Infection with the pathogenic fungus Histoplasma capsulatum can result in systemic disease in several species of animals, including dogs and cats. While the exact prevalence of this infection is unknown for these animals, large areas of endemicity exist throughout the world. In the United States, the largest number of cases occurs in the Ohio, Missouri, and Mississippi river valleys. One study reported histoplasmosis as the second most common fungal infection in cats.¹ Infected dogs and cats may present with a myriad of clinical signs, commonly including pyrexia, anorexia, and lethargy. Histoplasmosis is an important differential diagnosis to consider in animals from endemic areas.

DISCOVERY

Histoplasmosis was first described at the Panama Canal in 1905 by the American physician Samuel Darling.² During an autopsy of a patient suspected of having miliary tuberculosis, microscopic examination revealed intracellular organisms in many tissues, including the lungs. Because of the organisms' encapsulated appearance, resemblance to plasmodia, and location within histiocytes, Darling named the agent Histoplasma capsulatum. Thinking that he had identified a new form of visceral leishmaniasis, Darling incorrectly identified the organism as protozoal. In 1912, the Brazilian physician and pathologist Henrique da Rocha-Lima reviewed Darling's slides and suspected the organism was a yeast.²

The biological basics of Histoplasma species

CLINICAL OUTCOME

Histoplasmosis may have an acute or chronic clinical course.^7-9 Depending on the fungal inoculum size and host immunocompetence, H. capsulatum infections may result in subclinical infection, dormant infection with subsequent reactivation and dissemination, or pulmonary or disseminated histoplasmosis.^10-13

CLINICAL FINDINGS

Although histoplasmosis has been reported in dogs from 5 months to 11 years old,^8,9 it is most commonly seen in dogs < 4.¹¹ No consistent sex predilection has been documented. Male dogs have been reported to be 1.2 times more frequently affected than females,^11,14 while in other reports, females are more frequently affected than males.^8,9 One study reported that pointers, Weimaraners, and Brittany spaniels had an increased risk of histoplasmosis.^11,14

Once thought to be rarely affected, cats are now considered as likely as dogs to develop clinical histoplasmosis.^10,11,15 In a retrospective study of 571 cats with deep mycotic infections, histoplasmosis was the second most commonly reported fungal infection (16.7% of cases) after cryptococcosis (46.1% of cases). The reported age of cats with histoplasmosis ranged from < 2 months to > 15 years, with most cases occurring in cats < 2 years old. Males and females were affected in about equal numbers, which¹ contrasts with a previous study of 18 cats with pulmonary histoplasmosis in which females outnumbered males 2 to 1.¹⁶ Thus, as in dogs, conclusions regarding sex predilection in feline histoplasmosis cannot be drawn.

In a review of 96 cats with histoplasmosis, Persians and Siamese were found to be overrepresented and underrepresented, respectively; this has not been reported elsewhere in the literature.¹ Of these 96 cats, 15% had positive test results for feline leukemia virus (FeLV) infection.¹ In contrast, a previous retrospective study of 12 cats with disseminated histoplasmosis revealed no association between FeLV infection and histoplasmosis.¹⁵