A vegetation is seen on this heart valve. Inflammatory debris coats the surface of the vegetation.
The underlying fibrinoproteinacous material is hypocellular compared to the surface. One can see negative images of round forms amidst if closely inspected.
GMS stain confirms numerous the rounded yeast forms (3-5 microns) compatible with Histoplasmosis organisms within the hypocellular areas.
Histoplasma capsulatum is one of the three major dimorphic fungi that can cause systemic disease in humans. The clinical manifestations are similar to that of tuberculosis in that the disease can be (1) asymptomatic and self-limited; (2) elicit granulomatous pulmonary lesions with cough, fever and night sweats; (3) lead to disseminated systemic disease, especially in immunocompromised individuals.1
The infectious spores are acquired via inhalation of contaminated soil containing bird or bat droppings. Thus, a pneumonia that occurs after spelunking (cave exploration) and cleaning bird coops should make you very suspicious for Histoplasmosis. This organism is also endemic in the Ohio and Mississippi rivers and in the Caribbean.1
Histoplasma is an uncommon cause of endocarditis, but this organism should be suspected in individuals with signs and symptoms of endocarditis, but with negative blood cultures. Culture-negative endocarditis can be due to non-infectious causes or due to organisms that are difficult to grow by routine blood culture (ie. HACEK organisms or fungi).
In disseminated disease, one can see collections of rounded yeast forms within macrophages.
In a study of 43 cases of Histoplasma endocarditis, 36 cases involved native valves and 7 cases involved prosthetic valves. Most cases involved left-sided heart valves (mitral and/or aortic) and there was a high rate of embolization (58%). Diagnosis was made via serology (serum antibody or urinary antigen). Blood cultures were only positive in less than 20% of cases (Bhatti).
Prolonged Amphothericin B treatment is the standard of treatment. Untreated cases are fatal (Bhatti).
Kumar V, Abbas AK, Fausto N. Robbins and Cotran Pathologic Basis of Disease. 7th Ed. Philadelphia, PA: Elsevier; 2005: 754.
2 Bhatti S, Vilenski L, Tight R et al. Histoplasma endocarditis: clinical and mycologic features and outcomes. J Infect. 2005 Jul;51(1):2-9. Epub 2004 Nov 5.