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:
- High-grade parasitemia
- Crescent-shaped gametocytes
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.
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.
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.
So, when using microscopy to make a diagnosis of Plasmodium falciparum malaria, remember to look for high-grade parasitemia and crescent-shaped gametocytes.
Identifying Plasmodium malariae on a blood smear
Plasmodium malariae can be identified by its three unique characteristics:
- Senescent RBC infection
- Band-like trophozoites
- 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.
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.
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.
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.
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:
- Reticulocyte infection
- Schuffner’s dots
- Oval shape
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.
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.
Schuffner’s dots will distort the parasitized RBC into an oval, giving P. ovale its distinctive namesake shape.
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.
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:
- Reticulocyte infection
- Schuffner’s dots
- Absence of RBC shape changes
P. vivax selectively infects reticulocytes, so we would expect to see a low-grade parasitemia, with ring trophozoites in the largest red blood cells.
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.
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.
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.
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