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.
Table 1. Summary of Samples Yielding a Definitive Diagnosis in Dogs and Cats with Disseminated Histoplasmosis
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.
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.