Most cases of Histoplasmosis are reported from Gangetic West Bengal. The isolation of H. capsulatum from local soil proved that the disease is endemic. The rarity of the disease from other parts of India may be due to under reporting. Histoplasmosis is confused with malignancy or nonmalignant tumors or granulomas. One has to think of Histoplasmosis when diseases not responding to routine treatment. Ecological and epidemiological studies has to be under taken for histoplasmosis.
Article Histoplasma capsulatum in the soil of Gangetic Plain in India
Histoplasmosis remains something of a mystery. Vast numbers of people appear to have been exposed to it, yet most remain asymptomatic. When histoplasmin was available for population-based surveys, it was found that 80% of population in parts of central and eastern USA showed previous infection. It is quite likely that co-factors probably play an important part in the regional differences in incidence of the clinical illness. Genetic factors, possibly, but environmental determinants are more likely. Exposure to bird (including poultry) or bat droppings seems important, and to soil that might contain this material and the organism. But incidence in Europe appears very low, despite the intensive small-scale gardening there which would suggest exposure to compost, garden soil, pigeons and poultry.
You wrote "The isolation of H. capsulatum from local soil proved that the disease is endemic". That would suggest only that the organism is present in the environment, but endemicity (symptomatic and asymptomatic) would have to be determined from population surveys, and without the histoplasmin reagent, that is virtually impossible. Clearly much more understanding is needed about a range of potential co-factors. Meanwhile, as you have commented, the apparent rarity of the disease from other parts of India may be due to under-reporting or misdiagnosis.
Dr Tim Sly Very informative answer. Skin sensitivity to histoplasmin in Calcutta and its neibourhood (Sanyal and Thammayya, 1980) showed skin sensitivity to histoplasmin in 9.4 per cent of 275 persons tested and 13.8 per cent doubtful reaction. The positive reactors were residents of Gangetic plain. Histoplasma capsulatum grows well in tropical regions where its growth factors present. It may be that incidence in Europe is low because cold climate prevailing there. There was misdiagnosis in most cases prior to attending our department.
Please have a look at this useful link.
https://www.who.int/medical_devices/diagnostics/selection_in-vitro/selection_in-vitro-meetings/00005_03_RandhawaOccurrence-or-histoplasmosis.pdf
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