Plasmodium cynomolgi is a species of malaria with Southeast Asian macaques as its natural host, transmitted by the bites of the Leucosphyrus forest group of Anopheles mosquitoes, which exhibit relapses upon activation of hypnozoites similar to P. vivax [4, 7, 11, 12]. This report describes three people enrolled in a malaria surveillance study in Thailand who were found to have P. cynomolgi infection, although after initial microscopic diagnosis and treatment for P. vivax. The morphological characteristics shown on the blood smears in Fig. 2 are present in both species, with structural similarities also evident, as described by Kosaisavee et al. . For case C, who had a co-infection with P. cynomolgi and P. vivax, it was not possible to accurately identify the individual parasite species even in the thin smear, and the parasitaemias in cases A and B were too low to confidently locate the parasites and characterize the morphology. Currently used malaria RDTs are not adequate diagnostic tools for P. cynomolgi. The test antigens are either pan-Plasmodium (for example, aldolase or lactate dehydrogenase (LDH)) or P. falciparum or P. vivax specific, and sensitivity in pan-Plasmodium RDTs detecting non-falciparum or non-vivax malaria species are quite variable . Cross-reactivity between P. vivax and P. cynomolgi LDH in the laboratory has recently been demonstrated , but it is not clear that this would translate to accuracy in a field-deployed TDR. In addition, the low parasitaemia observed in P. cynomolgi may further reduce the sensitivity of the TDR. With the difficulties of blood film diagnosis, even for skilled/experienced microscopists, and the lack of utility of RDTs, diagnostic testing by PCR or other molecular methods will likely be required.
The only other publication on P. cynomolgi prevalence in Thailand carried out PCR tests on 1152 archived samples of malaria patients in the provinces of Tak, Ubon Ratchathani, Chanthaburi, Yala and Narathiwat during the period of 2007 to 2017 . There were nine P. cynomolgi infections detected, all co-infections: P. cynomolgi with P. vivax (n = 7), with P. falciparum (n = 1), or with both P. vivax and P. knowlesi (n=1). The cases were spread over several years, diagnosed between April and December (the rainy season is May to October) and found in all provinces, although Yala had five of the nine cases (55%). In these P. cynomolgi clinical cases as of 2021, two of the three were mono-infections, which is the first time this has been reported in Thailand. There is a case report of P. cynomolgi mono-infection of a European tourist traveling through Thailand (Surat Thani province) and Malaysia . However, the origin of the infection could not be confirmed.
With a first microscopic diagnosis of P. vivax, the patients were not asked about a history of contact with macaques. During follow-up visits by the Yala study team, cases A and B reported the presence of macaques near their homes. In Thailand, the main hosts of P. cynomolgi, P. knowlesi, Plasmodium inuiand Plasmodium coatneyi are macaca fascicularis and macaca nemestrinawith recent reports in stump-tailed macaques, macaca arctoides . Simian malaria co-infections are not uncommon in macaques, with the presence of two or three species detected simultaneously in 18% to 40% of monkeys [16, 17]which may explain why some human studies report more co-infections than mono-infections [2, 5]. Plasmodium cynomolgi was first reported as a mono-infection in a Malaysian woman in 2014 and so far cases have been reported from both Peninsular Malaysia and Borneo in Malaysia, the latter where P. knowlesianother simian malaria is endemic [5, 7]. There have been six other studies reporting the prevalence of P. cynomolgi in humans in Southeast Asia, shown in Table 1.
To date, most publications reporting on the P. cynomolgi infections are retrospective tests of blood samples. In both clinical case reports of mono-infection and past experimental infections in humans [1, 2, 18], undifferentiated flu-like symptoms were present, with symptoms occurring at very low parasitaemias and not progressing in severity. In humans, the antimalarial treatment required to P. cynomolgi is not well studied, but the macaques in P. cynomolgi drug and vaccine studies respond well to chloroquine and primaquine, diet P. vivax in Thailand . All Yala patients recovered quickly and there were no recurrences during the three months of active follow-up. The low prevalence of monkey malarias infecting humans means that the parasites are not subject to frequent selection pressure from antimalarial drugs and should remain susceptible to treatment. . In the study by Imwong et al. two Cambodian individuals were found to have P. cynomolgi again three months after the initial diagnosis, but it could not be determined whether it was a relapse, a new infection or a persistent blood stage infection.
the P. cynomolgi survey by Putaporntips et al.  demonstrated that P. cynomolgi has been infecting humans in Thailand for 15 years and is likely underdiagnosed. However, of the published studies reviewed here, the prevalence of P. cynomolgi was less than 1.5% in the samples tested. In Thailand, the first clinical case P. knowlesi was reported in 2004, and in 2017 cases began to be regularly reported by the Thai Ministry of Public Health, peaking at 53 cases in 2021 [20, 21]. It is not yet known whether the increase in human infections with simian malaria is due to better detection methods, the result of human encroachment on macaque habitats, or both. The three Yala patients were diagnosed separately in time and space, although Yala province borders Malaysia’s Perak and Kelantan states where P. cynomolgi has been documented [7, Fig. 1]. Whole genome sequencing of the isolates is planned, which will allow lineage comparisons between these three cases as well as with available data on cases in neighboring Malaysian states. . To mitigate the potential spread of P. cynomolgi and P. knowlesi and stay on track to eliminate malaria, heightened vigilance will be required for any signs of increased transmission in Yala and other areas of Thailand where exposure to macaques is possible.