Plasmodium ovale is one of five parasite species that cause malaria in humans, and it belongs to a small club of two, alongside Plasmodium vivax, that can hide dormant in the liver and trigger new bouts of illness months or even years after the original mosquito bite. Though it causes far fewer cases globally than Plasmodium falciparum, P. ovale’s ability to relapse makes it a persistent challenge for travelers, clinicians, and malaria-elimination programs. What makes the picture even more interesting is that “P. ovale” is not really one parasite at all, but two genetically distinct species circulating side by side, sometimes even within the same person.
Two Species Hiding Under One Name
For decades, scientists treated P. ovale as a single species with some genetic variation. Molecular analysis eventually revealed that what had been called P. ovale actually comprises two separate species: Plasmodium ovale curtisi and Plasmodium ovale wallikeri. The two look virtually identical under a microscope, which is why they went unrecognized for so long. But genetically, they are distinct enough that they do not recombine with each other, even when both circulate in the same communities at the same time. Research in Uganda, Congo-Brazzaville, and Equatorial Guinea showed that P. o. curtisi and P. o. wallikeri coexist in exactly the same places and time periods, which points to a biological barrier between them rather than geographic separation keeping them apart.1PubMed Central. Plasmodium ovale curtisi and Plasmodium ovale wallikeri circulate simultaneously in African communities
In a study of P. ovale samples from western Kenya, roughly 59% were identified as P. o. curtisi and 41% as P. o. wallikeri, based on two independent gene targets that agreed on every classification.2PLOS Neglected Tropical Diseases. Characterization of Plasmodium ovale curtisi and P. ovale wallikeri in Western Kenya Utilizing a Novel Species-specific Real-time PCR Assay Similar splits, sometimes favoring one species over the other depending on the region, have been documented across sub-Saharan Africa. For the average patient, the distinction might seem academic, but it turns out the two species differ in at least one clinically relevant way: the time it takes for a relapse to appear.
How Relapses Happen
When an infected mosquito bites you, it injects sporozoites that travel to your liver. Most of those parasites develop immediately, bursting out of liver cells and invading red blood cells within a couple of weeks. But some P. ovale sporozoites take a different path. They become hypnozoites: dormant forms that sit quietly inside liver cells for weeks, months, or sometimes years. When a hypnozoite eventually “wakes up,” it completes development, releases parasites into the bloodstream, and causes a new episode of malaria that is clinically indistinguishable from the original infection. This relapse can occur even if the blood-stage infection was successfully treated, because standard antimalarials like chloroquine kill parasites in the blood but cannot reach hypnozoites in the liver.
The timing of relapses differs between the two P. ovale species. A study of British travelers found that P. o. wallikeri had a geometric mean latency of about 41 days before illness appeared, while P. o. curtisi took more than twice as long at roughly 86 days.3BMJ Open. An observational study of malaria in British travellers: Plasmodium ovale wallikeri and Plasmodium ovale curtisi differ significantly in the duration of latency That difference matters for clinicians evaluating a returning traveler: a patient who fell ill six weeks after leaving a malaria zone might be carrying P. o. wallikeri, while someone who presents three months later is more likely to have P. o. curtisi.
How Often Do Relapses Actually Occur?
The relapse rate for P. ovale is lower than for P. vivax, which is the other relapsing species. In a large retrospective study from a non-endemic area, relapses were recorded in about 4% of P. ovale episodes compared to about 9% of P. vivax episodes. Among patients who were not given any relapse-preventing drug, the gap was starker: a third of P. vivax patients relapsed versus 10% of P. ovale patients. P. vivax carried roughly 3.5 times the risk of relapse.4PubMed Central. Relapse of Plasmodium vivax and Plasmodium ovale Malaria With and Without Primaquine Treatment in a Nonendemic Area
When P. ovale relapses did occur without treatment, they appeared at a median of 59 days after the primary infection, though the range was enormous, stretching from 28 days to over five years. In patients who received primaquine (the standard anti-relapse drug), the median time to relapse was longer at around 210 days, suggesting the drug delayed but did not always eliminate every hypnozoite.4PubMed Central. Relapse of Plasmodium vivax and Plasmodium ovale Malaria With and Without Primaquine Treatment in a Nonendemic Area A separate analysis of imported cases in China found that the overwhelming majority of P. ovale relapses, about 86%, happened within a year of the first episode.5PubMed Central. Analysis of the relapse of imported Plasmodium vivax and Plasmodium ovale in five provinces of China
These numbers underscore why clinicians in non-endemic countries need to keep P. ovale on their radar. A patient who traveled to West Africa a year ago and now presents with cyclical fevers could easily be experiencing their first relapse. The long tail of possible relapse timing makes the exposure history critical.
Why Diagnosis Is So Difficult
P. ovale infections are routinely missed by the two most common diagnostic tools in malaria care: microscopy and rapid diagnostic tests (RDTs). The parasite tends to produce low-level infections in the blood, making the parasites hard to find on a blood smear. Under the microscope, P. ovale-infected red blood cells have a few distinguishing features. They tend to be slightly enlarged, show fine stippling called Schüffner’s dots, and, most distinctively, develop a fimbriated (ragged-edged) oval shape that gives the parasite its name.6American Society for Clinical Laboratory Science. Malaria rapid diagnostic test and Giemsa – stained peripheral blood smear discrepancies in the diagnosis of Plasmodium ovale infection in New England But these features are subtle, and less experienced microscopists often confuse P. ovale with P. vivax or miss it entirely.
RDTs are even worse. These finger-prick tests work by detecting parasite proteins in the blood, and most were designed primarily to catch P. falciparum. A study in Japanese travelers found that the overall sensitivity of RDTs for P. ovale malaria was just 22%, compared to 94% for P. vivax.7PubMed Central. Performance of Rapid Diagnostic Tests for Plasmodium ovale Malaria in Japanese Travellers Another study tested three commercially available RDTs on 100 confirmed P. ovale samples and found that the best-performing device caught only 70 of them, while the worst caught a mere 18. All three exhibited very high false-negative rates.8PLoS Neglected Tropical Diseases. Assessment of false negative rates of lactate dehydrogenase-based malaria rapid diagnostic tests for Plasmodium ovale detection The practical consequence is serious: in field settings across Africa where RDTs are the primary diagnostic tool, P. ovale infections slip through undetected at alarming rates.
Molecular methods like PCR are far more reliable. A photo-induced electron transfer PCR assay designed specifically for P. ovale achieved a sensitivity of about 98% and specificity above 99%, with a detection limit of just one parasite per microliter of blood.9PLOS ONE. Molecular diagnosis of Plasmodium ovale by photo-induced electron transfer fluorogenic primers: PET-PCR PCR can also distinguish between P. o. curtisi and P. o. wallikeri, though at least one large cross-sectional study of German travelers concluded that routine species-level discrimination does not change clinical management in practice, since both are treated the same way.10PubMed Central. A comparison of two PCR protocols for the differentiation of Plasmodium ovale species and implications for clinical management in travellers returning to Germany: a 10-year cross-sectional study PCR is not available everywhere, of course, which means P. ovale remains chronically underdiagnosed in the regions where it is most common.
Where P. Ovale Circulates
P. ovale is found predominantly in sub-Saharan Africa, with scattered reports from Southeast Asia and the western Pacific. A systematic review covering two decades of data estimated a global pooled prevalence of P. ovale at about 0.8%, with the highest rates in the African region.11PubMed Central. Global trend of Plasmodium malariae and Plasmodium ovale spp. malaria infections in the last two decades (2000-2020): a systematic review and meta-analysis A focused survey in the Democratic Republic of the Congo tested over 18,000 adults and found a P. ovale prevalence of about 0.8%, broadly distributed across the country with a cluster of higher infections in the south-central region.12PubMed Central. Under the Radar: Epidemiology of Plasmodium ovale in the Democratic Republic of the Congo
These numbers almost certainly undercount the true burden. As the diagnostic section above makes clear, most surveillance tools miss P. ovale. When molecular methods are applied, the prevalence jumps. In southwestern Nigeria, PCR-based screening of asymptomatic schoolchildren found that 24% carried P. ovale, nearly always as part of a mixed infection with P. falciparum. By contrast, microscopy in the same population detected malaria in only 9% of children.13PubMed. High prevalence of Plasmodium malariae and Plasmodium ovale in co-infections with Plasmodium falciparum in asymptomatic malaria parasite carriers in southwestern Nigeria P. ovale, in other words, is more of a “background parasite” than the case counts suggest, quietly persisting in communities dominated by falciparum malaria.
Treatment and the G6PD Problem
Treating the acute blood-stage infection from P. ovale is straightforward. Chloroquine remains effective, as P. ovale has not developed the widespread drug resistance seen in P. falciparum. The challenge lies in the radical cure: eliminating the hypnozoites in the liver so the patient does not relapse. For that, only drugs in the 8-aminoquinoline class work. Primaquine has been the standard option for decades, given as a 14-day course alongside or after the blood-stage treatment.14PubMed Central. Use of primaquine and glucose-6-phosphate dehydrogenase deficiency testing: Divergent policies and practices in malaria endemic countries
In 2018, the FDA approved tafenoquine, a newer 8-aminoquinoline that can be given as a single dose instead of a two-week course. It was approved both for the prevention of all malaria species and for the radical cure of relapsing malaria caused by P. vivax and P. ovale.15Journal of Travel Medicine. Tafenoquine for travelers’ malaria: evidence, rationale and recommendations A single-dose cure is a major practical improvement, since many patients struggle to complete a 14-day primaquine course, especially after they start feeling better.
Both drugs share a dangerous side effect: they can trigger severe hemolysis (destruction of red blood cells) in people with glucose-6-phosphate dehydrogenase (G6PD) deficiency, the most common enzyme deficiency in humans. G6PD deficiency is particularly prevalent in the same tropical and subtropical populations where P. ovale circulates, which creates a painful clinical dilemma. Primaquine is the main tool for preventing relapses, but G6PD screening is usually unavailable at the point of care in endemic settings. The result is that radical cure is greatly underused, and there is still no global consensus on whether G6PD testing should be mandatory before prescribing it.14PubMed Central. Use of primaquine and glucose-6-phosphate dehydrogenase deficiency testing: Divergent policies and practices in malaria endemic countries Some countries require testing, others recommend it, and still others leave the decision to individual clinicians. That inconsistency means many P. ovale patients walk away without a drug that could prevent their next episode.
Evidence from a trial in Papua New Guinean children showed just how effective hypnozoite-clearing therapy can be. Primaquine treatment reduced the risk of recurrent P. vivax blood-stage infections by over 80% compared to placebo.16PLoS Medicine. Strategies for Understanding and Reducing the Plasmodium vivax and Plasmodium ovale Hypnozoite Reservoir in Papua New Guinean Children: A Randomised Placebo-Controlled Trial and Mathematical Model Though that trial focused on P. vivax, the same pharmacological principle applies to P. ovale hypnozoites. Getting radical cure to the people who need it remains one of the bigger gaps in malaria control.
Clinical Severity and When Things Get Serious
P. ovale has a reputation as the mildest of the human malaria parasites, and for most patients that reputation holds. A systematic review and meta-analysis of over 1,300 P. ovale cases found that severe disease occurred in only about 3–5% of infections. The most common serious complications were jaundice, severe anemia, and lung problems. The mortality rate was about 0.15%, based on two deaths among the 1,365 cases reviewed.17PubMed Central. Severity and mortality of severe Plasmodium ovale infection: A systematic review and meta-analysis Compared to P. falciparum, P. ovale patients were significantly less likely to develop severe infections.
That low severity profile can lull clinicians into complacency. Although rare, severe P. ovale cases do happen, and they tend to occur in patients who are already vulnerable: those with other chronic illnesses, pregnant women, and people who are immunosuppressed. A delayed diagnosis, which is common given the diagnostic difficulties discussed earlier, can also allow the infection to worsen. The message from the literature is not that P. ovale is harmless but that its dangers are concentrated in specific populations and specific clinical scenarios, particularly delayed recognition.
The Hidden World of Coinfections
In the real world, P. ovale rarely acts alone. In endemic areas, it typically coexists with P. falciparum and often with P. malariae as well. The Nigerian school study found that most asymptomatic malaria infections involved two or more species: the most common combination was P. falciparum plus P. malariae (35%), followed by all three species together (21%), and P. falciparum plus P. ovale (6%). Notably, while multispecies infections dominated among asymptomatic carriers, symptomatic patients at a nearby hospital overwhelmingly had P. falciparum alone (91%).13PubMed. High prevalence of Plasmodium malariae and Plasmodium ovale in co-infections with Plasmodium falciparum in asymptomatic malaria parasite carriers in southwestern Nigeria That pattern suggests that mixed infections may be associated with a lower likelihood of clinical illness, though the mechanism is not fully understood.
Research into coinfections involving the two P. ovale species and P. falciparum found that the clinical picture in mixed infections was broadly similar to P. falciparum alone, with some subtle laboratory differences. P. o. wallikeri coinfections showed higher white blood cell counts and lower parasitemia indices, while P. o. curtisi coinfections were associated with more frequent low platelet counts. The clinical significance of these differences remains uncertain, but the study did note that P. o. wallikeri was more common than P. o. curtisi in mixed falciparum infections, hinting at some kind of negative interaction between P. falciparum and P. o. curtisi.18PubMed. Comparison of routine clinical profiles of patients with imported P. falciparum malaria and co-infection with Plasmodium ovale wallikeri or Plasmodium ovale curtisi
Mosquito Transmission and Cotransmission of Both Species
P. ovale is transmitted by Anopheles mosquitoes, and quite a range of them. At least ten different Anopheles species have been shown to be susceptible to P. ovale infection in laboratory or field settings, including major African vectors like A. gambiae and Asian species like A. stephensi and A. dirus.19PubMed Central. Plasmodium ovale: parasite and disease This broad vector range helps explain why the parasite persists across such a wide geographic area even though it produces low-level blood infections that might seem unlikely to infect feeding mosquitoes.
A feeding study in Tanzania confirmed that P. ovale can be transmitted from asymptomatic carriers to mosquitoes. Six of eight P. ovale-infected asymptomatic individuals successfully transmitted the parasite to A. gambiae mosquitoes in direct feeding assays.20PubMed Central. Seasonality and transmissibility of Plasmodium ovale in Bagamoyo District, Tanzania Even more striking, when researchers used PCR to examine the mosquito midguts from these experiments, mixed P. o. curtisi and P. o. wallikeri infections were found in 79% of oocyst-positive mosquitoes. Eight of nine human carriers transmitted both species to mosquitoes, even though both species could only be detected in the blood of two of those carriers.21PubMed Central. Real-time PCR detection of mixed Plasmodium ovale curtisi and wallikeri infections in human and mosquito hosts The implication is that coinfection with both P. ovale species is far more common than blood tests alone would suggest, and that a single mosquito bite can deliver both.
P. Ovale and Returning Travelers
For clinicians in non-endemic countries, P. ovale is most often encountered in travelers and immigrants returning from sub-Saharan Africa. It poses a unique challenge in this population because of the latency period. Travelers who took antimalarial prophylaxis during their trip may still present with P. ovale weeks or months after returning home, because most prophylactic drugs kill blood-stage parasites but leave hypnozoites untouched. The British traveler study found that P. ovale was disproportionately represented among malaria cases in patients who reported using chemoprophylaxis, with odds more than seven times higher than for P. falciparum.3BMJ Open. An observational study of malaria in British travellers: Plasmodium ovale wallikeri and Plasmodium ovale curtisi differ significantly in the duration of latency
This makes P. ovale easy to miss in the travel medicine context. A patient who took their prophylaxis faithfully and returns well may assume they are in the clear. Their doctor, if unfamiliar with the relapsing species, may not think to ask about travel when the patient shows up with fevers three months later. And as the RDT data shows, even if malaria is suspected and a rapid test is performed, P. ovale is likely to be missed. The index of suspicion needs to remain high for at least a year after travel to an endemic area, and ideally longer given the occasional reports of relapses occurring years after exposure.
An Evolutionary Connection to Chimpanzees
The evolutionary origins of P. ovale trace back to a shared ancestor with parasites found in African great apes. Researchers analyzing Plasmodium strains from chimpanzees in Cameroon found two parasites closely related to human P. ovale. Phylogenetic analysis showed that these chimpanzee parasites formed a well-supported monophyletic group with both the curtisi and wallikeri types of human P. ovale, meaning all four descended from a common ancestor.22PLoS ONE. Chimpanzee Malaria Parasites Related to Plasmodium ovale in Africa The split between the two human P. ovale species appears to be ancient, predating the separation from the chimpanzee lineage in some analyses. This deep divergence is another piece of evidence supporting the classification of curtisi and wallikeri as genuinely separate species rather than mere variants of one parasite. It also raises the theoretical possibility of occasional cross-species transmission between apes and humans, though there is no evidence this occurs with any meaningful frequency today.