Persistence of Plasmodium cynomolgi hypnozoites in cynomolgus monkey iPS-derived hepatocytes

Plasmodium cynomolgi (Pc) is one of the few parasite species that forms quiescent liver stage parasites known as hypnozoites and is therefore a suitable model for Plasmodium vivax. Very little is known about liver stage dormancy, which hampers the search for compounds with anti-hypnozoite activity. Here, we present the development of a Pc in vitro infection model using stem cell-derived hepatocytes from Macaca fascicularis. IPS cells were established on feeder free condition and differentiated into hepatocytes via inducible overexpression of key transcription factors. The generated hepatocytes were infected with Pc sporozoites and hypnozoite formation as well as schizont development were confirmed by immunofluorescence. This system is a promising tool to study the mechanisms underlying liver stage dormancy and facilitate drug discovery against hypnozoites.

Zoonotic Transmission and Host Switches of Malaria Parasites

Malaria is a deadly disease that affects the health of hundreds of millions of people annually. Five Plasmodium parasite species naturally infect humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale, and Plasmodium knowlesi. These parasites can also infect various non-human primates. Parasites mainly infecting monkeys, such as Plasmodium cynomolgi and P. knowlesi, the latter of which was considered to be a monkey parasite for years, can also be transmitted to human hosts. Recently, many new Plasmodium species have been discovered in African apes, some of which may be transmitted to humans in the future. Here, we searched PubMed and the internet via Google and selected articles on the zoonotic transmission and evolution of selected malaria parasite species. We review current advances in the relevant topics, emphasizing the transmission of malaria parasites between humans and non-human primates. We also briefly discuss the transmission of some avian malaria parasites between wild birds and domestic fowls. Zoonotic malaria transmission is widespread, thus posing a threat to public health. More studies on parasite species, including their identification in non-human primates, transmission, and evolution, are needed to decrease or prevent the transmission of malaria parasites from non-human primates to humans.

The New Zoonotic Malaria: Plasmodium cynomolgi

Plasmodium cynomolgi is a simian malaria parasite that has been a central model parasite since it was first described in 1907. Recently it has made the zoonotic jump and started naturally infecting humans. In this paper, the interactions between Plasmodium cynomolgi and humans, the environment and the non-human animal intermediates or definitive host will be discussed, with a particular focus on the clinical implications of infection and approaches to management of this novel zoonotic parasite.

Preliminary review on the prevalence, proportion, geographical distribution, and characteristics of naturally acquired Plasmodium cynomolgi infection in mosquitoes, macaques, and humans: a systematic review and meta-analysis

Background Plasmodium cynomolgi is a simian malaria parasite that has been reported as a naturally acquired human infection. The present study aims to systematically review reports on naturally acquired P. cynomolgi in humans, mosquitoes, and macaques to provide relevant data for pre-emptive surveillance and preparation in the event of an outbreak of zoonotic malaria in Southeast Asia. Methods The protocol of the systematic review was registered at PROSPERO with approval ID CRD42020203046. Three databases (Web of Science, Scopus, and MEDLINE) were searched for studies reporting the prevalence of P. cynomolgi infections in Southeast Asian countries between 1946 and 2020. The pooled prevalence or pooled proportion of P. cynomolgi parasitemia in humans, mosquitoes, and macaques was estimated using a random-effects model. Differences in the clinical characteristics of P. cynomolgi infections were also estimated using a random-effects model and presented as pooled odds ratios (ORs) or mean differences (MDs) with 95% confidence intervals (CIs). Results Thirteen studies reporting on the prevalence of naturally acquired P. cynomolgi in humans (3 studies, 21 cases), mosquitoes (3 studies, 28 cases), and macaques (7 studies, 334 cases) were included. The results demonstrated that the pooled proportion of naturally acquired P. cynomolgi in humans was 1% (95% CI, 0.1%, I2, 0%), while the pooled proportion of P. cynomolgi infecting mosquitoes was 18% (95% CI, 10–26%, I2, 32.7%). The pooled prevalence of naturally acquired P. cynomolgi in macaques was 47% (95% CI, 27–67%, I2, 98.3%). Most of the cases of naturally acquired P. cynomolgi in humans were reported in Cambodia (62%) and Malaysia (38%), while cases of P. cynomolgi in macaques were reported in Malaysia (35.4%), Singapore (23.2%), Indonesia (17.3%), Philippines (8.5%), Laos (7.93%), and Cambodia (7.65%). Cases of P. cynomolgi in mosquitoes were reported in Vietnam (76.9%) and Malaysia (23.1%). Conclusions This study demonstrated the occurrence of naturally acquired P. cynomolgi infection in humans, mosquitoes, and macaques. Further studies of P. cynomolgi in asymptomatic human cases in areas where vectors and natural hosts are endemic are extensively needed if human infections with P. cynomolgi do become public health problems.

Plasmodium knowlesi infecting humans in Southeast Asia: What’s next?

Plasmodium knowlesi, a simian malaria parasite, has been in the limelight since a large focus of human P. knowlesi infection was reported from Sarawak (Malaysian Borneo) in 2004. Although this infection is transmitted across Southeast Asia, the largest number of cases has been reported from Malaysia. The increasing number of knowlesi malaria cases has been attributed to the use of molecular tools for detection, but environmental changes including deforestation likely play a major role by increasing human exposure to vector mosquitoes, which coexist with the macaque host. In addition, with the reduction in human malaria transmission in Southeast Asia, it is possible that human populations are at a greater risk of P. knowlesi infection due to diminishing cross-species immunity. Furthermore, the possibility of increasing exposure of humans to other simian Plasmodium parasites such as Plasmodium cynomolgi and Plasmodium inui should not be ignored. We here review the current status of these parasites in humans, macaques, and mosquitoes to support necessary reorientation of malaria control and elimination in the affected areas.

Safety, Pharmacokinetics, and Activity of High-Dose Ivermectin and Chloroquine against the Liver Stage of Plasmodium cynomolgi Infection in Rhesus Macaques

ABSTRACT Previously, ivermectin (1 to 10 mg/kg of body weight) was shown to inhibit the liver-stage development of Plasmodium berghei in orally dosed mice. Here, ivermectin showed inhibition of the in vitro development of Plasmodium cynomolgi schizonts (50% inhibitory concentration [IC50], 10.42 μM) and hypnozoites (IC50, 29.24 μM) in primary macaque hepatocytes when administered as a high dose prophylactically but not when administered in radical cure mode. The safety, pharmacokinetics, and efficacy of oral ivermectin (0.3, 0.6, and 1.2 mg/kg) with and without chloroquine (10 mg/kg) administered for 7 consecutive days were evaluated for prophylaxis or radical cure of P. cynomolgi liver stages in rhesus macaques. No inhibition or delay to blood-stage P. cynomolgi parasitemia was observed at any ivermectin dose (0.3, 0.6, and 1.2 mg/kg). Ivermectin (0.6 and 1.2 mg/kg) and chloroquine (10 mg/kg) in combination were well-tolerated with no adverse events and no significant pharmacokinetic drug-drug interactions observed. Repeated daily ivermectin administration for 7 days did not inhibit ivermectin bioavailability. It was recently demonstrated that both ivermectin and chloroquine inhibit replication of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in vitro. Further ivermectin and chloroquine trials in humans are warranted to evaluate their role in Plasmodium vivax control and as adjunctive therapies against COVID-19 infections.