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Characteristic features of Plasmodium falciparum parasites on a blood smear (1)

1. P. falciparum rings have delicate cytoplasm and

2. Rings with two dots of chromatin

3. Multiple rings in an individual RBC

4. Presence of appliqué/accolé forms (arrow) rings that appear to be protruding from the RBC; (this can be an important finding to support the diagnosis of P. falciparum)

5.  Absence of forms other than rings.

6. RBCs that are infected are not enlarged

7. Presence of Crescent-shaped gametocytes.

Morphologically, P. falciparum and Babesia ring forms may appear similar, leading to diagnostic uncertainty for patients.

Babesia replicates by binary fission inside red blood cells (RBC) so multiple ring forms in an individual RBC is quite common. In contrast, each Plasmodium ring in a single RBC represents a separate infection event. Therefore, the observation of more than two Plasmodium rings inside an RBC is rare, and the presence of four rings in a single RBC is usually considered diagnostic for Babesia infection (2)

However in our case, despite polyinfection, several features taken together supported the diagnosis of Plasmodium falciparum infection. ( Refer to the image )

1. Rings were mostly uniform in size and shape, in contrast to the pleomorphic ring forms more generally seen in Babesia.

2. Presence of Large numbers of marginal rings (so-called appliqué/accolé forms).

3. Rings with two chromatin dots (headphones) were common.

4. Extracellular ring forms, found frequently in Babesia infection were absent.

5. presence of tetrad or “Maltese cross” forms ( though Pathognomonic, do not appear in every species of Babesia) were absent 

6. Presence of very occasional Crescent-shaped gametocytes confirmed the diagnosis of P. falciparum malaria. 

The thick film provides the greatest sensitivity for parasite detection and is preferred for screening, while the thin film provides the best morphology for the differentiation between Babesia and Plasmodium parasites and Plasmodium species determination. Antigen detection methods are also widely used for the detection of malaria and molecular amplification methods have been described for both organisms.

References:

1. Centers for Disease Control and Prevention. (n.d). Laboratory diagnosis of malaria Plasmodium falciparum. DPDx Laboratory identification of parasites of public health concern. Retrieved from https://www.cdc.gov/dpdx/resources/pdf/benchAids/malaria/Pfalciparum_benchaidV2.pdf

2. Bryan AW Jr, Qian Q, Kirby JE. Answer to February 2017 Photo Quiz. J Clin Microbiol. 2017;55(2):660-661. doi:10.1128/JCM.00194-15