As mentioned above, E. hartmanni closely resembles the small race of E. histolytica. It can be differentiated by careful examination of hematoxylin-stained preparations. Burrows (1959) compared the two species. Most trophozoites of E. hartmanni are smaller than those of E. histolytica. Rounded trophozoites of E. hartmanni range from 3 to 10.5 u in diameter, while those of E. histolytica are 6.5 u or more in diameter. The trophozoite nucleus of E. hartmanni is usually 2.0 to 2.5 u in diameter but may range from 1.5 to 3.2 u, while that of E. histolytica is usually 3.0 to 3.5 u in diameter but may range from 2.8 to 3.8 u. The peripheral chromatin of E. hartmanni is more variable in its arrangement than that of E. histolytica and may consist of discrete granules with wide spaces between them, a crescent of granules on one side of the nucleus, or a single large bar of chromatin with several small granules around the membrane; the peripheral chromatin of E. histolytica is generally distributed uniformly along the nuclear membrane.
Most cysts of E. hartmanni are smaller than those of E. histolytica. They range from 3.8 to 8.0 u in diameter while those of small race E. histolytica are 5.5 u or more in diameter. The cyst nuclei of E. hartmanni are 1.8 to 3.0 u in diameter in uninucleate cysts, 1.3 to 2.0 u in binucleate cysts and 0.7 to 1.7 u in tetranucleate cysts; those of small race E. histolytica are 2.4 to 2.8 u in diameter in uninucleate cysts, 2.0 to 2.8 u in binucleate cysts and 1.4 to 2.2 u in tetranucleate cysts. The cysts of E. hartmanni seldom contain large glycogen bodies, but nearly all of them have a few to many small vacuoles; the cysts of E. histolytica generally have one large glycogen vacuole or no vacuoles. The chromatoid bodies of the two species are similar.
Freedman and Elsdon-Dew (1959) suggested that, until an accurate, practical method of separation is devised, mean sizes of 12 u for trophozoites and 10 u for cysts be used as the dividing line between E. histolytica and E. hartmanni. The latter criterion has been used for some time to distinguish between the cysts of large and small race E. histolytica by those who do not accept the name E. hartmanni (Shaffer et al., 1958).
The incidence of E. hartmanni in animals and man is unknown because in the past it has ordinarily been lumped with E. histolytica. According to Burrows (1957, 1959) about half of the reported cases of E. histolytica in the United States were actually E. hartmanni. Further studies in which the two species are separated will throw light on this point, which is important because E. hartmanni is non-pathogenic.