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. Author manuscript; available in PMC: 2022 Jul 20.
Published in final edited form as: Adv Parasitol. 2021 Sep 1;113:45–76. doi: 10.1016/bs.apar.2021.08.004

Clinical management of Plasmodium knowlesi malaria

Bridget E Barber a,b,*, Matthew J Grigg b, Daniel J Cooper b,c, Donelly A van Schalkwyk d, Timothy William e,f, Giri S Rajahram f,g, Nicholas M Anstey b
PMCID: PMC9299581  NIHMSID: NIHMS1821601  PMID: 34620385

Abstract

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.

1. Introduction

The zoonotic parasite Plasmodium knowlesi is endemic throughout Southeast Asia, with human cases reported in all countries where both the Anopheles leucosphyrus mosquito vector and the simian hosts reside. In Malaysia P. knowlesi accounts for nearly all cases of human malaria, with over 3000 cases reported in 2019 (World Health Organization, 2020). The 24h asexual replication cycle of P. knowlesi is the shortest of any human malaria, and high parasitaemias can develop rapidly. In adults the rate of severe disease from P. knowlesi is at least as high as that of falciparum malaria (Barber et al., 2013a), and fatal cases have been reported (Rajahram et al., 2019). Prompt initiation of effective treatment is therefore essential to avoid poor outcomes. This review discusses the treatment of uncomplicated and severe knowlesi malaria, highlighting the benefits of artemisinin-combination therapy (ACT) for uncomplicated knowlesi malaria, and the importance of prompt initiation of intravenous artesunate for all severe malaria regardless of species. The role of adjunctive paracetamol in severe knowlesi malaria is also discussed.

2. Diagnosis of Plasmodium knowlesi

The diagnosis of P. knowlesi is limited by the difficulties with distinguishing the parasite from other Plasmodium species by microscopy. Ring forms of P. knowlesi may resemble those of P. falciparum, while P. knowlesi trophozoites and schizonts are nearly indistinguishable from P. malariae (Lee et al., 2009). Misidentification of P. knowlesi as P. vivax, and vice-versa, is also common in co-endemic areas (Barber et al., 2013c; Coutrier et al., 2018). Misdiagnosis of P. knowlesi as the more benign P. malariae has been associated with failure to recognise severe disease, with subsequent delay in administering parenteral therapy (Rajahram et al., 2012). For this reason, in areas where P. knowlesi is prevalent, parasites resembling P. malariae should be reported as P. knowlesi. Rapid diagnostic tests evaluated to date are insensitive for P. knowlesi, particularly at low parasitaemias, and are not able to distinguish P. knowlesi from other non-falciparum species (Barber et al., 2013b; Grigg et al., 2014). Given these limitations of microscopy and rapid diagnostic tests, species confirmation using molecular methods such as PCR should be performed where possible (Grigg et al., 2021). In addition to facilitating accurate reporting and surveillance, PCR confirmation of species enables any patient with P. vivax misdiagnosed with P. knowlesi to receive appropriate anti-hypnozoital treatment with primaquine for prevention of relapses.

3. In vitro susceptibility of P. knowlesi to antimalarial agents

The recent adaption of P. knowlesi to long-term in vitro culture (Grüring et al., 2014; Lim et al., 2013; Moon et al., 2013) has enabled the detailed evaluation of the susceptibility of this species to both approved and investigational antimalarial agents. Importantly, P. knowlesi is closely related phylogenetically to P. vivax and other non-falciparum species (Loy et al., 2017), and thus susceptibility of P. knowlesi to antimalarial agents may be indicative of the drug susceptibility of other non-falciparum Plasmodium species. This is particularly important given that the development of novel antimalarial agents generally involves extensive testing only on P. falciparum. Demonstrating susceptibility of P. knowlesi to antimalarial agents will support the further development of these agents and their use in areas endemic for non-falciparum species.

The susceptibilities of culture-adapted A1–H1 P. knowlesi (isolated in the 1960s) to a range of approved and investigational antimalarial agents have been recently evaluated in a series of studies by van Schalkwyk et al., with the authors comparing drug susceptibilities to those of P. falciparum 3D7 using identical assay conditions (van Schalkwyk et al., 2017, 2019, 2020). The EC50 values reported in these and other studies (Arnold et al., 2016; Burns et al., 2020; Fatih et al., 2012) are listed in Table 1. The large majority of antimalarial agents evaluated by van Schalkwyk et al. were highly potent against P. knowlesi, with EC50 values similar to those of P. falciparum 3D7, including chloroquine (EC50 < 30 nM), quinine (EC50 < 55 nM), amodiaquine (EC50< 10 nM), the artemisinin derivatives (EC50 values < 11 nM), and the synthetic endoperoxides arterolane (OZ277) and artefenomel (OZ439) (EC50 values both < 5 nM) (van Schalkwyk et al., 2019). Compared to P. falciparum, P. knowlesi was significantly more susceptible to several artemisinin partner drugs, including mefloquine (EC50 < 11 nM), lumefantrine (EC50 < 91 nM), and piperaquine (EC50 < 22 nM) (van Schalkwyk et al., 2017). This is in contrast to an earlier study which assessed P. knowlesi growth using a modified schizont maturation assay and reported reduced susceptibility of P. knowlesi to mefloquine (Fatih et al., 2013). However, this earlier study included only six clinical isolates, and did not assess stage specificity of the drugs, which is an important confounder of susceptibility of non-falciparum Plasmodium species (Kerlin et al., 2012; Russell et al., 2008).

Table 1.

In vitro susceptibility of P. knowlesi and P. falciparum to approved and investigational antimalarial agents

Compound EC50 values (nM) Fold difference (Pk/Pf) P value Refs.
P. knowlesi P. falciparum 3D7
Artemisinins and synthetic endoperoxides
Dihydroartemisinin 2.35±0.23 5.16±0.62 0.46 0.0017 van Schalkwyk et al. (2019)
2.0±0.3 4.2±0.5 0.48 0.0098 van Schalkwyk et al. (2017)
2.1 (0.4)a van Schalkwyk et al. (2019)
1.6±0.92b Fatih et al. (2012)
0.79 (0.62–1.0)c Fatih et al. (2012)
1.52±0.07 3.64±0.42 0.42 0.0112 Van Schalkwyk et al. (2020)
2.4±1d 0.8±0.1d 3.0 ND Burns et al. (2020)
Artemisinin 7.39±1.87 11.00±1.32 0.67 0.1667 van Schalkwyk et al. (2019)
2.1±0.99b Fatih et al. (2012)
0.80 (0.35–1.9)c Fatih et al. (2012)
Artemisone 0.47±0.14 0.72±0.15 0.65 0.2701 van Schalkwyk et al. (2019)
Artesunate 10.30±1.9 8.28±1.05 1.24 0.3552 van Schalkwyk et al. (2019)
10.9±1.7 9.0±1.5 1.20 0.4280 van Schalkwyk et al. (2017)
13.8 (3.6)a van Schalkwyk et al. (2019)
0.90±0.12b Fatih et al. (2012)
2.0 (0.93–4.2)c Fatih et al. (2012)
Artemether 4.56±0.55 6.93±0.73 0.66 0.0413 van Schalkwyk et al. (2019)
0.90±0.19b Fatih et al. (2012)
0.84 (0.34–2.1)c Fatih et al. (2012)
Arterolane (OZ277) 2.27±0.42 4.03±0.59 0.56 0.0407 van Schalkwyk et al. (2019)
Artefenomel (OZ439) 4.76±0.40 4.82±0.62 0.99 0.9350 van Schalkwyk et al. (2019)
6.6±1.4 7.4±1.2 0.89 0.6750 van Schalkwyk et al. (2017)
4.4 (1.6)a van Schalkwyk et al. (2019)
Deoxyartemisinin >10,000 >10,000 ND ND van Schalkwyk et al. (2019)
Aminoquinolines and amino-alcohols
Chloroquine 29.3±4.7 15.9±3.0 1.85 0.0303 van Schalkwyk et al. (2017)
23±4.8b Fatih et al. (2012)
3.2 (2.2–4.7)c Fatih et al. (2012)
10.9±3.1e Arnold et al. (2016)
33.1±2.0 17.7±1.3 1.87 <0.0001 Van Schalkwyk et al. (2020)
21.5 (4.9)a van Schalkwyk et al. (2019)
17±5d 52±6d 0.34 ND Burns et al. (2020)
38±8f 53±7f Gilson et al. (2019)
Amodiaquine 9.3±1.7 5.9±0.6 1.59 0.0662 van Schalkwyk et al. (2017)
Desethylamodiaquine 12.4±1.4 12.4±3.1 1.00 0.9973 van Schalkwyk et al. (2017)
Quinine 54.8±3.0 57.9±6.9 0.94 0.7177 van Schalkwyk et al. (2017)
40.3 0.4)a van Schalkwyk et al. (2019)
Quinidine 38.4±10.5 52.9±10.7 0.73 0.3704 van Schalkwyk et al. (2019)
AQ 13 11.4±2.82 5.1±1.15 2.23 0.1091 van Schalkwyk et al. (2019)
Mefloquine 10.9±1.1 26.2±4.2 0.42 0.0090 van Schalkwyk et al. (2017)
26±3.1b Fatih et al. (2012)
25 (7.4–81)c Fatih et al. (2012)
13.1 2.5)a van Schalkwyk et al. (2019)
3±1f 18±4f Gilson et al. (2019)
Primaquine 3871±887 5627±1195 0.69 0.2847 van Schalkwyk et al. (2017)
Bisquinoline 2.2±1.4 2.3±0.94 0.97 0.9715 van Schalkwyk et al. (2019)
Lumefantrine 90.4±13 152±26 0.60 0.0424 van Schalkwyk et al. (2017)
Piperaquine 21.0±3.1 39.8±4.9 0.53 0.0115 van Schalkwyk et al. (2017)
Pyronaridine 10.7±1.6 4.4±1.6 2.44 0.0268 van Schalkwyk et al. (2017)
4.9 (0.84) van Schalkwyk et al. (2019)
Ferroquine 12.2±1.6 4.7±0.6 2.60 0.0068 van Schalkwyk et al. (2017)
9.8 (2.32)a van Schalkwyk et al. (2019)
Halofantrine 0.92±0.25 3.60±0.40 0.31 0.0174 van Schalkwyk et al. (2019)
Naphthoquine 117±83 111±23 1.05 0.5544 van Schalkwyk et al. (2019)
New Isoquine 15.9±2.4 11.4±2.38 1.39 0.2474 van Schalkwyk et al. (2019)
DHFR inhibitors
Pyrimethamine 5.1±0.8 54.0±5.0 0.09 <0.0001 van Schalkwyk et al. (2017)
3.2 (0.5)a van Schalkwyk et al. (2019)
Cycloguanil 1.3±0.3 11.8±0.6 0.11 <0.0001 van Schalkwyk et al. (2017)
3.9±0.7e Arnold et al. (2016)
0.7 (0.1)a van Schalkwyk et al. (2019)
Trimethoprim 265±47 3098±229 0.09 <0.0001 van Schalkwyk et al. (2017)
137 (54)a van Schalkwyk et al. (2019)
P218 4.1±0.7 3.5±0.2 1.18 0.4884 van Schalkwyk et al. (2017)
0.68 (0.04)a van Schalkwyk et al. (2019)
Quinolones
Atovaquone 2.6±0.4 2.3±0.5 1.13 0.6366 van Schalkwyk et al. (2017)
0.71±0.02g 0.74±0.09 0.99 0.1211 Van Schalkwyk et al. (2020)
4.1 (0.04)a van Schalkwyk et al. (2019)
Endochin 18.9±1.2 18.1±0.5 1.04 0.6233 van Schalkwyk et al. (2020)
ELQ-300 15.4±0.9 23.1 ±1.2 0.67 0.0215 van Schalkwyk et al. (2020)
ATP4 inhibitors
Cipargamin (KAE609) 6.1±0.5 0.89±0.08 6.83 <0.0001 van Schalkwyk et al. (2019)
3.8 (1.6)a van Schalkwyk et al. (2019)
SJ733 386±34 64.3±4.3 6.00 <0.0001 van Schalkwyk et al. (2019)
PA21A092 63.8±7.6 10.2±1.4 6.25 0.0002 van Schalkwyk et al. (2019)
35.7 (2.8)a van Schalkwyk et al. (2019)
Others
Clindamycin >10,000 Arnold et al. (2016)
>10,000 >10,000 ND ND van Schalkwyk et al. (2017)
Doxycycline >10,000 >10,000 ND ND van Schalkwyk et al. (2017)
Azithromycinh 5662±725 6003±323 0.94 ND van Schalkwyk et al. (2017)
16,000±1800 11,310±490 1.41 ND Burns et al. (2020)
Proguanil 2461±236g 228±29 10.79 0.0007 Van Schalkwyk et al. (2020)
AN13762 2762±296 41.3±3.8 66.88 <0.0001 van Schalkwyk et al. (2019)
3618 (110)a van Schalkwyk et al. (2019)
Pentamidine 331±20 99±4 3.34 0.0003 van Schalkwyk et al. (2019)
Cladosporin 411±134 133±10 3.09 0.0831 van Schalkwyk et al. (2019)
Ganaplacide (KAF156) 1.7±0.16 7.9±0.12 0.22 <0.0001 van Schalkwyk et al. (2019)
Methylene Blue 5.38±1.8 3.31±0.7 1.63 0.3543 van Schalkwyk et al. (2019)
NMT MMV884705 46.2±16 206±44 0.22 0.0266 van Schalkwyk et al. (2019)
MMV253 13.0±1.9 7.4±1.5 1.76 0.0810 van Schalkwyk et al. (2019)
MMV048 17.2±0.9 29.5±2.5 0.58 0.0103 van Schalkwyk et al. (2019)
Cyclohexamide 117±40 188±35 0.62 0.2544 van Schalkwyk et al. (2019)
Benzylquine 31.53±10.88 22.05±2.02 1.43 0.3469 van Schalkwyk et al. (2019)
WR194965 282±111 539±236 0.52 0.3633 van Schalkwyk et al. (2019)
BIX-01294 22.6±4.4 21.2±4.1 1.07 0.8207 van Schalkwyk et al. (2019)
Sitamaquine 112±25 72±15 1.56 0.2350 van Schalkwyk et al. (2019)
Methotrexate 1991±125 693±48 2.87 0.0006 van Schalkwyk et al. (2019)
MK-4815 47.6±6 127±16 0.37 0.0100 van Schalkwyk et al. (2019)
MMV688558 17.5±2.0 42.4±1.6 0.41 0.0006 van Schalkwyk et al. (2019)
DSM1 509±11 149±5 3.4 ≤0.0018 van Schalkwyk et al. (2017)
DSM265 303±15 37±3 8.2 ≤0.0018 van Schalkwyk et al. (2017)
170 (66)a van Schalkwyk et al. (2019)
DSM421 194±23 72±5 2.7 ≤0.0018 van Schalkwyk et al. (2017)
142 (71)a van Schalkwyk et al. (2019)
2-Anilinoquinazoline compounds WEB-484, 485, 486 and 487 59±10 to 95±8 87±1 to 110±37 Gilson et al. (2019)
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a

EC50 of UM01 clinical isolate. Data are the mean of 2 independent experiments, each performed in duplicate. Numbers in parenthesis are the range/2.

b

Mean from 6 clinical isolates, schizont maturation assay.

c

Mean (95% CI) from a single experiment repeated 5 times, laboratory H strain, pLDH assay.

d

In-cycle assay using the P. knowlesi YH1 strain, and the P. falciparum PHG strain.

e

Utilising a [3H]hypoxanthine uptake assay.

f

Evaluated by flow-cytometry after 48h exposure for P. knowlesi YH1, or 72h exposure for P. falciparum 3D7.

g

Drug exposure over 2.5 life-cycles.

h

The susceptibility of P. knowlesi YH1 to a number of azithromycin analogues has also been evaluated, with EC50 values of 12–360 nM (Burns et al., 2020). Numbers are mean±SEM unless otherwise specified. The P. knowlesi strain is A1-H1 unless otherwise specified. Drug exposure is over one life-cycle unless otherwise specified.

Although the clinical significance is uncertain, van Schalkwyk et al. did find that compared to P. falciparum, P. knowlesi had reduced susceptibility to inhibitors of dihydroorotate dehydrogenase (DHODH), a target of several new antimalarials under development including DSM421 and DSM265 (van Schalkwyk et al., 2017). DSM265 in particular was 8-fold less potent against P. knowlesi than P. falciparum. Importantly, compared to P. falciparum this agent has also been demonstrated to be 5-fold less potent against P. vivax field isolates ex vivo (Phillips et al., 2016). Furthermore, in a phase 2a study in Peru, DSM265 rapidly cleared most P. falciparum infections in patients while none of the doses tested cleared P. vivax infections (Llanos-Cuentas et al., 2018). This highlights the utility of P. knowlesi as a model to test drug susceptibilities of non-falciparum species. P. knowlesi also had reduced susceptibility to ATP4 inhibitors cipargamin, SJ733 and PA21A092, with EC50 values ~6 fold higher than those of P. falciparum (van Schalkwyk et al., 2017). The greatest difference in susceptibility between P. knowlesi and P. falciparum was observed with the oxaborole AN13762 (van Schalkwyk et al., 2019). This agent was 67-fold less potent against P. knowlesi than against P. falciparum, suggesting either differences in the drug target or a species-specific resistance mechanism.

In a more recent study van Schalkwyk et al. also evaluated the susceptibility of P. knowlesi to quinolones, including atovaquone as well as the more recently developed endochin-like quinolones (ELQs) (Van Schalkwyk et al., 2020). The quinolones were found to be equally potent against P. knowlesi (EC50 < 117 nM) as P. falciparum. However, P. knowlesi was 10-fold less susceptible to the quinolone partner drug proguanil, with an EC50 of 2461 nM compared to 228 nM for P. falciparum. Furthermore, in contrast to P. falciparum, no synergy against P. knowlesi was observed between the quinolones and proguanil. Importantly however, synergy between proguanil and the novel ELQ-300 was demonstrated against both species. In addition, ELQ-300 was synergistic with atovaquone against both P. falciparum and P. knowlesi.

Given the potential limitations of using a culture-adapted strain of P. knowlesi to evaluate drug susceptibilities, van Schalkwyk et al. also evaluated the drug susceptibilities of a new culture-adapted P. knowlesi line, UM01, isolated from a human host in Malaysia in 2013 (Amir et al., 2016; van Schalkwyk et al., 2019). Compared to the A1-H1 strain, EC50 values for the UM01 line were similar or lower to all antimalarial agents evaluated (Table 1), suggesting that the 1960s culture-adapted A1-H1 strain may continue to be suitable for evaluating drug susceptibilities of P. knowlesi. Van Schalkwyk et al. also evaluated several drug combinations against the P. knowlesi A1-H1, and showed similar drug interactions to those with P. falciparum (van Schalkwyk et al., 2019).

4. Drug resistance mutations

The emergence and spread of drug resistant mutations in P. falciparum and P. vivax presents a major threat to malaria control. However, given that transmission of P. knowlesi is largely, if not exclusively, zoonotic (Cuenca et al., 2021; Imai et al., 2014; Lee et al., 2011), drug resistant mutations are not expected to emerge. The lack of orthologous P. knowlesi drug resistant mutations has been confirmed by a number of studies. These include a study by Assefa et al., where 48 isolates from Sarawak, Malaysia, were analysed for mutations in the chloroquine resistance transporter (CRT) gene, the multidrug resistant protein 1 (MDR1) gene, the dihydrofolate reductase gene (DHFR), the dihydropteroate synthase (DHPS) gene, and the kelch 13 gene (Assefa et al., 2015). No drug resistant mutations were found, suggesting lack of drug pressure from chloroquine, mefloquine, pyrimethamine, sulphadoxine, and artemisinin derivatives, respectively. An additional 6 clinical isolates from Sarawak were analysed by Fatih et al., who also found no drug-resistance mutations in either the CRT or the MDR1 gene (Fatih et al., 2013). In over 400 samples from Sabah, Malaysia, Grigg et al. analysed the sequences of the DFHR gene, and although non-synonymous pkdhfr polymorphisms were frequently present, homology modelling demonstrated that these were not associated with pyrimethamine drug binding (Grigg et al., 2016a). Tyagi et al. also found no drug resistance mutations in either the DHFR gene or the CRT gene in 53 isolates from the Andaman and Nicobah Islands in India (Tyagi et al., 2013). Finally, although Pinheiro et al. reported dimorphism and polymorphism among P. knowlesi isolates in the multidrug resistance associated protein MRP1 and the multidrug resistance protein MDR2, both members of the ATP-binding cassette transporter family of genes associated with mefloquine resistance in P. falciparum (Pinheiro et al., 2015), the functional implications in P. knowlesi are not clear.

5. Treatment of uncomplicated knowlesi malaria

Uncomplicated knowlesi malaria is defined as detectable P. knowlesi parasitaemia without any of the clinical or laboratory criteria meeting the definition of severe knowlesi malaria (Table 2). For the purposes of treatment recommendations, this also incorporates asymptomatic parasitaemia. Treatment should be initiated as soon as a diagnosis is confirmed. The preferred treatment regimen consists of an oral artemisinin combination therapy (ACT). While chloroquine is also effective against P. knowlesi, ACT is associated with faster parasite clearance times (detailed below). Furthermore, administration of chloroquine to those with a diagnosis of knowlesi malaria is associated with a risk of inadvertently administering chloroquine to those with misdiagnosed vivax or falciparum malaria. Given the high prevalence of chloroquine-resistant vivax and falciparum malaria in knowlesi-endemic regions (Grigg et al., 2016b; Price et al., 2014), use of chloroquine as an antimalarial treatment is not recommended.

Table 2.

Severe malaria definitions.

(1) Epidemiological and Research definition for severe knowlesi malaria.
One or more of the following criteria (World Health Organization, 2014, 2021):
Unrousable comaa Glasgow coma scale < 11
Respiratory distress Oxygen saturation < 92% with respiratory rate > 30 breaths/min
Shock Systolic blood pressure < 80 mmHg with cool peripheries or impaired capillary refill
Jaundice Bilirubin >50 μmol/L, with parasitaemia >20,000/μL and/or creatinine >132 μmol/L
Severe anaemia Haemoglobin <7.0 g/dL (adults)
Haemoglobin <5.0 g/dL (children)a
Significant abnormal bleeding
Hypoglycaemia Blood glucose <2.2 mmol/L
Metabolic acidosis Bicarbonate <15 mmol/L or lactate >5 mmol/L
Acute kidney injury Creatinine >265 μmol/L
Hyperparasitaemia Parasite count >100,000/μL (or > 2% or infected red blood cells)
(2) Clinical definition requiring treatment with intravenous artesunate:
One or more of the following criteria (World Health Organization, 2014) in
settings where clinical or laboratory criteria of severity cannot be fully assessed.

  • Inability to tolerate oral therapy

  • Warning signs or clinical severity criteria as above

  • Parasitaemia >20,000/μL, particularly in settings where clinical or laboratory criteria of severity cannot be fully assessed
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a

Not yet reported in PCR-confirmed knowlesi malaria.

5.1. Artemisinin combination treatment (ACT)

Artemether-lumefantrine is the most widely used ACT for the treatment of knowlesi malaria, and is listed by the Malaysian Ministry of Health as the preferred treatment (Ministry of Health Malaysia, 2014). Artemether-lumefantrine has been evaluated in one of only two randomised controlled trials evaluating antimalarial treatment for uncomplicated knowlesi malaria (Table 3). In this study, 123 patients with PCR-confirmed uncomplicated knowlesi malaria in 3 district hospitals in Sabah, Malaysia, were randomised to receive either artemether-lumefantrine (total dose 12 mg/kg of artemether and 60 mg/kg of lumefantrine) or chloroquine (25 mg/kg) (Grigg et al., 2018b). To ensure applicability of results to a real-world setting, co-administration of artemether-lumefantrine with fatty food (a biscuit or milk) was encouraged but not mandated. Participants of all ages were included if they weighed >10 kg, had a parasite count of <20,000/μL, and had a negative rapid diagnostic test result for P. falciparum. The primary outcome was parasite clearance at 24h, which was achieved in 76% (95% CI 63–86%) of patients administered artemether-lumefantrine, compared to 60% (95% CI 47–72%) of those administered chloroquine (P = 0.06). Median parasite clearance was shorter after artemether-lumefantrine than chloroquine (18 vs. 24h, P = 0.02), and all patients were aparasitaemic by 48h. Patients were followed up to day 42, and there were no treatment failures with either artemether-lumefantrine or chloroquine. Adverse events were similar between the two groups, although dyspepsia occurred more commonly in those who received artemether-lumefantrine (7% vs. 0%, P = 0.03). There were no serious adverse events. When applying the Malaysian national hospital policy of 2 negative blood films on 2 consecutive days prior to discharge, the predicted bed occupancy was 2414 days per 1000 patients treated with artemether-lumefantrine compared to 2800 days per 1000 patients treated with chloroquine (incidence rate ratio 0.86, 95% CI 0.82–0.91, P < 0.001).

Table 3.

Studies reporting antimalarial chemotherapy used in the treatment of knowlesi malaria in adults and children in endemic areas.a

Study design Number
of
patients		 Percent
male		 Median
age (range)		 Median
(range)
parasitaemia
(parasites/μL)		 PCT50
(hours),
median		 PCT90 (hours),
median		 Parasite
reduction
ratio at 24h
(%, 95% CI)		 % negative
at 24h
(95% CI)		 Time to
parasite
clearance
(<5/μL),
median
(range)		 Died Refs.
Uncomplicated malaria
Chloroquine and primaquine Prospective observational 33 58 46 3724 (1845–7480) 3.1 (range 2.8–3.4) 10.3 (range 9.4–11.4) 99.4 (97.0–99.9) 33 36 (31–51) hours 0 Daneshvar et al. (2010)
Chloroquine Randomised controlled trial 125 75 32 (7–85) 1329 (33–35,873) 6.3 (95% CI 5.1–7.5) 14.8 (95% CI 13.4–16.1) 97.5 (96.6–98.4) 55 (45–64) 24 (6–60) hours 0 Grigg et al. (2016c)
Chloroquine Retrospective 16 50 9 (4–14) 2240 (200–14,400) NR NR NR 6b 2 days (range 1–5)b Barber et al. (2011)
Chloroquine Randomised controlled trial 65 71 31 (4–75) 1485 (89–32,660) 8.2 (95% CI 6.4–10.1) 15.6 (95% CI 3.6–17.7) 98.1 (97.2–99.1)c 60 (47–72) 24 (12–48) hours Grigg et al. (2018b)
Artesunate/mefloquine Randomised controlled trial 115 81 33 (3–82) 1457 (36–35,008) 3.4 (95% CI 2.9–4.0) 8.9 (95% CI 8.2–9.6) 99.8 (99.7–100) 84 (76–91) 18 (6–48) hours 0 Grigg et al. (2016c)
Artemether-lumefantrine Prospective observational 27 70 31 (17–70) 4066 (232–60,840) NR NR NR 33b 2 days (range 1–3)b 0 Barber et al. (2013a) c
Artemether-lumefantrine Retrospective 6 100 30 NR NR NR NR 66b 1 day (range 0–3)b 0 William et al. (2011) c
Artemether-lumefantrine Randomised controlled trial 58 85 30 (7–79) 1437 (57–44,744) 7.2 (95% CI 5.6–8.9) 13.7 (95% CI 11.8–14.7) 99.6 (99.2–100)c 76 (63–86) 24 (12–42) hours Grigg et al. (2018b)
Oral quinine Retrospective 11 83 45 NR NR NR NR 12b 2.5 days (range 1–3)b 0 William et al. (2011) c
Severe malaria
Intravenous artesunate Prospective observational 36 75 55 (20–74) 100,995 (32–584,015) NR NR NR 33b NRb,d 0 Barber et al. (2013a) c
Intravenous artesunate Retrospective 6 63 56 NR NR NR NR 50% 2 days (range 1–3)b 1 (17%) William et al. (2011) c
Intravenous quinine Prospective observational 10 30 64 (36–73) NR NR NR NR NR NR 2 (20%) Daneshvar et al. (2010)
Intravenous quinine Retrospective 16 69 55 4+ NR NR NR 30% 4 days (range 2–7)b 5 (27%) William et al. (2011)
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a

Does not include case reports.

b

Based on daily routine hospital microscopy.

c

Supplementary data.

d

NR, not recorded.

In the only other randomised controlled trial to have evaluated antimalarial treatment for uncomplicated knowlesi malaria, 252 patients older than 1 year presenting to the same district hospitals as the study above were randomised to receive either artesunate-mefloquine (12 mg/kg artesunate and 25 mg/kg mefloquine) or chloroquine (25 mg/kg) (Grigg et al., 2016c). The primary endpoint of parasite clearance at 24h was achieved in 84% (95% CI 76–91) of patients in the artesunate-mefloquine group, compared to 55% (95% CI 45–64) in the chloroquine group (P < 0.0001). Median parasite clearance was faster with artesunate-mefloquine compared to chloroquine (18 vs 24h, P < 0.0001). The risk of anaemia within 28 days was also lower in the artesunate-mefloquine group compared to the chloroquine group (62% vs 71%, P = 0.035). Two patients had serious adverse events. The first was in a 41 year old man in the artesunate-mefloquine group who developed an acute psychosis on day 3, with auditory hallucinations, nausea, dizziness, and subsequent minor attempts at self-harm. The symptoms resolved over a period of 2 weeks, and the patient remained well over the next 6 months of follow-up. The event was considered probably related to mefloquine. The second serious adverse event also occurred in the artesunate-mefloquine arm, in a 55 year old man with cardiovascular risk factors who died after being readmitted to hospital with severe pneumonia 24 days after his enrolment in the trial. The event was considered unrelated to the study drug. Adverse events were otherwise similar between the two groups.

A meta-analysis of the two studies above found that compared to chloroquine, both ACTs were associated with a similar increase in the proportion of patients who were parasite negative at 24h, and there were no significant differences in treatment outcomes between artemether-lumefantrine and artesunate-mefloquine (Grigg et al., 2018b). The results of these randomised controlled trials are consistent with earlier non-randomised studies reporting efficacy of artemether-lumefantrine and artesunate-mefloquine for the treatment of uncomplicated malaria at a tertiary referral hospital in Sabah, Malaysia (Table 3; Barber et al., 2013a; William et al., 2011).

Several other ACTs have been used successfully for the treatment of uncomplicated knowlesi malaria, including dihydroartemisinin-piperaquine (Setiadi et al., 2016). Given the similar in vitro susceptibilities of P. knowlesi and P. falciparum to artemisinin derivatives (detailed above) and most partner drugs (Table 1), it is highly likely that most ACTs effective against artemisinin-sensitive strains of P. falciparum will be also highly effective against P. knowlesi.

5.2. Chloroquine

Prior to the increasing recognition of knowlesi malaria in Malaysia over the past decade, knowlesi malaria was commonly diagnosed as the microscopically near-identical P. malariae, and was thus treated with chloroquine, previously the first-line treatment for P. malariae in Malaysia. The large majority of patients included in early case series of knowlesi malaria therefore received treatment with chloroquine. Consistent with the findings from the randomised controlled trials described above, these early studies demonstrated chloroquine to be effective for the treatment of uncomplicated knowlesi malaria. In the first report of a large focus of human infections with knowlesi malaria, Singh et al. reported 92 patients with knowlesi malaria treated with chloroquine at a district hospital in Sarawak, Malaysia, with primaquine given at 24 and 48h (Singh et al., 2004). Median parasite clearance time was 2.4 days (range 1–5 days) and no deaths were reported. In a prospective observational study at the same hospital, chloroquine was administered to 96 adults with uncomplicated knowlesi malaria (Daneshvar et al., 2010). All patients had cleared their parasites by day 3, and all were negative by PCR on day 7, 14, 21 and 28.

Despite the efficacy of chloroquine for treating uncomplicated knowlesi malaria, administration of chloroquine to patients with unsuspected severe knowlesi malaria has resulted in adverse outcomes, including deaths (Rajahram et al., 2012). For this reason, in addition to the risk of administering chloroquine to patients with misdiagnosed vivax or falciparum malaria, ACT is the preferred option for treatment of uncomplicated knowlesi malaria.

5.3. Other agents

Atovaquone-proguanil has been used for the treatment of uncomplicated knowlesi malaria in returned travellers, with rapid recovery reported in all cases (Ehrhardt et al., 2013; Figtree et al., 2010; Hoosen and Shaw, 2011; Mackroth et al., 2016). However, although atovaquone is highly potent in vitro against P. knowlesi (EC50 < 5 nM) (van Schalkwyk et al., 2017, 2019), proguanil has been shown to be more than 10-fold less potent against the laboratory adapted P. knowlesi A1-H1 strain than P. falciparum (EC50 of 2461 nM compared to 228 nM for P. falciparum) (Van Schalkwyk et al., 2020). Furthermore, the synergy between proguanil and atovaquone that occurs with P. falciparum was not observed with P. knowlesi (Van Schalkwyk et al., 2020). Therefore, atovaquone-proguanil should be used for the treatment of knowlesi malaria only if ACTs are not readily available.

5.4. Primaquine

Primaquine is not generally indicated for the treatment of knowlesi malaria. P. knowlesi does not form hypnozoites, thus anti-relapse therapy is not required. The use of primaquine to reduce transmission is also not considered necessary, given that P. knowlesi exists primarily as a zoonosis without evidence of substantial human-human transmission. Furthermore, gametocytes have been shown to be cleared rapidly in knowlesi malaria without use of primaquine. In a clinical trial, gametocytes were detected by PCR at day 7 in only 2/48 (4%) and 3/49 (6%) patients treated with chloroquine and artesunate-mefloquine respectively, despite gametocytes having been present at baseline in 39/48 (81%) and 43/49 (88%) of these patients (Grigg et al., 2016c). In another clinical trial, although PCR was not done, gametocytes were present by microscopy at baseline in 15/58 (26%) and 17/65 (26%) of patients treated with artemether-lumefantrine and chloroquine, respectively, and negative by microscopy in all patients at all follow-up time points (including 6 hourly blood films until parasite clearance) (Grigg et al., 2018b). Primaquine should be used to prevent P. vivax relapses in cases of P. knowlesi/P. vivax mixed infections, and should also be used to prevent transmission in cases of P. knowlesi/P. falciparum mixed infections.

6. Clinical management of severe knowlesi malaria

Although the majority of patients with knowlesi malaria have uncomplicated disease, in Malaysia severe disease has been shown to occur in ~6–9% of patients at district hospitals (Daneshvar et al., 2009; Grigg et al., 2018a), and up to 29% of patients at a tertiary referral hospital (Barber et al., 2013a). In the tertiary hospital study, risk of severe disease was at least as high as that of P. falciparum. Age and parasitaemia have both been shown to be independent risk factors for severe disease (Barber et al., 2017; Grigg et al., 2018a). In a recent review of knowlesi malaria cases in Sabah, Malaysia, during 2010–2017, the overall case-fatality rate was 2.5/1000: 6.0/1000 for women, and 1.7/1000 for men (Rajahram et al., 2019). Independent risk factors for death included female sex (OR 2.6 [95% CI 1.0–6.7], P = 0.04) and age > 45 years (OR 4.7 [95% CI 1.8–12.5], P < 0.01). Parasitaemia as a risk factor for death was not able to be evaluated in this analysis, although this association could be assumed, given the strong association between parasitaemia and disease severity. Other factors contributing to fatal outcomes included cardiovascular comorbidities, microscopic misdiagnoses, and delays in administering intravenous therapy. Thus, early recognition and diagnosis of severe disease, and early initiation of appropriate treatment is essential to avoid fatal outcomes.

Severe knowlesi malaria has been defined according to modified criteria for severe falciparum malaria, with a lower parasite count of 100,000/μL used as a cut-off to define hyperparasitaemia. In adults, the most common severity criteria include hyperparasitaemia, jaundice, acute kidney injury, shock, and respiratory distress (Barber et al., 2013a; Daneshvar et al., 2009; Grigg et al., 2018a). Anaemia occurs less commonly. Metabolic acidosis is common in fatal cases (Rajahram et al., 2019).

6.1. Intravenous artesunate

The WHO recommends intravenous artesunate for all patients with severe malaria regardless of species (World Health Organization, 2021), based on 8 randomised controlled trials in African and Asian countries in adults (n = 1664) and children (n = 5765) demonstrating reduced mortality with intravenous artesunate compared to intravenous quinine for the treatment of severe falciparum malaria (Sinclair et al., 2012). Intravenous artesunate has also been shown to be highly effective for the treatment of severe knowlesi malaria, with no deaths occurring in a prospective tertiary hospital involving 38 patients with severe knowlesi malaria in Sabah, Malaysia (Barber et al., 2013a). In an earlier retrospective study conducted at the same hospital, 1/6 (17%) patients treated with intravenous artesunate died, compared to 5/16 (31%) treated with intravenous quinine (William et al., 2011). In another prospective observational study in Sarawak, Malaysia, 2/10 (20%) patients with severe knowlesi malaria treated with intravenous quinine died (Daneshvar et al., 2010). This lower case-fatality rate associated with the use of intravenous artesunate was also supported by a state-wide study conducted in Sabah, that demonstrated a fall in the P. knowlesi case-fatality rate from 9.2/1000 case notifications in 2010 (when local guidelines changed to recommend intravenous artesunate) to 1.6/1000 case notifications in 2014 (Rajahram et al., 2016). It is likely that this reduction was due at least in part to the increasing use of intravenous artesunate for severe knowlesi malaria.

While intravenous artesunate should clearly be administered to patients with clinical or laboratory evidence of severe knowlesi malaria (Table 1), and to those not tolerating oral medications, the optimal treatment for patients with moderately high parasite counts in the absence of other evidence of severe disease is less certain. In a tertiary hospital study, severe disease occurred in 24/45 (53%) patients with a parasite count of >20,000/μL, including in 9/27 (33%) of those who had parasite counts of between 20,000 and 100,000/μL. The sensitivity and specificity of 20,000/μL as a cut-off for predicting severe disease was 75% and 79%, respectively (Barber et al., 2013a). Similar results were observed in a district hospital study, where severe disease occurred in 22/54 (41%) patients with parasite counts over 15,000 parasites/μL, with this cut-off having a sensitivity and specificity of 74% and 87%, respectively (Grigg et al., 2018a). In a third study, severe disease occurred in 11/25 (44%) in patients with a parasite count above 35,000 parasites/μL, including 3/9 (33%) with a parasitaemia between 35,000 and 100,000/μL; the 35,000/μL parasitaemia cut-off had a sensitivity and specificity of 65% and 85%, respectively, for predicting severe disease (Willmann et al., 2012). In the two randomised controlled trials by Grigg et al. evaluating artemether-lumefantrine (Grigg et al., 2018b) or artesunate-mefloquine (Grigg et al., 2016c) versus chloroquine for the treatment of uncomplicated knowlesi malaria, patients with parasite counts >20,000/μL on screening hospital microscopy were excluded due to uncertainty about the safety of administering oral treatment to these patients. However, research cross check of enrolment slides and recalculation of parasite counts using individual patient WBC measurements indicated that 17 patients had parasite counts of between 20,000 and 45,000 parasites/μL and no other severity criteria and were safely treated with either chloroquine (n = 7), artesunate-mefloquine (n = 9), or artemether-lumefantrine (n = 4) (Grigg et al., 2016c, 2018b). Nonetheless, because of the higher risk of severe disease at these moderately high parasitaemias, WHO has recommended intravenous artesunate for patients with parasitaemias >20,000/μL, particularly if other laboratory criteria for severe disease cannot be evaluated (World Health Organization, 2014). More recently, based on the largest of the studies above (Grigg et al., 2018a), a lower parasitaemia cut-off of >15,000/μL has also been recommended (Anstey et al., 2021; World Health Organization & Regional Office for the Western Pacific, 2017).

For patients fulfilling criteria for severe knowlesi malaria, intravenous artesunate should be given for a minimum of 3 doses (2.4 mg/kg), 12h apart (World Health Organization, 2021). This should be followed by a 3 day course of oral ACT such as artemether-lumefantrine once oral intake is tolerated, according to local guidelines and availability. For patients commenced on intravenous artesunate in the absence of criteria for severe malaria, oral ACT should be substituted as soon as oral intake is tolerated.

In severe falciparum malaria intravenous artesunate has in some cases been associated with post-treatment haemolysis, presenting as severe anaemia 7–14 days after treatment (Fanello et al., 2017; Gómez-Junyent et al., 2015; Jauréguiberry et al., 2014). While early haemolysis with haemoglobinuria has been reported following artesunate use in severe knowlesi malaria (Barber et al., 2016b), post-artesunate delayed haemolysis has not yet been reported in P. knowlesi infection. WHO recommends that all hyperparasitaemic patients who have received intravenous artesunate be monitored for delayed haemolytic anaemia (World Health Organization, 2021).

6.2. Paracetamol as a renoprotective agent

Acute kidney injury (AKI) is a common complication of knowlesi malaria. In a tertiary-hospital study, AKI by KDIGO criteria (with baseline creatinine estimated by the MDRD equation) was present on admission in 44/154 (29%) non-severe cases, and 40/48 (83%) severe cases (Barber et al., 2018). In another district hospital study where severe malaria

was less common, AKI by the same criteria was present in 109/396 (30%) hospitalised patients with knowlesi malaria (Cooper et al., 2018a). As with falciparum malaria, haemolysis is increased in severe knowlesi malaria, and is thought to be a key contributing factor to AKI (Barber et al., 2018). Haemolysis is associated with release of cell-free haemoglobin (CFHb), leading to rapid oxidation of ferrous (Fe2+) to ferric (Fe3+) haemoglobin. Further oxidation of ferric to ferryl (Fe4+) haemoglobin generates lipid radical species, leading to oxidative stress and oxidative injury.

In falciparum malaria, acute kidney injury has been shown to be associated with both CFHb and measures of lipid peroxidation, supporting the hypothesis that CFHb-induced lipid peroxidation contributes to AKI in falciparum malaria (Plewes et al., 2017). Paracetamol inhibits CFHb-mediated lipid peroxidation by reducing heme-ferryl radicals, and hence has been proposed as a renoprotective agent in severe malaria complicated by haemolysis. Building on this hypothesis, a Phase 2 RCT evaluating the ability of regularly dosed paracetamol (1 g 6 hourly) to improve renal function was conducted in 62 Bangladeshi adults with moderate to severe falciparum malaria (Plewes et al., 2018). After 72h creatinine had reduced by 23% (IQR 37–18%) in patients randomised to paracetamol, compared to 14% (IQR 0–29%) in those randomised to no paracetamol (P = 0.043). The difference was more marked in those with CFHb > 45,000 ng/mL compared to those with a CFHb < 45,000 ng/mL (37% [IQR 22–48%] reduction vs 14% [IQR −71–30%), P = 0.01).

In a larger RCT in Malaysian adults with knowlesi malaria of any severity, 396 hospitalised patients were randomised to paracetamol (1 g 6 hourly for 72h), or no paracetamol (Cooper et al., 2018a, b). While the primary endpoint of improved change in creatinine at 72h was not met in the overall group, regularly-dosed paracetamol was associated with greater improvements in creatinine in certain pre-defined subgroups, including those with severe malaria, and in those with AKI and haemolysis (defined in this study as a CFHb ≥77,600 ng/mL). Regularly dosed paracetamol was shown to be safe and well-tolerated. While the median alanine transaminase (ALT) was higher following treatment in the paracetamol arm compared to the control arm, no patient met the criteria for Hy’s law for hepatotoxicity (Temple, 2006). Based on these results, regularly-dosed paracetamol may be considered as a renoprotective agent for patients with severe knowlesi malaria, and is recommended in current guidelines (Antibiotic Expert Group, 2019). Regularly dosed paracetamol may also be considered for patients with knowlesi malaria complicated by AKI, even in the absence of other criteria for severe malaria.

6.3. Other adjunctive and supportive treatment

Like falciparum malaria, severe knowlesi malaria is frequently associated with multiorgan failure requiring intensive supportive management. However, with the exception of the paracetamol trial discussed above, there have been no other clinical trials of adjunctive therapy in severe knowlesi malaria, and no other adjunctive therapy that has proven effective in severe falciparum malaria (World Health Organization, 2021). Supportive treatment for severe knowlesi malaria is therefore limited to inotropic and ventilatory support (Barber et al., 2013a; William et al., 2011), haemodialysis for acute kidney injury (Barber et al., 2013a, William et al., 2011), and blood transfusions as required (World Health Organization, 2021). Thrombocytopenia is universal in severe knowlesi malaria; however, platelet counts recover rapidly after commencement of antimalarial treatment (Barber et al., 2013a), and significant abnormal bleeding is uncommon (Barber et al., 2013a; Cox-Singh et al., 2008; Daneshvar et al., 2009; Grigg et al., 2018a; Lee et al., 2010; Ninan et al., 2012). Thus, platelet transfusions are not generally required. There have been no clinical trials to guide intravenous fluid management in knowlesi malaria. In severe falciparum malaria liberal administration of intravenous fluids has been shown to be deleterious (Hanson et al., 2013, 2014), while conservative fluid regimens have been shown to be safe (Aung et al., 2015; Ishioka et al., 2020). Because the risk of acute respiratory distress syndrome and non-cardiogenic pulmonary oedema is at least as high in severe knowlesi malaria as in severe falciparum malaria (Barber et al., 2013a), a conservative intravenous fluid regimen is generally recommended (Barber et al., 2016a; Hanson et al., 2014; World Health Organization, 2021).

There are no clinical trials to guide the use of empirical antibiotics in severe knowlesi malaria. Concurrent bacteraemia, predominantly gram negative, occurs in up to 15% of adults hospitalised with falciparum malaria (Aung et al., 2015, 2018); in knowlesi malaria, Enterobacter bacteremia has been reported in two series of severe knowlesi malaria (Barber et al., 2013a, William et al., 2011), and Neisseria meningitides in another (Grigg et al., 2018a). Empirical intravenous broad-spectrum antibiotics are commonly administered in severe knowlesi malaria with multiorgan failure until blood cultures are negative (Barber et al., 2013a).

7. Treatment of knowlesi malaria in children

Plasmodium knowlesi occurs less commonly in children than in adults. In Sabah, Malaysia, children aged < 5 years accounted for only 25/3262 (0.8%) of PCR confirmed P. knowlesi cases reported during 2015–2017, while children 5–13years accounted for another 170 (5%) cases (Cooper et al., 2019). The disease is also of lower severity. Parasite counts are lower (Grigg et al., 2018a), and the multi-organ failure that characterises severe disease in adults does not appear to occur in children, nor the severe anaemia commonly seen with falciparum and vivax malaria in this age group (Douglas et al., 2013). Indeed, there have been no cases of severe paediatric PCR-confirmed knowlesi malaria reported to date (Barber et al., 2011; Grigg et al., 2018a; Singh and Daneshvar, 2013). Moderate severity disease is however not uncommon. In a recent district hospital in Sabah, children < 13 years accounted for 44/481 (9%) of patients hospitalised with knowlesi malaria (Grigg et al., 2018a). Abdominal pain was more common in children than in adults (43% vs 23%), as was anaemia, occurring in 82% of children compared to 36% of adults. AKI was common in both children and adults (26% vs 19%).

The treatment recommendations for knowlesi malaria in children are the same as for adults. In the two RCTs conducted by Grigg et al., 12, 13 and 7 children ≤12years old were treated with artesunate-mefloquine, chloroquine, and artemether-lumefantrine, respectively (Grigg et al., 2016c, 2018b). Median (range) parasite counts in these children were 377 (36–9998), 2455 (274–32,320), and 487 (183–9229) parasites/μL, respectively, and all were aparasitaemic by 48h (Grigg et al., 2016c). The use of chloroquine in an additional 16 children with uncomplicated knowlesi malaria was reported in a retrospective study; median parasite clearance time was 2 days, although a parasite clearance time of 5 days was reported in an 8 year old boy with an admission parasitaemia of 14,400/μL (Barber et al., 2011). Successful use of oral and intravenous quinine for knowlesi malaria in children was also reported (Barber et al., 2011). In the absence of reported severe knowlesi malaria in children, the use of intravenous artesunate for this indication has not been described, but would be recommended based on its known efficacy in children with severe falciparum malaria (Dondorp et al., 2005, 2010), and in adults with severe knowlesi malaria (Barber et al., 2013a). If artesunate is required, children weighing <20 kg should receive a higher dose (3 mg/kg per dose) to ensure adequate drug exposure (World Health Organization, 2021).

8. Treatment of knowlesi malaria in pregnancy

In contrast to P. falciparum and P. vivax, P. knowlesi infection during pregnancy appears to be relatively rare, with only 6 cases reported to date (Barber et al., 2014; Rajahram et al., 2019; William et al., 2011). However, despite its rarity, adverse maternal and infant outcomes have been reported, including severe maternal malaria, foetal loss and low birth weight (Barber et al., 2014; William et al., 2011), and a single maternal fatality, occurring in a 32 year old woman at 35 weeks gestation (Rajahram et al., 2019). Prompt and effective treatment is therefore essential, and any woman with severe malaria in pregnancy should be treated with immediate intravenous artesunate, regardless of species and including those in the first trimester (World Health Organization, 2021). Women with uncomplicated knowlesi malaria in the second or third trimester of pregnancy may be treated with oral ACT, as per the guidelines for treatment of uncomplicated falciparum malaria in pregnancy (World Health Organization, 2021). In the first trimester of pregnancy uncomplicated knowlesi malaria may be treated with chloroquine. If there is any doubt as to the microscopic diagnosis, alternative treatments include quinine and clindamycin, as per WHO guidelines for falciparum and vivax malaria (World Health Organization, 2021). These guidelines are based on published prospective data from 700 women exposed to ACT in the first trimester of pregnancy (Dellicour et al., 2015; McGready et al., 2012; Moore et al., 2016; Mosha et al., 2014), which indicate no adverse effects on the pregnancy or the health of the foetus or neonate, and are sufficient to exclude a ≥4.2-fold increase in risk of any major birth defect (background prevalence assumed to be 0.9%), if half the exposures occur during the embryo-sensitive period (4–9 weeks post conception) (World Health Organization, 2021). These data can be used to reassure women who are accidentally exposed to ACT during the first trimester of pregnancy.

9. Conclusions

The zoonotic parasite P. knowlesi has emerged as a major cause of human malaria in Southeast Asia, particularly in eastern Malaysia. Diagnosis by microscopy can be challenging, and for treatment purposes, in knowlesi endemic areas any patient with parasites resembling P. malariae should be considered to have P. knowlesi. Recognition of the ability of P. knowlesi to cause severe disease is paramount for ensuring prompt initiation of effective treatment to prevent adverse outcomes. Given the zoonotic nature of the parasite, drug resistance has not emerged as a concern, and the first line ACTs artesunate-mefloquine and artemether-lumefantrine have both been shown to be highly effective for treatment of uncomplicated disease. For severe disease, intravenous artesunate must be initiated without delay. Recent data also indicate a role for paracetamol as a renoprotective agent for those with severe disease.

Acknowledgements

This work was supported by the Australian National Health and Medical Research Council (grant numbers 1037304 and 1045156; fellowships to NMA [1042072], BEB [1088738]; MJG [1138860] and ‘Improving Health Outcomes in the Tropical North: A multidisciplinary collaboration ‘Hot North’, [grant 1131932]). The Sabah Malaria Research Program is supported by the US NIH, and by the Australian Centre for International Agricultural Research and Department of Foreign Affairs, Australian Government (#LS-2019-116).

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