Plasmodium knowlesi
- Parasite
- Exam priority: medium
A monkey malaria parasite of Southeast Asia that also infects people. It multiplies every 24 hours and can become severe quickly.
Also called: monkey malaria, fifth human malaria
Quick facts
- Type
- Parasite · Apicomplexans
- Gram / shape
- Rings like P
- Spread
- Bite of forest Anopheles mosquitoes · Like any malaria
- Causes
- Knowlesi malaria · Severe knowlesi malaria
At a glance
- Shape
- Rings like P. falciparum; band trophozoites and schizonts like P. malariae
- Family
- Plasmodiidae
- Type
- Intracellular protozoan (sporozoan) of red blood cells
- Stain
- Giemsa thick and thin smear; PCR needed to name the species
Morphology
- Early rings look like P. falciparum: double chromatin dots, appliqué forms and several parasites per cell.
- Older trophozoites form bands across the cell, like P. malariae.
- Mature schizonts have up to 16 merozoites; infected red cells are not enlarged.
Transmission
- Bite of forest Anopheles mosquitoes of the Leucosphyrus group that have fed on infected macaques.
- Human-to-human transmission by mosquitoes has not been shown in nature: it is a zoonosis.
- Like any malaria, also possible by blood transfusion.
Reservoir: Long-tailed and pig-tailed macaques (and leaf monkeys) in Southeast Asian forests
Vector: Anopheles of the Leucosphyrus group (for example An. balabacensis in Malaysian Borneo), which bite in forests and forest edges.
Isolation precautions
What the care team adds to standard precautions.
- Standard
Life cycle
- An infected Anopheles injects sporozoites while biting a macaque or a person in the forest.
- Sporozoites multiply in the liver; P. knowlesi has no dormant hypnozoites.
- Merozoites invade red cells and complete a cycle about every 24 hours.
- Some parasites become gametocytes in the blood.
- A mosquito takes up gametocytes from a monkey, and the sexual cycle ends with new sporozoites in its salivary glands.
Virulence factors
- The 24-hour blood cycle lets parasitemia rise fast, so high counts can be reached within days.
- Severe disease with jaundice, kidney injury, respiratory distress and shock, mostly in adults with high parasitemia.
- Thrombocytopenia is common.
Diseases it causes
- Knowlesi malaria: daily fever with chills, headache and myalgia after forest exposure in Southeast Asia. Malaria →
- Severe knowlesi malaria with acute kidney injury and high parasitemia; can be fatal if recognized late. Acute kidney injury →
Clinical picture
- Most cases come from Malaysia (especially Sabah and Sarawak in Borneo), but it occurs across Southeast Asia.
- Travelers who visited forests in the region can present at home weeks later.
- Daily (quotidian) fever is a clue, unlike the 72-hour rhythm of P. malariae.
Diagnosis
- Thick and thin Giemsa smears show the parasite but often misread it as P. malariae (or P. falciparum in early stages).
- PCR is the definitive way to identify the species.
- Rapid antigen tests may detect it poorly; a 'P. malariae' with high parasitemia in a traveler from Southeast Asia is P. knowlesi until proven otherwise.
Treatment
Uncomplicated disease responds to artemisinin-based combinations or chloroquine; severe disease is treated with intravenous artesunate, as for severe falciparum malaria. No primaquine is needed for relapse because it has no hypnozoites.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Repellents, long clothing and treated nets when entering forests in Southeast Asia.
- Chemoprophylaxis for travelers as for other malaria in the region.
- Bed nets and indoor spraying are less useful because vectors bite outdoors in forests.
Test yourself
Think of the answer, then tap the question to see it.
What is the natural reservoir of Plasmodium knowlesi?
Long-tailed and pig-tailed macaques in Southeast Asian forests.
Why is P. knowlesi often missed on microscopy?
Its late stages look like P. malariae and its rings like P. falciparum; PCR is needed to identify it.
What is the length of its blood cycle and why does it matter?
About 24 hours: it causes daily fever and lets parasitemia rise quickly to dangerous levels.
Does P. knowlesi need primaquine to prevent relapses?
No. It does not form hypnozoites in the liver.
Connections
← Back to the group: Apicomplexans
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: CDC - DPDx - Malaria (CDC, 2024); Zoonotic Malaria – Global Overview and Research and Policy Needs (Frontiers in Public Health (PMC), 2014); Three Divergent Subpopulations of the Malaria Parasite Plasmodium knowlesi (Emerging Infectious Diseases (CDC), 2017).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.