This picture is small to really tell if those are yeast forms or not, but the attached picture is what Histoplasmosis would look like on a blood smear.
I think the key here is in the history/presentation. Histoplasmosis does not classically cause leukopenia. Additionally, we are told that the man is a hiker. This should clue us in on some sort of tick borne disease or arbovirus type infection.
I did not know that this was Erlichiosis, but since the question was asking about treatment, I was able to guess right with Doxycycline. As a general rule, many of the rare infections such as those that are tick borne (lyme, RMSF, etc) are usually treated with Doxycycline.
