Why is it important to differentiate histoplasma and leishmania in histology sections, blood and bone marrow smears?
Histoplasma yeast form and leishmania donovani amastigote form are particularly difficult to distinguish. We will discuss further how to differentiate between the two; since treatments differ.
It is not uncommon for someone who has worked long enough in a hematology laboratory to have encountered histoplasmosis wrongly diagnosed as leishmania or vice versa, based on a bone marrow aspiration or biopsy.
Leishmania infection can mimic anything form malignancy to autoimmune diseases, yet histoplasmosis is among the top differentials. Hence caution is warranted.
Histoplasma is predominantly intracellular. Extracellular organisms are very rare.
Leishmamia is intracellular as well as extracellular.
2. BACKGROUND INFLAMMATION
Histoplasma has very minimal background inflammation. No neutrophils, eosinophils and plasma cells.
Leishmania often shows an inflammatory background, especially dense infiltration of plasma cells, often leading to misdiagnosis.
3. PERIODIC ACID SCHIFF (PAS) POSITIVITY
Histoplasma stains positive with PAS, whereas Leishmania does not stain PAS positive.
The nucleus of leishmania donovani has a small kinetoplast near the nucleus. Kinetoplast is composed of mitochondrial DNA. (black arrow)
Histoplasma has a characteristic crescent shaped nucleus.(green arrow)
There is a very slight variation in size. Histoplasma is 2-5 microns and Leishmania is 1- 4 microns. Though not very significant, this can be helpful at times.
That’s it- hopefully the confusion is cleared.
For a quick summary click below.