Things worth knowing about "Plasmodium ovale"
Plasmodium ovale is a malaria-causing parasite transmitted by Anopheles mosquitoes. It causes a mild form of malaria and can remain dormant in the liver for years.
What is Plasmodium ovale?
Plasmodium ovale is a single-celled parasite belonging to the genus Plasmodium and is one of five species known to cause malaria in humans. It is transmitted through the bite of infected female Anopheles mosquitoes. P. ovale is divided into two genetically distinct subspecies: Plasmodium ovale curtisi and Plasmodium ovale wallikeri. The disease it causes is referred to as tertian malaria and generally follows a milder course than malaria caused by Plasmodium falciparum.
Geographic Distribution
Plasmodium ovale is found predominantly in sub-Saharan Africa, but also occurs in parts of Southeast Asia, the western Pacific region, and occasionally South America. Travelers to these regions are at increased risk of infection, particularly if appropriate malaria prophylaxis is not taken.
Life Cycle and Mechanism of Action
The life cycle of P. ovale takes place in two main phases:
- Liver stage (exoerythrocytic phase): Following a mosquito bite, sporozoites enter the bloodstream and migrate to liver cells. There they develop into schizonts, which later release merozoites. A key characteristic of P. ovale is its ability to form hypnozoites -- dormant liver-stage parasites that can remain inactive for months to years and later cause relapses.
- Blood stage (erythrocytic phase): Merozoites invade red blood cells, multiply within them, and destroy them after approximately 48 hours, producing the characteristic 48-hour fever cycle.
Symptoms
Symptoms of a P. ovale infection resemble those of other malaria types and typically appear 10 to 17 days after the infective bite:
- Episodic fever with chills occurring in a 48-hour cycle
- Profuse sweating following the fever episode
- Headache and muscle pain
- Fatigue and general malaise
- Nausea and vomiting
- Mild enlargement of the liver and spleen (hepatosplenomegaly)
Unlike P. falciparum, P. ovale rarely leads to severe complications. However, due to hypnozoite formation, relapses can occur months or even years after the initial infection.
Diagnosis
Diagnosis is established through several methods:
- Light microscopy (thick and thin blood smear): The gold standard for identifying the parasite. Infected red blood cells appear enlarged and often show a characteristic oval or fimbriated shape, as well as Schuffner stippling (fine granular inclusions within the red blood cell).
- Rapid Diagnostic Tests (RDTs): Antigen-detection tests that are easy to use, though they may have lower sensitivity for P. ovale compared to microscopy.
- Polymerase Chain Reaction (PCR): A highly sensitive molecular method for species identification and for distinguishing between the subspecies curtisi and wallikeri.
Treatment
Treatment of a P. ovale infection involves two components:
- Chloroquine: The drug of choice for treating the blood stage. P. ovale generally remains sensitive to chloroquine, unlike P. falciparum.
- Primaquine: Administered in addition to eliminate hypnozoites in the liver and thereby prevent relapses. Before use, a G6PD deficiency (glucose-6-phosphate dehydrogenase deficiency) must be ruled out, as it can cause hemolytic anemia.
In patients with G6PD deficiency, tafenoquine (approved in certain countries) or an adjusted primaquine regimen may be considered as an alternative.
Prevention
Prevention of P. ovale and other malaria infections includes:
- Taking malaria prophylaxis medications (e.g., atovaquone/proguanil, mefloquine, or doxycycline) as recommended by a travel medicine specialist
- Using insect repellents containing DEET or icaridin
- Wearing long, protective clothing during dusk and nighttime hours
- Sleeping under insecticide-treated bed nets
References
- World Health Organization (WHO): World Malaria Report 2023. Geneva: WHO Press, 2023.
- Sutherland CJ et al.: Two nonrecombining sympatric forms of the human malaria parasite Plasmodium ovale occur globally. Journal of Infectious Diseases, 2010; 201(10): 1544-1550.
- Centers for Disease Control and Prevention (CDC): Malaria -- Biology. Atlanta: CDC, 2023. Available at: https://www.cdc.gov/malaria