Canine and feline histoplasmosis: A review of a widespread fungus - Veterinary Medicine
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Canine and feline histoplasmosis: A review of a widespread fungus
Infection with this pathogenic fungus most commonly results from inhaling spores from contaminated soil. The infection should be treated promptly to avoid dissemination, which carries a poorer prognosis.


VETERINARY MEDICINE


Fungal culture


Table 1. Summary of Samples Yielding a Definitive Diagnosis in Dogs and Cats with Disseminated Histoplasmosis
A definitive diagnosis may be made through fungal culture of tissue, body fluid, or fine-needle aspirates of affected organs.10,11 However, culturing H. capsulatum outside of professional diagnostic laboratories is not recommended, as incubation at room temperature can result in the growth of the mycelial phase within seven to 10 days, posing a risk for personnel.10 In addition, fungal cultures may be falsely negative,10,61-63 and growth of H. capsulatum may require up to four weeks.

Polymerase chain reaction testing

To date, polymerase chain reaction (PCR) testing has been used primarily in research settings to confirm a diagnosis of histoplasmosis. A nested PCR technique has been applied in situ to DNA extracted from pus and paraffin-embedded tissue samples of dogs with histoplasmosis.64,65 PCR testing is not commercially available for diagnostic testing for histoplasmosis in cats and dogs.

Serology

In people, standard serologic tests include complement fixation and immunodiffusion, which use yeast and mycelial antigens to detect antibodies against H. capsulatum.61 Complement fixation is more sensitive.62 However, when complement fixation was performed in nine cases of canine histoplasmosis, only one titer of 1:8 was obtained.9 Similarly, serologic testing performed in nine cats with histoplasmosis had positive results in only four of the cats (44%).1 Because of a myriad of false negative and false positive results, serology for antibody detection is not considered a reliable method of diagnosis in cats and dogs.10,11

Antigen detection tests are used to detect histoplasmosis in human serum, urine, bronchoalveolar lavage fluid, and cerebrospinal fluid.63 Cross reactivity occurs with other agents of disseminated mycoses, such as Blastomyces species.66 Research on the use of antigen detection tests to diagnose canine and feline histoplasmosis is limited. Full validation of these tests for dogs and cats is pending.


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