P. knowlesi occurs in long-tailed and pig-tailed macaques monkeys commonly found in forested areas of South East Asia. It was considered rare for P. knowelsi to occur in humans, however, recent studies have found large numbers of infected patients in Malaysia, especially Sarawak and Malaysian Borneo, as well as Thailand, the Philippines, Singapore and Myanmar.

It may be that more cases have been misdiagnosed. P. malariae and P.knowlesi are difficult to distinguish under the microscope. Unlike the relatively benign form of P. malariae, P. knowlesi causes a high parasitaemia (lots of parasites in the blood) and replicates every 24 hours. This means that is has the potential to cause death in humans in a short period of time. P. malariae on the other hand, replicates every 72 hours and causes low levels of parasitaemia.

It is of paramount importance (as there have been a few cases of P.knowlesi even in short term travellers) that physicians attending to patients who have visited SE Asia, particularly forested areas, and have diagnosed P. malariae, consider P.knowlesi in a differential diagnosis.

P.knowlesi resolves rapidly following treatment with appropriate antimalarial
medication. In patients that are severely ill due to high parasitaemia, management and treatment will be the same as for P.falciparum.

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