How to identify the type of malaria on a blood smear

In this Medmastery Clinical Guide article, learn how to identify the subtype of malaria from a blood smear. See photos.
Last update14th Jul 2021

There are a number of ways to make a diagnosis of malaria, but one of the fastest is to look at a patient’s blood smear under a microscope. This allows us to determine the presence of malaria and the type of malaria. If we use a blood sample with more than 50 parasitized red blood cells / µL, we can diagnose malaria with about 70% sensitivity.

So how do we use a blood smear to distinguish between the different types of malaria? Well, each type of malaria displays a unique set of characteristics in infected red blood cells (RBCs) that can be seen under a microscope. Let’s take a closer look at how to identify each of the four main types of malaria—Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, and Plasmodium vivax.

Identifying Plasmodium falciparum on a blood smear

Let’s review the main characteristics of Plasmodium falciparum that can be identified with microscopic examination of a blood smear:

  1. High-grade parasitemia
  2. Crescent-shaped gametocytes

High-grade parasitemia

Figure 1 highlights the classic presentation of falciparum malaria in a blood smear. Notice just how many RBCs are infected, including some RBCs that are doubly parasitized with two ring trophozoites in one cell. Remember that P. falciparum can infect RBCs of any age. So, seeing a high-grade parasitemia, or a lot of infected cells, should immediately cue us to consider a diagnosis of falciparum malaria.

Figure 1. Ring trophozoites can be seen in a blood smear when viewed under a microscope. The high proportion of infected red blood cells seen here, including doubly parasitized cells, is characteristic of Plasmodium falciparum. Image source: CDC Public Health Image Library (PHIL), ID#5856

Crescent-shaped gametocytes

The most definitive finding of P. falciparum is the shape of the gametocytes. Unlike what we see in the other species of malaria, they are crescent-shaped or banana-shaped.

Figure 2. A definitive finding of Plasmodium falciparum in a blood smear is the presence of crescent-shaped gametophytes. Image source: CDC Public Health Image Library (PHIL), ID#5856

Appliqué forms

There is one other feature of P. falciparum to be aware of. Some of the trophozoites may look like they are on the surface of the RBC. These are the appliqué forms. They are much more common to see with falciparum than other types of malaria.

Figure 3. In Plasmodium falciparum infection it is common to see appliqué forms where the ring trophozoites appear on the periphery of the red blood cell. Image source: CDC Public Health Image Library (PHIL), ID#5856

So, when using microscopy to make a diagnosis of Plasmodium falciparum malaria, remember to look for high-grade parasitemia and crescent-shaped gametocytes.

Figure 4. When looking at a blood smear under a microscope, Plasmodium falciparum can be identified by the presence of a high proportion of infected red blood cells and crescent-shaped gametocytes.

Become a great clinician with our video courses and workshops

Identifying Plasmodium malariae on a blood smear

Plasmodium malariae can be identified by its three unique characteristics:

  1. Senescent RBC infection
  2. Band-like trophozoites
  3. Rosette forms

Senescent RBC infection

Remember that P. malariae primarily infects senescent cells, which are typically smaller than other red blood cells. If we identify a ring trophozoite, the only way to determine if it is P. malariae is to decide if the infected RBC is smaller than the surrounding cells.

Figure 5. Microscopic view of the ring trophozoite of Plasmodium malariae. Image source: CDC Public Health Image Library (PHIL), ID#5127

Band-like trophozoites

Now as the P. malariae trophozoites mature, they become more distinctive with a band-like or sash-like structure. This is unique to malariae malaria.

Figure 6. Developing Plasmodium malariae trophozoites can be distinguished by their band-like structure. Image source: CDC Public Health Image Library (PHIL), ID#639

Rosette forms

Lastly, we may find merozoites lined up around the perimeter of the schizont with pigment in the center. This is called the rosette form of P. malariae malaria.

Figure 7. The rosette form of a Plasmodium malariae schizont, where the merozoites line up around the perimeter with pigment in the center of the cell, can be seen under a microscope. Image source: CDC Public Health Image Library (PHIL), ID#5860

So, for Plasmodium malariae, look for infection in smaller cells, as well as the presence of a band-like structure in the developing trophozoite, and a rosette form of the schizont.

Figure 8. Plasmodium malariae can be identified using a microscope by looking for infection in the smallest red blood cells, band-like trophozoites, and rosette schizonts.

Identifying Plasmodium ovale on a blood smear

Let's consider Plasmodium ovale. There are four main features that will be useful to consider when looking at the blood smear:

  1. Reticulocyte infection
  2. Schuffner’s dots
  3. Oval shape
  4. Feathering

Reticulocyte infection

Remember that P. ovale likes young red blood cells—reticulocytes—which are generally bigger than the other forms of red blood cells. So, look for ring forms inside large cells on the blood smear.

Figure 9. When viewing a blood smear under a microscope, ring trophozoites seen inside large reticulocytes are characteristic of Plasmodium ovale infection. Image source: CDC Public Health Image Library (PHIL), ID#5056

Schuffner's dots

Another common feature of P. ovale is the eosinophilic dots, or Schuffner’s dots, that you see throughout the cytoplasm of an infected RBC. Schuffner's dots are classic findings of P. ovale and P. vivax infection.

Oval shape

Schuffner’s dots will distort the parasitized RBC into an oval, giving P. ovale its distinctive namesake shape.

Figure 10. A developing Plasmodium ovale trophozoite, with Schuffner’s dots in the cytoplasm of the oval-shaped red blood cell, is visible under a microscope. Image source: CDC Public Health Image Library (PHIL), ID#5934

Feathering

Lastly, close inspection of an RBC infected with P. ovale will reveal the cell’s edges are feathered.

When diagnosing Plasmodium ovale, remember that infection occurs in larger cells. Look for the presence of Schuffner’s dots inside cells that are distorted into an oval shape with feathering around the edges of the cell.

Figure 11. Look for infection in large red blood cells, Schuffner’s dots in the cytoplasm, oval-shaped cells, and feathering around the cell’s edges when identifying Plasmodium ovale in a blood smear.

Become a great clinician with our video courses and workshops

Identifying Plasmodium vivax on a blood smear

Finally, let's look at the second most common type of malaria worldwide—Plasmodium vivax. There are three things to look for in blood smears with P. vivax infection:

  1. Reticulocyte infection
  2. Schuffner’s dots
  3. Absence of RBC shape changes

Reticulocyte infection

P. vivax selectively infects reticulocytes, so we would expect to see a low-grade parasitemia, with ring trophozoites in the largest red blood cells.

Figure 12. Plasmodium vivax ring trophozoites can be seen in the large reticulocytes in a blood smear. Image source: CDC Public Health Image Library (PHIL), ID#3709

Schuffner's dots

And like P. ovale, many infected red blood cells will have Schuffner’s dots.

Absence of RBC shape changes

But unlike P. ovale infections, the Schuffner’s dots seen in P. vivax don’t distort the red blood cell. It actually stays more rounded without a feathered edge.

Figure 13. Under a microscope, Schuffner’s dots can be seen in a developing Plasmodium vivax trophozoite. The cell stays rounded and lacks feathered edges which distinguishes it from Plasmodium ovale infection. Image source: CDC Public Health Image Library (PHIL), ID#5819

So, like P. ovale, Plasmodium vivax affects larger reticulocytes and develops Schuffner’s dots. But in P. vivax infections the cells are more rounded and do not show feathering around the edges.

Figure 14. In Plasmodium vivax infections, the blood smear will show characteristic low-grade parasitemia with infected reticulocytes only. Schuffner’s dots will be present but the red blood cells will remain round and will not have feathered edges.

While microscopy is a quick and sensitive way to diagnose malaria, don’t forget that there are other options. Rapid diagnostic tests (RDTs) and polymerase chain reaction (PCR) are both effective. RDTs are often used when microscopic analysis is unavailable. PCR is the most sensitive option but requires specialized equipment and is much slower to produce results.

That’s it for now. If you want to improve your understanding of key concepts in medicine and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.

Recommended reading

  • Ashley, EA, Phyo, AP, and Woodrow, CJ. 2018. Malaria. Lancet391:1608­–1621. PMID: 29631781
  • Fairhurst, RM and Wellems, TE. 2014. “Malaria (Plasmodium Species)”. In: Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, edited by Bennett, JE, Dolin, R, Blaser, MJ. 8thedition. Philadelphia: Elsevier Saunders. (Fairhurst and Wellems 2014, 3070–3090)
  • Phillips, MA, Burrows, JN, Manyando, C, et al. 2017. Malaria. Nat Rev Dis Primers3:17050. PMID: 28770814
  • World Health Organization. 2015. Guidelines for the treatment of malaria third edition. World Health OrganizationWorld Health Organization
  • World Health Organization. 2019. World malaria report 2019. World Health Organizationhttps://www.who.int

About the author

John F. Fisher, MD MACP FIDSA
Professor of Medicine (Infectious Diseases) at the Medical College of Georgia, Augusta University, USA.
Author Profile

Become an expert

BMA Highly recommendedComenius EduMedia Siegel 2017
Highly commended by the British Medical Association
Awarded in the “digital” category of the BMA Book Awards - London 2017