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Malaria and Babesia on blood films

16 min

  • Distinguish the roles of thick and thin blood films for parasite examination
  • Recognize malaria species features that need confirmatory identification
  • Tell Babesia from malaria and blood-film artifacts

Read the full reference

Try first

Try first

Malaria parasites are seen on the thick film of a returning traveler. Which film gives the species identification and the percent parasitemia?

The next section explains it.

The next section explains it.

Right. The next section explains why.

The next section explains it.

Get the idea

Two films, two jobs

At least 2 thick and 2 thin films are made from EDTA or fingerstick blood, preferably within 1 hour of collection.1,2 Only the thin film is fixed in methanol. The thick film is left unfixed so that its red cells lyse in the stain. That concentrates the parasites and makes the thick film the more sensitive screen. The thin film keeps each red cell intact, so species identification and percent parasitemia come from it.2 At least 100 thick-film oil-immersion fields are examined before a film is called negative. If malaria stays possible, films are repeated every 12 to 24 hours for a total of 3 sets.2,3

Read the whole population

One parasite rarely names a species. The infected cell's size, the ring's shape, stippling, pigment and the stages present are read together.4

FeatureP. falciparumP. vivaxP. malariaeBabesia
Infected cellNormal size, often several ringsEnlarged, fine Schüffner dotsNormal or smallerNormal size
Forms seenDelicate rings, crescent gametocytesAll stages, ameboid trophozoitesBand forms, rosette schizontsPleomorphic rings, pear-shaped pairs, cells outside red cells
PigmentIn mature stages and gametocytesPresent in growing stagesPresentNone

Some patterns overlap. Early P. knowlesi rings look like P. falciparum, and its later band forms look like P. malariae. When exposure makes P. knowlesi possible, the report gives a provisional identification and species-level PCR confirms it.4

Babesia rings vary

Babesia rings vary in size and shape, sit in normal-sized cells, can lie outside cells and carry no pigment. The tetrad, or Maltese cross, is diagnostic when present. It is uncommon, so its absence leaves Babesia possible. Morphology does not assign a Babesia species.5 When travel keeps malaria in question, molecular testing covers both.6

References
  1. Miller JM, Binnicker MJ, Campbell S, et al. Guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update by the Infectious Diseases Society of America and the American Society for Microbiology. Clin Infect Dis. 2024;ciae104. doi:10.1093/cid/ciae104
  2. Centers for Disease Control and Prevention. DPDx: blood specimens. Accessed September 27, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/blood/index.html
  3. Centers for Disease Control and Prevention. Evaluation and diagnosis of malaria. Accessed September 27, 2026. https://www.cdc.gov/malaria/hcp/clinical-guidance/evaluation-diagnosis.html
  4. Centers for Disease Control and Prevention. DPDx: malaria. Accessed September 27, 2026. https://www.cdc.gov/dpdx/malaria/
  5. Centers for Disease Control and Prevention. DPDx: babesiosis. Accessed September 27, 2026. https://www.cdc.gov/dpdx/babesiosis/index.html
  6. Krause PJ, Auwaerter PG, Bannuru RR, et al. Clinical practice guidelines by the Infectious Diseases Society of America (IDSA): 2020 guideline on diagnosis and management of babesiosis. Clin Infect Dis. 2021;72(2):e49-e64. doi:10.1093/cid/ciaa1216

Watch one

A 52-year-old man has fever 10 days after returning from forest trekking in Malaysian Borneo. His Giemsa-stained thin film shows delicate rings, some with double chromatin dots, in normal-sized and slightly small red cells. Several cells hold more than one ring. A few parasites stretch across the cell as bands with brown pigment. No crescents are seen. How is the film reported?

  1. Confirm real parasites: each ring has blue cytoplasm with a chromatin dot, repeated across many cells.

    Stain precipitate, platelets and Howell-Jolly bodies can mimic parasites.

  2. Check the infected cells: normal-sized or smaller, with no enlargement or stippling.

    Cell size separates P. vivax and P. ovale from the others.

  3. Read the rings: they fit P. falciparum.

    Delicate rings with double dots and multiple infection suggest P. falciparum.

  4. Read the older stages: bands fit P. malariae, and P. falciparum rarely shows them.

    Band forms belong to P. malariae and to later P. knowlesi.

  5. Put the pattern with the exposure: P. knowlesi is possible.

    The mix of falciparum-like rings and malariae-like bands is the P. knowlesi pattern, and Borneo is where it is acquired.

Report Plasmodium species seen, with P. knowlesi possible, as a provisional identification, together with the percent parasitemia. Species-level PCR confirms the species.

Your turn

Problem 1 of 3

A thin film from a patient in the northeastern United States shows pleomorphic rings in normal-sized erythrocytes, some paired pear-shaped forms, an occasional extracellular parasite, and no pigment. No tetrad is seen. What does the laboratory report?

Incorrect. P. falciparum rings are more uniform, and hemozoin appears in mature stages and gametocytes. Marked pleomorphism, pyriform pairs, extracellular forms, and absent pigment point to Babesia.

Incorrect. The features support Babesia, but the species cannot be assigned by morphology, and a species-specific B. microti PCR can miss other Babesia species.

Correct. Marked pleomorphism, pyriform pairs, extracellular forms, and absent pigment support babesiosis. The Maltese cross is diagnostic when present but uncommon, so its absence has little exclusion value. When travel leaves malaria possible, molecular testing covers both groups.

Hint
  1. List the features that differ from a P. falciparum ring population.
  2. Ask what morphology can and cannot say about the species.

Review Blood-film findings

Problem 2 of 3

A thin film shows infected red cells about 1.5 to 2 times the size of their neighbors, filled with fine pink stippling. The trophozoites are irregular and ameboid. Schizonts with about 16 merozoites are also present. Which species fits best?

P. falciparum leaves red cells normal in size, and its schizonts are seldom seen in blood.

P. ovale enlarges the cell only to about 1.25 times, often oval with ragged edges, and its trophozoite stays compact.

Enlargement of 1.5 to 2 times, Schüffner dots, ameboid trophozoites and schizonts of about 16 merozoites fit P. vivax.

P. malariae leaves the cell normal or small, with compact band forms and about 8 merozoites in a schizont.

Hint
  1. Start with the size of the infected cells.
  2. Two species enlarge the cell. Compare how much each enlarges it and how their trophozoites look.

Review Species morphology on thin films

Problem 3 of 3

A new technologist fixes both the thick and the thin films in methanol before Giemsa staining. What happens to the thick film?

Fixation holds the red cells in place, so the stain cannot lyse them.

A thick film works only when its red cells lyse. Fixed, the stacked cells hide the parasites, so a new unfixed thick film is made.

A thick film is many cells deep. Species features are read on the thin monolayer.

Expected a thick film to preserve red-cell morphology

The thick film lyses the red cells to concentrate parasites for detection, so infected-cell size, stippling, and ring position are lost. Species identification and percent parasitemia come from the thin film.

Review Blood-film preparation and examination

Use it

  • Hal Lindqvist, 74, a retired ornithologist, bands birds every summer on an island off Massachusetts. He has not left the United States in 10 years.
  • He has fever, fatigue and dark urine in July.
  • Thin film: small rings of many sizes and shapes in normal-sized red cells, some cells with 3 or 4 rings, pear-shaped pairs and two rings lying outside cells. No pigment and no crescents. No tetrad in 300 fields.
  • The thick film shows only a few faint dots.
Decision 1 of 3

What does the thin film show?

Pleomorphic rings, pear-shaped pairs, extracellular forms and no pigment support Babesia. His exposure fits, and he has not traveled to a malaria area.

P. falciparum rings are more uniform. Pear-shaped pairs, rings outside cells and no pigment point to Babesia.

Reported Babesia rings as P. falciparum

Babesia rings are pleomorphic, sit in normal-sized cells, can lie outside cells, and carry no pigment. P. falciparum rings are more uniform, and pigment appears in mature stages. The Maltese cross is diagnostic but uncommon, so its absence leaves Babesia possible.

The Maltese cross is uncommon. The other features already support Babesia.

Review Blood-film findings

Decision 2 of 3

How is the identification worded?

Morphology cannot assign the species. B. microti is common in this region, and other species occur.

This names the genus the film supports. PCR can go further if the species matters.

The film has no Plasmodium features, and his history holds no malaria exposure.

Review Blood-film findings

Decision 3 of 3

The thick film shows only faint dots. How are the two films used?

Tiny Babesia forms are hard to see once the red cells lyse. The thin film already shows them.

The thick film has no intact red cells to count. The percent comes from the thin film.

Expected a thick film to preserve red-cell morphology

The thick film lyses the red cells to concentrate parasites for detection, so infected-cell size, stippling, and ring position are lost. Species identification and percent parasitemia come from the thin film.

The thin film keeps the red cells whole, so it shows the forms and gives the count of infected cells.

Review Blood-film preparation and examination

The clue that settled this case is the ring population together with the missing pigment. Varied rings, pear-shaped pairs and extracellular forms with no pigment mark Babesia, and his summers on the island fit it.

Keep

Sources checked