As noted by Anon, Lee, and Marco Ligozzi, the diagnostic features include:
- Enlarged size of infected RBCs compared to uninfected cells
- Amoeboid trophozoites
- Schüffner's dots/stippling of infected RBCs
- Molding of infected cells to the contours of the surrounding cells
The primary differential based on the morphology is Plasmodium ovale, which also infects younger (thus larger) RBCs and exhibits Schüffner's stippling. It can sometimes be challenging to differentiate between the two species, particularly when only a few parasites are present. The main differentiating features that P. ovale exhibits are oval shape (seen in approximately 1/3 of infected cells), darker/coarser hemozoin pigment, fimbriated (ragged) edges of the infected RBCs, and more compact trophozoite forms. The P. ovale schizont stage also has fewer merozoites than P. vivax.