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Malaria and Babesia on blood films

18 min

Skills you'll build3 skills
  • 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

At the bench: Read an organism stain and communicate a significant preliminary finding

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?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Thick and thin films

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 is still suspected, films are repeated every 12 to 24 hours for a total of 3 sets.2,3

Recognizing parasites

A Plasmodium ring has delicate blue cytoplasm with one or two red chromatin dots, and the same forms repeat across many cells.4 A dark dot with no cytoplasm around it is not counted. Films are read in an area that is well stained and free of stain precipitate.7

Species identification

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

Six thin-film fields. P. falciparum: thin rings with small chromatin dots in normal-sized red cells, two cells holding more than one ring. A single crescent-shaped falciparum gametocyte with dark pigment along its middle. P. vivax: two enlarged red cells filled with fine stippling around irregular ameboid trophozoites. P. malariae: one parasite forming a straight band across a normal-sized red cell with dark pigment along it. Babesia: a cross-shaped tetrad of four small parasites inside one red cell. Babesia: rings of different sizes and shapes, some paired, in normal-sized red cells with no pigment and one extracellular form.
Dark pigment lies along the crescent and the band. Neither Babesia panel has any.4,5

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

Babesia rings vary in size and shape, are found in normal-sized cells, can be extracellular and have no pigment. The tetrad, or Maltese cross, is diagnostic when present. It is uncommon, so its absence does not exclude Babesia. Morphology does not assign a Babesia species.5 When travel history makes malaria possible, molecular testing detects both.6

Parasitemia and reporting

Percent parasitemia is infected red cells ÷ red cells counted × 100, read on the thin film across 500 to 2,000 red cells. A cell with several rings is counted once, and gametocytes are counted separately.7 The film gives three results: parasites present, the species and the percentage. They are reported to the provider within hours. PCR confirms the species once the diagnosis is made.8

Follow the bench demonstration

Sources for this step
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
  7. Centers for Disease Control and Prevention. DPDx: blood specimens, microscopic examination. Accessed October 2, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/blood/microexam.html
  8. Centers for Disease Control and Prevention. Malaria diagnostic tests. Accessed October 2, 2026. https://www.cdc.gov/malaria/hcp/diagnosis-testing/malaria-diagnostic-tests.html

Watch one

Read each decision and its explanation.

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 contain 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 they are 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 distinguishes 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 are seen in P. malariae and in later P. knowlesi stages.

  5. Combine 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 an area 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.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A new laboratory professional 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 can be read only when its red cells lyse. When fixed, the stacked cells obscure 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.

Hint
  1. Think about what has to happen to the thick film's red cells during staining.
  2. Ask what methanol does to a red cell.

Review Blood-film preparation and examination

Problem 2 of 3 · Hints available

Three patients with fever have these thin films. Point to the film whose parasite cannot be Babesia.

Three unlabeled thin-film photomicrographs, two above and one below, each with parasites in red cells.

Image sources and licenses

  1. Top left
  2. Top right
  3. Bottom
Hint
  1. Look for something Babesia never makes.
  2. Look for dark granules inside a parasite.
Show the answer

P. malariae band with dark pigment

The band in the lower film contains dark pigment, and Babesia never makes pigment. Rings without pigment, as in the two upper films, leave both genera possible until the rest of the film is read.

Review Blood-film findings

Problem 3 of 3 · On your own
  • Hal Triplett, 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.

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 suggest Babesia.

Reported Babesia rings as P. falciparum

Babesia rings are pleomorphic, sit in normal-sized cells, can lie outside cells, and have 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

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Efua Spinner, 38, a choir director who sings at every family gathering, came home 9 days ago from three weeks with family in Kumasi, Ghana.
  • She has fever and shaking chills. Her EDTA blood reaches the laboratory at 22:10.
  • Thick film: many rings.
  • Thin film count: 2,000 red cells in monolayer fields, 512 of them infected, with 680 rings in all. The infected cells are normal sized. Rings only, with no gametocytes or older stages.
Decision 1 of 4

Before you count, point to a dark dot in this field that you would not count as a parasite.

A Giemsa-stained thin blood film at oil immersion: overlapping red cells with small dark dots on and between them.

Image sources and licenses

  1. Top, center
  2. Top, right of center
  3. Upper right
  4. Middle, left of center
  5. Middle right
  6. Lower center
Show the answer

Precipitate between cells, Precipitate on a cell edge, Two precipitate specks

Stain precipitate is black, sits on top of the cells with a bright rim and has no blue cytoplasm. Each parasite here is a red chromatin dot on a thin blue ring, and those are the ones counted.

Review Blood-film findings

Decision 2 of 4

Which identification does the film support?

Rings only, often more than one to a cell, in normal-sized cells fit P. falciparum. Her exposure in Ghana fits it too.

P. vivax enlarges the infected cell and shows growing stages. These cells are normal sized, with rings only.

P. malariae shows compact bands and older stages. This film shows rings only.

Babesia comes from ticks, and her exposure was in a malaria area. The film also has none of its pear-shaped pairs, tetrads or rings outside cells.

Review Species morphology on thin films

Decision 3 of 4

What is the percent parasitemia?

Show the answer

25.6 %

512 infected cells ÷ 2,000 red cells × 100 = 25.6%. A cell with two or three rings is counted once.

Review Parasitemia

Decision 4 of 4

The count is finished at 22:40. What do you do with the result?

Presence, species and percentage are reported to the provider within hours. PCR confirms the species once the diagnosis is made.

PCR results often come too late to help an acutely ill patient. The film result goes out now.

A crescent is not needed. The rings and her exposure already support P. falciparum, and PCR confirms it.

Repeat films are for a negative first set. This film is positive now.

Review Species morphology on thin films

The film shows rings only, often several to a cell, in normal-sized cells, and the exposure was in Ghana.

For reference

Sources checked