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L. Joseph Wheat

and Lynn Guptill

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date: 24 January 2022

Histoplasma was initially described from a lesion in a horse by Rivolta in 1873, who named the organism Cryptococcus farciminosum. In 1905, Samuel Darling noted the presence of intracellular organisms in many tissues, including the lungs, of a patient suspected of succumbing to miliary tuberculosis (Darling 1906). Darling named the organism Histoplasma capsulatum , because it appeared to be an encapsulated protozoan-like organism. In 1912, mycologist Henrique da Rocha-Lima reviewed Darling’s slides and noted the cytological similarities between Darling’s Histoplasma organism and Cryptococcus farciminosum. Cryptococcus farciminosum was reclassified as Histoplasma farciminosum in 1934, and in 1985 it was again reclassified as a variant of Histoplasma capsulatum (var. farciminosum ) (Weeks et al. 1985).

William De Monbreun cultured the organism from the blood of a child suffering from an unexplained febrile disease in 1934, and demonstrated it to be a dimorphic fungus (De Monbreun 1934). De Monbreun and others reported naturally occurring histoplasmosis in a dog in 1939, and subsequently demonstrated experimentally that clinically inapparent histoplasmosis occurred in dogs (De Monbreun 1939). De Monbreun and others speculated that animals might serve as the source of histoplasmosis in human beings. However, C.W. Emmons demonstrated in 1949 that Histoplasma capsulatum is a soil saprophyte, and that inhalation of aerosolized microconidia and mycelial fragments served as the source of infection (Emmons 1949).

The prevalence of histoplasmosis in endemic regions was estimated to be more than 50% based on positive skin tests for histoplasmin (Edwards et al. 1969). Active histoplasmosis has been identifi ed in up to 50% of dogs in endemic regions based on culture at necropsy of healthy animals (Turner et al. 1972a). The case prevalence of disseminated histoplasmosis at a veterinary teaching hospital in an endemic region of the mid-western USA of 43 cases in cats and 12 cases in dogs per 100,000 hospital records per year has been estimated (Clinkenbeard et al. 1988; Kaplan 1973). Dogs and cats with outdoor exposure are reportedly at greater risk for histoplasmosis than those with minimal time outdoors. However, some completely indoor cats become ill with histoplasmosis (Davies and Troy 1996; Johnson et al. 2004). Young to middle-aged dogs of hunting and sporting breeds have historically been reported at greatest risk for acquiring histoplasmosis (Selby et al. 1981). Risk factors for cats have not been systematically studied.

Infection by Histoplasma capsulatum var. capsulatum is not contagious except in unusual situations. Rare cases of horizontal transmission have been reported. Horizontal transmission is associated with conjugal contraction of individuals with cutaneous lesions of the genitalia (Sills et al. 1973) and by solid organ transplantation of infected organs (Limaye et al. 2000). No documented cases of transmission from animals to human beings or vice versa have been reported. In contrast to Histoplasma capsulatum var. capsulatum, equine infection by Histoplasma capsulatum var. farciminosum is contagious and is transmitted by bites of contaminated flies or ticks as well as through skin traumatized with contaminated tack (Kohn 2006).

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