Malaria is caused by the protozoan parasite members of the genus Plasmodium and over 250 Plasmodium spp. have been described in mammals, birds, rodents and reptiles [1, 2, 3]. More than 30 malaria species have been reported in non-human primates and the following have been either naturally acquired or experimentally transmitted to humans by mosquitoes: P. cynomolgi, P. knowlesi and P. inui from Old World monkeys, P. brasilianum and P. simium from New World monkeys and P. schwetzi from chimpanzees [2, 4–7].
Plasmodium knowlesi was first reported as a significant cause of human malaria in Malaysia in 2004 . Subsequently, naturally-acquired human infections with P. knowlesi were documented in several other countries in Southeast Asia including Thailand [9, 10], Indonesia , Philippines , Singapore , Vietnam , Cambodia , Laos  and Myanmar . Furthermore, travelers have returned to their home countries after visiting Thailand and other Southeast Asian countries with knowlesi [18, 19]. In Thailand, the first locally acquired natural infection with P. knowlesi was reported in 2004, in a patient who had visited the forest in Prachuap Kiri Khan Province, Southern Thailand near the Myanmar border . Subsequently, P. knowlesi infected patients have been reported in Tak, Chantaburi, Yala, Narathiwat, Prachuap Kiri Khan and Ranong Provinces [20–22]. These areas are located near the borders of Cambodia, Myanmar and Malaysia. Tourists visiting Ranong Province, and South Western Thailand have also returned to their home countries in Germany [23–25] and France  with knowlesi malaria. Besides P. knowlesi, naturally acquired human P. cynomolgi infections have recently been reported in Peninsular Malaysia , Malaysian Borneo [28, 29] and Cambodia  and in a Danish tourist who had visited Peninsular Malaysia and Thailand . Although naturally-acquired human infections with P. inui have not been described, P. inui can cause malaria in humans by blood passage  or through mosquito bites in the laboratory .
The main natural hosts of P. knowlesi, P. cynomolgi and P. inui are long-tailed macaques (Macaca fascicularis) and pig-tailed macaques (M. nemestrina) which are found in nature in Southeast Asia . There have also been reports of a P. knowlesi infection in a banded leaf monkey (Presbytis melalophos) in Peninsular Malaysia  and a dusky leaf monkey (Semnopithecus obscurus) in Thailand . The other natural hosts of P. cynomolgi are M. radiata, M. cyclopis, M. sinica, M. mulatta, Presbytis cristasus and P. entellus . Plasmodium inui naturally infects many other monkey species too, including M. cyclopis, M. mulatta, M. radiata, Presbytis cristasus, P. obscurus and Cynopithecus niger . Besides the natural hosts described above, a number of non-human primate hosts can be experimentally infected with P. knowlesi and P. cynomolgi. Experimental hosts of P. knowlesi described to date have included Callithrix jacchus, Cebus spp., Cercocebus fuliginosus, C. cephus, Cynochepalus papio, Hoolock hoolock, M. radiata, M. arctoides, Papio doguera, P. jubilaeus, P. papio, Presbytis cristatus, Saimiri sciureus and Semnopithecus entellus . For P. cynomolgi experimental hosts have included M. mulatta, Cercopithecus aethiops, Cebus capucinus and P. papio .
In Thailand, six species of macaques have been identified in nature; M. fascicularis, M. nemestrina, northern pig-tailed macaque (M. leonina), rhesus macaque (M. mulatta), stump-tailed macaque (M. arctoides) and assamese macaque (M. assamensis) [35–39]. There have been only two studies undertaken to determine the prevalence of malaria in non-human primates in Thailand. Using molecular techniques, P. inui and P. coatneyi were identified in wild-caught M. fascicularis in Ranong Province, near the Myanmar border . Then in 2010, P. inui, P. coatneyi and P. knowlesi were described in wild-caught M. fascicularis and M. nemestrina in Yala and Narathiwat Provinces, in Southern Thailand near the Malaysian border . It is important to determine the geographical range of malaria-positive non-human primates to inform the public of the risks of acquiring zoonotic malaria. The objective of this study was to determine the prevalence of malaria parasites in non-human primates from 4 new locations in Thailand.