Plasmodium Species - Malaria | Choose the Right Test

Malaria is a serious and potentially fatal disease caused by various parasites within the Plasmodium species, specifically P. falciparum, P. vivax, P. ovale, and P. malariae, and less commonly, P. knowlesi via infected macaque monkeys. Malaria is most often spread by infected Anopheles mosquitos; human-to-human transmission is rare and only possible in cases of perinatal transmission, blood transfusion, organ transplantation, or contaminated needle use. Malaria is not currently endemic to North America and generally only occurs in subtropical or tropical regions at elevations below 1,500 meters. Malaria may present with a standard set of symptoms (uncomplicated malaria) or with more serious complications (severe malaria). Symptoms are often nonspecific and flu-like (eg, fever, fatigue, chills, headache, muscle aches). Malaria is diagnosed by microscopy on thick and thin blood smears.

Quick Answers for Clinicians

Which factors influence treatment decisions for malaria?

The selection of antimalarial drugs for a patient with malaria is determined by the following factors:

The specific Plasmodium species responsible for infection (ie, P. falciparum, P. vivax, P. ovale, P. malariae, or P. knowlesi)
Clinical status of the patient (ie, uncomplicated malaria or severe malaria)
Expected drug susceptibility (based on geographic location)
History of antimalarial use (including drugs used for prophylaxis)

For detailed information on the selection of antimalarial drugs, Refer to the CDC's Malaria in the United States: Treatment Tables.

What are the differences between uncomplicated malaria and severe malaria?

Uncomplicated malaria generally presents with a combination of symptoms such as fever, sweats, chills, headaches, nausea, and general fatigue. More rarely, it may present with classic malaria attacks, which last 6-10 hours and occur every second or third day. These attacks consist of a cold stage, a hot stage, and a sweating stage.

Severe malaria occurs when complications such as organ failure, blood abnormalities, or metabolic abnormalities are present. Patients with one or more of the following criteria are considered to have severe disease:

Parasite density of ≥5%
Impaired consciousness or coma
Severe anemia (hemoglobin <7 g/dL)
Acute kidney injury
Acute respiratory distress syndrome
Circulatory collapse or shock
Disseminated intravascular coagulation
Acidosis
Jaundice (in the presence of one or more additional signs of severe malaria)

What is the incubation period for malaria, and how does it impact laboratory testing?

The incubation period for malaria is typically 7-30 days. Plasmodium falciparum is associated with the shorter incubation periods and P. malariae is associated with the longer incubation periods. However, the appearance of symptoms can be delayed for weeks or months by antimalarial drugs taken for prophylaxis, especially in cases of infection with P. vivax and P. ovale. Thus, when assessing a patient with symptoms of malaria, all travel within the last 12 months to areas where malaria is endemic should be taken into consideration.

How are malaria cases evaluated for treatment resistance?

All cases of malaria in the United States should be evaluated for treatment resistance by the CDC. After molecular testing to confirm infection with Plasmodium species, one or more of the following techniques will be used to assess the sample for drug resistance:

In vitro tests may be performed on pretreatment specimens within 72 hours of collection. The blood sample is cultured, and the resulting parasites are exposed to different antimalarial drugs.
Molecular testing (eg, polymerase chain reaction [PCR], gene sequencing) may identify resistance to certain drugs for which molecular markers that confer resistance have been identified.
Drug levels may aid in distinguishing between drug nonadherence and drug resistance in patients who developed malaria while taking malaria prophylaxis drugs.

Indications for Testing

Testing for malaria should be considered in individuals with an applicable travel history who present with symptoms of either uncomplicated malaria (eg, fever, fatigue, chills, headache) or severe malaria (eg, coma, confusion, neurologic focal signs, anemia).

Laboratory Testing

Blood Smears

Microscopy is the gold standard for the diagnosis of malaria. Blood smears should be performed and read within 24 hours of a patient’s initial presentation so that treatment, if needed, can be initiated quickly. False-negative results can occur due to low parasitic densities. To rule out a malaria diagnosis if the initial test is negative, blood smears should be repeated every 12-24 hours until three sets have been performed.

Microscopy may be performed on either thick or thin blood smears; thick smears are used to detect parasites and thin smears are used for species identification via morphologic analysis. Giemsa, Wright, or Wright-Giemsa stains may all be used, but Giemsa is preferred because it allows more morphologic features to be identified, which aids in determining the specific Plasmodium species, an important factor in treatment decision-making.^,^,

After diagnosis and species identification, microscopy should be used for quantification (ie, to determine parasitic density). Quantification may be performed on either thick or thin smears and may help guide treatment.^,

Microscopy should also be performed after treatment has been initiated to monitor the patient’s parasitologic status. Blood smears should be repeated every 12-24 hours for infections caused by P. falciparum, P. knowlesi, or suspected chloroquine-resistant P. vivax. A negative smear should be obtained after treatment.

Molecular Testing

Although microscopy on blood smears generally allows for the identification of Plasmodium species, it may fail to differentiate between species with overlapping morphologic characteristics or in cases of altered parasite morphology (which typically occurs due to drug treatment or improper sample storage). In these cases, molecular testing, such as polymerase chain reaction (PCR), may be used for species identification. Additionally, PCR may be used to diagnose malaria when the parasitic density is too low to be detected by blood smear.

Rapid Antigen Testing

Rapid antigen testing provides results within 15 minutes and may be useful if a blood smear cannot be read in a timely fashion. However, the test offers relatively low sensitivity and cannot differentiate between species of Plasmodium. A negative antigen test must be confirmed by microscopy, and a positive test must be followed up by microscopy to determine which species was detected and to quantify the percentage of infected red blood cells (RBCs).

Serology

Because of the time needed to develop antibodies to Plasmodium, serology is generally not recommended for the diagnosis of malaria. However, it may be useful in the following situations:

Retroactively diagnosing a patient who has been treated for malaria but whose diagnosis was uncertain
Testing blood donors in cases of transfusion-induced malaria when parasitic density is too low to be detected by blood smear

Testing is performed using indirect fluorescent antibody (IFA) methodology and is available for each of the malaria-causing Plasmodium species. Cross-reactivity between Plasmodium species and Babesia species is common. Serology testing is primarily available through the CDC.