Plasmodium Knowlesi - Epidemiology

Epidemiology

P. knowlesi infection is normally considered a parasite of long-tailed (Macaca fascicularis) and pig-tailed (Macaca nemestrina) macaques but humans who work at the forest fringe or enter the rainforest to work are at risk of infection. With the increasing popularity of deforestation and development efforts in South East Asia, many macaques are now coming in close and direct contact with humans. Hence more and more people who live in the semi-urban areas are being found to be infected with knowlesi malaria.

This parasite is mostly found in South East Asian countries particularly in Borneo, Cambodia, Malaysia, Myanmar, Philippines, Singapore, Thailand and neighboring countries and it appears to occur in regions that are reportedly free of the other four types of human malaria. Infective mosquitoes are restricted to the forest areas. Non-infective mosquitoes are typically found in the urban areas but transmission may occur due to the abundance of mosquitoes in this region. particularly Malaysia, but there are also reports on the Thai-Burmese border. One fifth of the cases of malaria diagnosed in Sarawak, Malaysian Borneo are due to P. knowlesi.

Plasmodium knowlesi is absent in Africa. This may be because there are no long-tailed and pig-tailed macaques (the reservoir hosts of P. knowlesi) in Africa and many West Africans lack the Duffy antigen - a protein on the surface of the red blood cell that the parasite to uses to invade.

P. knowlesi is the most common cause of malaria in childhood in the Kudat district of Sabah, Malaysia.