diarrhea with tenemus, mucus, and freash blood in the stool are the most common clinical findings.

Diagnosis:

Currently, no consistently reliable immunodiagnostic test is available for identification of histoplasmosis in companion animals. Intradermal skin tests for reactivity to histoplasmin are unreliable in companion animals and cannot be used to confirm the diagnosis of histoplasmosis. Serologic tests for antibodies directed against Histoplasma antigens are often falsely negative in animals with active naturally occurring disease. Also, test results may be false positive in an animal with prior exposure that has recovered from infection.
Conventionally, infection by Histoplasma capsulatum could only be confirmed by culturing the fungus. However, attempts to culture Histoplasma capsulatum in a routine practice setting are not recommended because of the pathogenic potential of this organism. Histoplasma capsulatum can be cultured from tissue specimens, fine-needle aspirates, and body fluids by specialized laboratories but the process may take anywhere from 7 to 10 days. The newest generation of DNA diagnostic tests are much more rapid (2-3 days) then conventional methods, and avoid the problems associated with serological testing since antibodies are not used. A DNA test for the detection of Histoplasma capsulatum in a sample is now available. This test offers veterinarians a sensitive, accurate and rapid means of detecting Histoplasma capsulatum, and confirming infection

Treatment:

The duration of antifungal treatment required for each patient is variable and is determined by the severity of infection and the patient’s clinical response. Itraconazole (ITZ) is currently the drug of choice for the treatment of histoplasmosis in animals. Treatment is usually initiated once daily; however, studies in cats indicate significant variability regarding per os (by mouth) absorption of ITZ, and twice daily dosing (at 10 mg/kg) may be required in some to achieve the desired therapeutic effects (Greene C. – Infectious diseases of the dog and cat. 1998).

Sample:

1. Nasal swabs or ocular swabs in a sterile container.
2. Lung aspirates in a sterile container

Special Handling:

Store samples at 4°C until pick up or shipment.

Test Code:

D413