Here is a write-up from the CDC on histoplasmosis. It is caused by a fungus that is pretty common, but the fungus thrives in nitrogen-rich soils, like those containing a lot of bird or bat droppings. It is present in a lot of soils but generally not in concentrations that can cause problems. It is not a disease that chickens catch and carry, but a fungus that can thrive in their droppings.
People with weakened immune systems are the ones primarily threatened. The article does mention the possibility of people going blind from it, but the way I read this article, someone getting vision problems from it is extremely rare. This is the only article I found that mentions the possibility of people having vision problems due to this fungus. Even a person getting sick from it is somewhat rare unless they have a weakened immune system. Apparently most people don’t show any symptoms from a casual exposure. It generally takes a concentration of the spores.
Like others, I'm guessing histoplasmosis may be what the good Doctor was talking about, but I’m a little confused on that. Particles from burning can irritate eyes and lungs, but wouldn’t the fungus be killed by burning?
I’m not a medical professional, but I don’t see the link between burning the bedding and histoplasmosis. I can see a link between eye irritation and burning, if you are close enough that the concentration of the burned ash and smoke is high enough. If you are allowed to burn, you are probably not in the middle of a city. But I agree, if you can manage it, composting is a better way to go, even if your composting technique is to pile it up and leave it alone and never turn it.
I imagine the doctor has seen vision problems caused by histoplasmosis at some point in his/her practice, but I’m not sure where the good doctor linked that to burning chicken litter. I’d think it would more likely come from someone working with unburned chicken manure or used litter, since the chicken industry is so prevalent here in Arkansas. Or maybe it was linked to the practice of spreading chicken manure on hay fields and pastureland as fertilizer. The link to burning is the one I have trouble making, not the link to chicken manure. If you get a chance, you might question the good doctor on that link to burning chicken litter.
The way I look at this, if your immune system is weakened, histoplasmosis can be a potential problem. If your immune system is in pretty good shape, there is not much risk from histoplasmosis, but it is always wise to take reasonable precautions. I’m more concerned with my allergies acting up when raking in the coop than with the danger to me from histoplasmosis.