Plasmodium knowlesi – an emerging pathogen

Ten years have passed since the publication of a large focus of Plasmodium knowlesi infections in the human population. The discovery was made during a molecular investigation of atypical P. malariae cases in the Kapit Health Division, Sarawak, Malaysian Borneo. Patients were more symptomatic with higher parasite counts than expected in P. malariae infections. The investigation found only P. knowlesi DNA present in patient blood samples. Morphological similarity had allowed P. knowlesi to masquerade as P. malariae during routine diagnostic microscopy for malaria. P. knowlesi, a malaria parasite of macaque monkeys, had entered the human population. The subsequent development of P. knowlesi species-specific PCR assays soon demonstrated that the entry was not confined to the Kapit Division but extended across island and mainland Southeast Asia. Relevant clinical descriptions and guidelines for the treatment and management of patents with P. knowlesi malaria were not available. Nor was it clear whether P. knowlesi had undergone a host switch event into the human population or if infections were zoonotic. The outputs of studies on P. knowlesi malaria during the past 10 years will be summarized, highlighting major findings within the context of pathophysiology, virulence, host switch events, treatment, control and importantly malaria elimination.