Infections Due to Histoplasma capsulatum var. capsulatum
Histoplasmosis due to H. capsulatum var. capsulatum has a wide range of clinical manifestations, presenting as mild respiratory distress, acute respiratory infection or life-threatening disseminated disease.
The outcome is variable according to the inoculum size and the strain virulence. Once inhaled, Histoplasma propagules convert to yeasts in the lungs. A granulomatous inflammatory response occurs that consists primarily of macrophages, variable numbers of lymphocytes and sometimes fibrosis. Some animals control the initial infection but remain latently infected with small numbers of yeast cells. Subsequent immune suppression can lead to reactivation of infection years later. As a consequence, the incubation period may range from 2 to 3 weeks to several years. Histoplasma organisms may be found in pulmonary lymph nodes, in reticuloendothelial organs (liver, spleen and bone marrow), in the skin, in the central nervous system and in the small and/or large intestinal tract.Clinical signs of histoplasmosis in cats and dogs are commonly non-specific, such as weight loss, inappetence, weakness, dehydration and fever (Bromel and Sykes 2005). Histoplasma capsulatum was first reported as a pathogen in a cat in 1949. In a review of 571 cats with deep mycotic infections in the USA, histoplasmosis was with 16.7% the second most commonly reported fungal disease after cryptococcosis (46.1%) (Davies and Troy 1996). Approximately 40% of infected cats have respiratory signs such as dyspnoea and tachypnoea and to a lesser extent cough and nasal discharge. Respiratory signs may be absent in some animals with dissemination of Histoplasma to non-pulmonary sites. Thoracic radiographs usually reveal diffuse, linear, nodular or miliary interstitial patterns, but mixed interstitial-alveolar-bronchial patterns and an absence of abnormal findings have also been reported.
Ocular signs (chorioretinitis, retinal detachment, optic neuritis, anterior uveitis or panophthalmitis) occur quite frequently (in more or less one-quarter of cats). In dogs, chronic diarrhoea (often with hematochezia or melena) and wasting (pale mucous membranes, weight loss and weakness) are frequently observed. Other clinical signs include cutaneous nodules, joint pain, lesions on the tongue, myosis, splenomegaly and ocular signs (such as anterior uveitis, chorioretinitis, optic neuritis and retinal detachment) (Bromel and Sykes 2005).Systemic histoplasmosis with intermittent cough, dyspnoea, lymphadenopathy, hyperthermia, anorexia, weight loss and diarrhoea has been described, mostly in horses (Rezabek et al. 1993), more rarely in cattle (Morgado et al. 1976) and the dromedary (Chandel and Khere 1994). Occasionally, H. capsulatum var. capsulatum is also responsible for abortion in mares with lesions of placentitis and invasion of the foetus; perinatal death of foals, usually due to severe granulomatous pneumonia (Saunders et al. 1983); or ocular mycosis with keratitis in horses (Richter et al. 2003).
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