Case Report: Cambodian National Malaria Surveillance Program Detection of Plasmodium knowlesiChristina Yek, Sreyngim Lay, Jennifer A. Bohl, Somnang Man, Sophana Chea, Chanthap Lon, Vida Ahyong, Cristina M. Tato, Joseph L. DeRisi, Siv Sovannaroth, and Jessica E. Manning
ASTMH, 2022Abstract: In May 2015, the WHO approved the Global Technical Strategy for Malaria (GTS) with the overarching goal of reducing the global malaria burden by 90% in 2030. The Greater Mekong subregion (GMS), encompassing Cambodia, China, Laos, Myanmar, Thailand, and Vietnam, was identified as an area of particular interest given the prevalence of multidrug-resistant strains of Plasmodium falciparum along the Cambodia–Thailand border.1 Public policy in the GMS has concentrated on the elimination of P. falciparum; in the 2010s, the Mekong Malaria Elimination Program introduced multifaceted measures including robust surveillance systems, outreach to rural and migrant populations, vector control, bed net distribution, and early combination treatment to address possible drug-resistant strains.2 Ongoing deforestation is also contributing to malaria case reduction given shrinking habitats for anopheline mosquito vectors, typically thought to be zoophilic, outdoor feeders that reside primarily in the forest or forest fringe.3 The recently published 2020 WHO World Malaria Report highlighted the interim success of regional efforts, with malaria cases in the GMS falling by 90% between 2000 and 2019 and cases of P. falciparum malaria falling by 97%, meeting GTS interim milestones.1
While elimination efforts for P. falciparum appear to be on track, reports of zoonotic malaria are emerging with increasing frequency in the GMS and surrounding countries.4–6 In Malaysia, where molecular diagnosis is part of national malaria surveillance and enables accurate species reporting, all indigenous malaria cases observed between 2018 and 2020 were attributed to P. knowlesi, and incidence appears to be increasing (1,600 to over 4,000 cases from 2016 to 2018).1 In contrast, Cambodia, which constitutes approximately 58% of all malaria cases in the GMS, has relatively few reports of zoonotic malaria. It is not clear which vectors primarily contribute to ongoing residual malaria transmission given Cambodia’s great diversity of known and cryptic vectors, but these are likely Anopheles spp. that predominate in the forest.7 One survey conducted in malaria clinics in Pailin Province over a decade ago identified P. knowlesi in two symptomatic Cambodian patients.8 In 2015, a malariometric survey in Pailin and Battambang provinces found 8 asymptomatic cases of P. knowlesi and 11 asymptomatic cases of P. cynomolgi.9 National surveillance of malaria in Cambodia is conducted by the National Center for Parasitology, Entomology, and Malaria Control, a central entity that collates data submitted by regional government-run healthcare facilities on monthly malaria cases.10 Cases are diagnosed by rapid diagnostic tests or blood microscopy, with a subset undergoing polymerase chain reaction (PCR) testing to confirm species classification (P. falciparum, P. vivax, P. ovale, and P. malariae). In the absence of routine national surveillance for P. knowlesi species, the true prevalence of disease in Cambodia remains unknown.
