Histoplasmosis is a mycosis caused by Histoplasma capsulatum (HC) which can be aggressive under certain conditions. HC is a spore-forming, dimorphic fungus that has a hyphal morphology in the natural environment and a yeast-like morphology in the human host (1). HC is generally present in the soil, particularly in soil rich in bird and bat guano; the endemic region for histoplasmosis includes the Mississippi River basin of the United States and parts of South America (2,3). Human-to-human transmission via a pulmonary route has not been reported, to the best of our knowledge (4). Only a few cases have been reported in Japan due to the rarity of HC in the natural environment (5). In Japan, the majority of the case reports are single cases only, and there are no publications summarizing the findings of these reports. HC infects the human lungs via the bronchi and clinical manifestations most often develop as pulmonary histoplasmosis. Several individuals have subclinical infections or exhibit mild, cold-like symptoms and heal spontaneously after 2-3 weeks (6). Infection with HC is characterized histopathologically by a granulomatous inflammation of the lung, with the fungal body being entrapped inside the granulomas in the lung (7). However, in immunosuppressed patients, such as those with acquired immunodeficiency syndrome (AIDS), organ transplant recipients and the elderly, the fungus may disseminate to other organs as visceral histoplasmosis (8). Sites of onset are diverse, including the skin, central nervous system and gastrointestinal (GI) tract. The development of lesions in the GI tract has been reported in several AIDS patients (9). The frequency of clinically diagnosed GI histoplasmosis is 3-12% of patients with disseminated histoplasmosis. GI lesions are found in 70% of disseminated histoplasmosis cases at autopsy (10). Rapid diagnosis and treatment are necessary as this infectious disease may be fatal. However, it can be difficult to diagnose accurately, due to a similarity in symptoms with bacterial and viral pneumonia, such as fever, chills, cough and shortness of breath, which can result in delayed treatment (11). There are several methods that facilitate diagnosis, including serological tests, culturing and morphological identification of fungal bodies by microscopy (8). HC must be differentiated from several other pathogens as the morphological findings may be inconclusive; thus, molecular techniques have been used to aid in identification. This molecular pathological methodology identifies specific DNA sequences from HC, potentially from very small amounts of specimen, and this method is the most effective approach for accurate diagnosis of HC infection (12).

The present report describes the case of a woman who was immunosuppressed due to an HIV infection, and a duodenal lesion was incidentally found by upper endoscopy. This report describes the diagnosis of the duodenal histoplasmosis lesion based on histopathological and molecular pathological studies.

HC must be differentiated from other pathogens using morphological and molecular based approaches. These other pathogens include Candida glabrata, Cryptococcus neoformans, Talaromyces marneffei, Paracoccidioides brasiliensis, Sporothrix schenkii and Leishmania donovani, all of which are very similar to HC (13). HC is phagocytosed into the cytoplasm of macrophages, whereas the yeast cells of C. glabrata are rarely found within macrophages. C. neoformans has a capsule which may be stained using a polysaccharide stain. As HC is not encapsulated, these organisms cannot be observed using such staining methods; however, halos surrounding the fungi can be observed by microscopy (14). C. neoformans, when phagocytosed by macrophages, loses its capsule, and thus often appear small in size. In such a case, the fungal walls can be visualized by staining using the Fontana-Masson stain (15). T. marneffei may have a yeast-like form similar to that of HC, but a septum can be found in these oval-shaped yeast-like cells (13). P. brasiliensis is characterized by a mariner's wheel, which is formed by a mother cell surrounded by peripheral daughter yeasts (16,17), and S. schenkii occasionally presents as an asteroid body (18). L. donovani is also similar to HC, but it is negative for the PAS reaction and Grocott staining (13). L. donovani is also characterized by its kinetoplast, an organelle which is not present in HC. Although the kinetoplast is difficult to detect by H&E staining, it is possible to visualize using Giemsa staining (14). However, it is difficult to provide a diagnosis based on morphology alone as there are relatively few differences between these pathogens.

As detailed above, there are certain characteristic differential morphological findings which can be used to identify HC, but definitive diagnosis is difficult in some cases. Thus, it may occasionally be necessary to base a diagnosis on the results of molecular techniques. The present case was unequivocally diagnosed by amplifying a specific genomic region of HC using nested PCR, followed by direct sequencing. The genomic DNA can be amplified and visualized readily, even from a small quantity of formalin-fixed, paraffin-embedded tissues, obtained from an endoscopic duodenal mucosal biopsy specimen. This is considered to be an extremely specific diagnostic method for differentiating HC from other pathogens, including other fungi.

The patient in the present report had not been abroad for 20 years and may already have been infected with HC when she came to Japan. Although she was in a clinically latent state of AIDS, it is likely that her immunocompromised condition led to the development of histoplasmosis. There are various considerations regarding the interrelationship between immunodeficiency caused by HIV infection and the development of histoplasmosis (19). HIV can infect not only CD4⁺ T cells but also CD4⁺ macrophages and dendritic cells. CD4⁺ T cells cannot differentiate following HIV, and a lack of CD4⁺ T cell helper function underlies the deficiency of cytotoxic production in patients infected with HIV (20,21). The phagocytic capacity of macrophages and the microbicidal ability of neutrophils are all impaired. CD4⁺ macrophages infected with HIV may be alive; however, their microbicidal activity is decreased (22). The number of CD4⁺ T cells was markedly reduced and a large number of macrophages phagocytosing HC were present in the duodenal mucosa of the patient. These findings support the hypothesis that macrophages are not destroyed by HIV infection but they may lose their ability to digest phagocytosed pathogens; accordingly, a large number of HCs would thus be present in the cytoplasm of these macrophages.

Histoplasmosis is characterized by the formation of granulomatous inflammatory lesions in the lung (5). However, no typical granulomatous changes were observed in the duodenal mucosa of this patient, and it is unclear why no granulomatous lesions were present there. One possible explanation is that the granuloma-forming activity of macrophages was decreased and differentiation into Th1 and Th17 cells from CD4⁺ T cells was impaired due to HIV infection (23,24).

In conclusion, a case of histoplasmosis that occurred as the initial lesion in the duodenum of an HIV-infected patient is reported. It can be difficult to distinguish HC from other fungi and protozoa based on morphology alone. Therefore, the final diagnosis of duodenal histoplasmosis should be based on additional auxiliary techniques, including histopathological analysis and molecular pathological methods.