Diagnosis of Histoplasmosis in Animals

A definitive diagnosis of histoplasmosis is made by cytologic or histopathologic identification of Histoplasma yeasts. The latter are typically observed within macrophages or, less frequently, free in pyogranulomatous exudates.

Culture is possible on Sabouraud dextrose agar without cycloheximide or on brain-heart infusion with blood. With an incubation temperature of 27-30 °C, Histoplasma fungi form whitish and slow-growing colonies. Incubation periods of 2-4 weeks may be required before growth is appreciated. Under these conditions, infective conidia (thick-walled, large, tuberculate macroconidia and small oval microconidia) are produced. Because these conidia may cause infection in laboratory personnel, the growth of the mycelial phase of Histoplasma poses a health hazard and should be performed in specialised laboratories with adapted levels of confinement.

PCR techniques have been developed for the diagnosis of human histoplasmosis (Muraosa et al. 2016). These techniques could also be applied in animals. Several serological techniques, including immunodiffusion and complement fixation tests, have been used to detect human antibodies to H. capsulatum, but serology does not seem to be a reliable diagnostic tool (Kauffman 2009). This also applies to dogs and cats. Serology for H. capsulatum antibodies in nine cases of canine disseminated histoplasmosis revealed a titer of 1:8 in one case (Mitchell and Stark 1980). Serology to detect antibodies against H. capsulatum was positive in only four out of nine feline cases (Davies and Troy 1996).

Antigen detection tests are now widely used for the diagnosis of human histo­plasmosis. The tests are usually performed on urine specimens. In a retrospective study, Cook et al. (2012) compared the results of a urine antigen assay with standard diagnostic methods in cats with clinical signs of histoplasmosis. Antigenuria was detected in 17 out of 18 infected cats. The histofarcin skin test was developed by Soliman et al. (1985) for horses. This test proved to be a valuable tool in diagnosing epizootic lymphangitis in the field (Ameni et al. 2006). A delayed, intradermal, type IV hypersensitivity reaction indicates previous exposure to Histoplasma.

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