Rapid-Antigen Test Negative Malaria in a Traveler Returning From Thailand, Molecularly Diagnosed as Plasmodium knowlesi

Plasmodium knowlesi (Pk) Malaria: A Review & Proposal of Therapeutically Rational Exchange (T-REX) of Pk-Resistant Red Blood Cells

Plasmodium knowlesi (Pk) causes zoonotic malaria and is known as the "fifth human malaria parasite". Pk malaria is an emerging threat because infections are increasing and can be fatal. While most infections are in Southeast Asia (SEA), especially Malaysia, travelers frequently visit this region and can present with Pk malaria around the world. So, clinicians need to know (1) patients who present with fever after recent travel to SEA might be infected with Pk and (2) Pk is often misdiagnosed as P. malariae (which typically causes less severe malaria). Here we review the history, pathophysiology, clinical features, diagnosis, and treatment of Pk malaria. Severe disease is most common in adults. Signs and symptoms can include fever, abdominal pain, jaundice, acute kidney injury, acute respiratory distress syndrome, hyponatremia, hyperparasitemia, and thrombocytopenia. Dengue is one of the diseases to be considered in the differential. Regarding pathophysiologic mechanisms, when Pk parasites invade mature red blood cells (RBCs, i.e., normocytes) and reticulocytes, changes in the red blood cell (RBC) surface can result in life-threatening cytoadherence, sequestration, and reduced RBC deformability. Since molecular mechanisms involving the erythrocytic stage are responsible for onset of severe disease and lethal outcomes, it is biologically plausible that manual exchange transfusion (ET) or automated RBC exchange (RBCX) could be highly beneficial by replacing "sticky" parasitized RBCs with uninfected, deformable, healthy donor RBCs. Here we suggest use of special Pk-resistant donor RBCs to optimize adjunctive manual ET/RBCX for malaria. "Therapeutically-rational exchange transfusion" (T-REX) is proposed in which Pk-resistant RBCs are transfused (instead of disease-promoting RBCs). Because expression of the Duffy antigen on the surface of human RBCs is essential for parasite invasion, T-REX of Duffy-negative RBCs-also known as Fy(a-b-) RBCs-could replace the majority of the patient's circulating normocytes with Pk invasion-resistant RBCs (in a single procedure lasting about 2 h). When sequestered or non-sequestered iRBCs rupture-in a 24 h Pk asexual life cycle-the released merozoites cannot invade Fy(a-b-) RBCs. When Fy(a-b-) RBC units are scarce (e.g., in Malaysia), clinicians can consider the risks and benefits of transfusing plausibly Pk-resistant RBCs, such as glucose-6-phosphate dehydrogenase deficient (G6PDd) RBCs and Southeast Asian ovalocytes (SAO). Patients typically require a very short recovery time (<1 h) after the procedure. Fy(a-b-) RBCs should have a normal lifespan, while SAO and G6PDd RBCs may have mildly reduced half-lives. Because SAO and G6PDd RBCs come from screened blood donors who are healthy and not anemic, these RBCs have a low-risk for hemolysis and do not need to be removed after the patient recovers from malaria. T-REX could be especially useful if (1) antimalarial medications are not readily available, (2) patients are likely to progress to severe disease, or (3) drug-resistant strains emerge. In conclusion, T-REX is a proposed optimization of manual ET/RBCX that has not yet been utilized but can be considered by physicians to treat Pk malaria patients.

Epidemiology of the zoonotic malaria Plasmodium knowlesi in changing landscapes

Within the past two decades, incidence of human cases of the zoonotic malaria Plasmodium knowlesi has increased markedly. P. knowlesi is now the most common cause of human malaria in Malaysia and threatens to undermine malaria control programmes across Southeast Asia. The emergence of zoonotic malaria corresponds to a period of rapid deforestation within this region. These environmental changes impact the distribution and behaviour of the simian hosts, mosquito vector species and human populations, creating new opportunities for P. knowlesi transmission. Here, we review how landscape changes can drive zoonotic disease emergence, examine the extent and causes of these changes across Southeast and identify how these mechanisms may be impacting P. knowlesi dynamics. We review the current spatial epidemiology of reported P. knowlesi infections in people and assess how these demographic and environmental changes may lead to changes in transmission patterns. Finally, we identify opportunities to improve P. knowlesi surveillance and develop targeted ecological interventions within these landscapes.

Clinical management of Plasmodium knowlesi malaria

The zoonotic parasite Plasmodium knowlesi has emerged as an important cause of human malaria in parts of Southeast Asia. The parasite is indistinguishable by microscopy from the more benign P. malariae, but can result in high parasitaemias with multiorgan failure, and deaths have been reported. Recognition of severe knowlesi malaria, and prompt initiation of effective therapy is therefore essential to prevent adverse outcomes. Here we review all studies reporting treatment of uncomplicated and severe knowlesi malaria. We report that although chloroquine is effective for the treatment of uncomplicated knowlesi malaria, artemisinin combination treatment is associated with faster parasite clearance times and lower rates of anaemia during follow-up, and should be considered the treatment of choice, particularly given the risk of administering chloroquine to drug-resistant P. vivax or P. falciparum misdiagnosed as P. knowlesi malaria in co-endemic areas. For severe knowlesi malaria, intravenous artesunate has been shown to be highly effective and associated with reduced case-fatality rates, and should be commenced without delay. Regular paracetamol may also be considered for patients with severe knowlesi malaria or for those with acute kidney injury, to attenuate the renal damage resulting from haemolysis-induced lipid peroxidation.

Quantification of the misidentification of Plasmodium knowlesi as Plasmodium malariae by microscopy: an analysis of 1569 P. knowlesi cases

Background

Plasmodium knowlesi is recognized as the fifth Plasmodium species causing malaria in humans. It is morphologically similar to the human malaria parasite Plasmodium malariae, so molecular detection should be used to clearly discriminate between these Plasmodium species. This study aimed to quantify the rate at which P. knowlesi is misidentified as P. malariae by microscopy in endemic and non-endemic areas.

Methods

The protocol of this systematic review was registered in the PROSPERO International Prospective Register of Systematic Reviews (ID = CRD42020204770). Studies reporting the misidentification of P. knowlesi as P. malariae by microscopy and confirmation of this by molecular methods in MEDLINE, Web of Science and Scopus were reviewed. The risk of bias in the included studies was assessed using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS). The pooled prevalence and 95% confidence interval (CI) of the misidentification of P. knowlesi as P. malariae by microscopy were estimated using a random effects model. Subgroup analysis of the study sites was performed to demonstrate any differences in the misidentification rates in different areas. Heterogeneity across the included studies was assessed and quantified using Cochran’s Q and I² statistics, respectively. Publication bias in the included studies was assessed using the funnel plot, Egger’s test and contour-enhanced funnel plot.

Results

Among 375 reviewed studies, 11 studies with a total of 1569 confirmed P. knowlesi cases in humans were included. Overall, the pooled prevalence of the misidentification of P. knowlesi as P. malariae by microscopy was estimated at 57% (95% CI 37–77%, I²: 99.3%). Subgroup analysis demonstrated the highest rate of misidentification in Sawarak, Malaysia (87%, 95% CI 83–90%, I²: 95%), followed by Sabah, Malaysia (85%, 95% CI 79–92%, I²: 85.1%), Indonesia (16%, 95% CI 6–38%), and then Thailand (4%, 95% CI 2–9%, I²: 95%).

Conclusion

Although the World Health Organization (WHO) recommends that all P. malariae-positive diagnoses made by microscopy in P. knowlesi endemic areas be reported as P. malariae/P. knowlesi malaria, the possibility of microscopists misidentifying P. knowlesi as P. malariae is a diagnostic challenge. The use of molecular techniques in cases with malariae-like Plasmodium with high parasite density as determined by microscopy could help identify human P. knowlesi cases in non-endemic countries.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03714-1.

Retrospective clinical case series study in 2017 identifies Plasmodium knowlesi as most frequent Plasmodium species in returning travellers from Thailand to Germany

Febrile illnesses are common in travellers returning from south-east Asia. However, malaria is a rare diagnosis in this population. A series of Plasmodium knowlesi infections was noted in German travellers returning from Thailand since 2012. Infectious disease and tropical medicine facilities registered by the German Society for Tropical Medicine and International Health were contacted in March 2017, and asked to report previous P. knowlesi cases. In addition, surveillance data from the Robert Koch-Institute were analysed. The facilities reported a total of six P. knowlesi-positive cases, all were returning travellers from Thailand. The P. knowlesi-positive cases made up 6/9 of all diagnosed malaria cases imported from Thailand in the time period 2012 to 2017. In 4/5 of cases where a malaria rapid diagnostic test had been applied it revealed a negative result. P. knowlesi is an important differential diagnosis in travellers returning from south-east Asia with itineraries that include Thailand. This study highlights the importance of this Plasmodium species in this patient subgroup. Whenever malaria is suspected in a returning traveller from Thailand, P. knowlesi should be taken into consideration and a differential PCR be executed as currently the unequivocal diagnosis of P. knowlesi is based on nuclear amplification techniques.

Characteristics and Severity of Disease among 100 Cases of Imported Malaria Seen at a U.S. University Hospital, 2000-2017

Malaria acquired in endemic areas poses a substantial risk to travelers arriving in or returning to the United States. Timely diagnosis and recognition of severe illness are crucial; however, many U.S.-based clinicians lack familiarity with this disease. We conducted a retrospective review of 100 cases of malaria in adults seen at a single urban university hospital during 2000-2017. Descriptive and analytical statistics were calculated, including logistic regression modeling case severity. Most of the patients presented with Plasmodium falciparum (76%), most commonly after travel from sub-Saharan Africa (94%). Prior malaria experience was common (50%), but adherence to a prophylactic regimen was exceedingly rare (4%). Twenty-one patients had severe malaria, including 10 with cerebral malaria. Severity was predicted by high parasitemia, bandemia, hypoglycemia, and hypotension at the time of presentation. In 24 patients, the initial treatment regimen was changed, usually because of the appearance of clinical deterioration or drug toxicity. One patient required intravenous artesunate. All patients survived, although one suffered fetal loss. Among 30 patients initially evaluated at other institutions, 43% had been treated for an alternative diagnosis. The most common reasons for transfer of patients to our hospital were inadequate facilities and lack of expertise with malaria. There needs to be increased awareness among U.S.-based travelers and clinicians regarding malaria as a potentially lethal condition, emphasizing the use of appropriate prophylaxis. Our simple model of disease severity could serve frontline physicians when deciding which patients should be admitted to the intensive care unit or transferred for higher level care.

Evaluation of the multiplex real-time PCR assays RealStar malaria S&T PCR kit 1.0 and FTD malaria differentiation for the differentiation of Plasmodium species in clinical samples

BACKGROUND

Two commercial PCR assays were assessed in a retrospective study to determine their reliability as tools for the differentiation of Plasmodium species in human blood.

METHODS

A total of 1022 blood samples from 817 patients with suspected or confirmed malaria submitted to the German National Reference Centre for Tropical Pathogens were subjected to malaria microscopy using thick and thin blood films as well as to a genus-specific malaria real-time PCR. Parasite-positive samples were analysed by RealStar Malaria S&T PCR Kit 1.0 (altona Diagnostics) and FTD Malaria Differentiation (Fast Track Diagnostics) multiplex real-time PCR assays targeting species-specific Plasmodium DNA.

RESULTS

Out of the 1022 blood samples, 247 (24.2%) tested positive for Plasmodium spp. The two multiplex assays showed rather similar performance characteristics and provided concordant species information in 98.9% of samples positive by malaria microscopy and in 95.1% (RealStar) and 96.8% (FTD) of samples positive by genus-specific PCR. Compared to FTD, RealStar revealed slightly reduced sensitivity for submicroscopic, low-level P. falciparum infections, while FTD was unable to detect P. knowlesi.

CONCLUSIONS

The two commercial malaria PCR assays assessed are suitable for discriminating Plasmodium species in clinical samples, and can provide additional information in cases of microscopically uncertain findings.

Plasmodium knowlesi infection in a returning German traveller from Thailand: a case report on an emerging malaria pathogen in a popular low-risk travel destination

BACKGROUND

Thailand is a major destination for German travellers with more than 760,000 arrivals in 2015. At the same time, malaria is a concern in travel recommendations with regard to this destination. The World Malaria Report of 2016 mentions only P. falciparum and P. vivax as prevalent species for Thailand, however, P. knowlesi infections have been occasionally reported in Thailand. In German travellers, only five cases of P. knowlesi infections have been reported to date.

CASE PRESENTATION

A 45-year-old German male tourist travelled to Thailand from 25 December 2016 to 13 January 2017. On 14 January he developed fever with no other symptoms, and presented on 17 January at the Division for Tropical Medicine and Infectious Diseases in Munich, Germany. Malaria was diagnosed, primarily based on a single parasite in the thin smear microscopy, while commercial rapid diagnostic testing remained negative. Only the result of a differential PCR assay revealed P. knowlesi infection.

CONCLUSIONS

P. knowlesi has to be considered in travellers returning from Thailand. Cases may present with an extremely low parasitaemia. This is in contrast to the assumption that P. knowlesi was likely to cause high parasitaemia due to its short replication cycle.

Clinical features and management of Plasmodium knowlesi infections in humans

Plasmodium knowlesi is a simian malaria of primarily the macaque species of South East Asia. While it was known that human infections could be induced during the years of malariotherapy, naturally occurring P. knowlesi human infections were thought to be rare. However, in 2004, knowlesi infections became recognized as an important infection amongst human populations in Sarawak, Malaysian Borneo. Since then, it has become recognized as a disease affecting people living and visiting endemic areas across South East Asia. Over the last 12 years, clinical studies have improved our understanding of this potentially fatal disease. In this review article the current literature is reviewed to give a comprehensive description of the disease and treatment.

The Treatment of Plasmodium knowlesi Malaria

Plasmodium knowlesi occurs across Southeast Asia and is the most common cause of malaria in Malaysia. High parasitaemias can develop rapidly, and the risk of severe disease in adults is at least as high as in falciparum malaria. Prompt initiation of effective treatment is therefore essential. Intravenous artesunate is highly effective in severe knowlesi malaria and in those with moderately high parasitaemia but otherwise uncomplicated disease. Both chloroquine and artemisinin-combination therapy (ACT) are highly effective for uncomplicated knowlesi malaria, with faster parasite clearance times and lower anaemia rates with ACT. Given the difficulties with microscope diagnosis of P. knowlesi, a unified treatment strategy of ACT for all Plasmodium species is recommended in coendemic regions.