Human malaria has been known to be caused by 4 Plasmodium species, with Plasmodium falciparum causing the most-severe disease. Recently, numerous reports have described human malaria caused by a fifth Plasmodium species, Plasmodium knowlesi, which usually infects macaque monkeys. Hundreds of human cases have been reported from Malaysia, several cases have been reported in other Southeast Asian countries, and a few cases have been reported in travelers visiting these areas. Similarly to P. falciparum, P. knowlesi can cause severe and even fatal cases of disease that are more severe than those caused by the other Plasmodium species. Polymerase chain reaction is of value for diagnosis because P. knowlesi infection is easily misdiagnosed as less dangerous Plasmodium malariae infection with conventional microscopy. P. knowlesi infection should be suspected in patients who are infected with malaria in Southeast Asia. If human-mosquito-human transmission were to occur, the disease could spread to new areas where the mosquito vectors live, such as the popular tourist areas in western India.
Four species have been known to bring on human malaria, the most severe disease being caused by Plasmodium falciparum. In 2007, after returning from Malaysia, a Finnish tourist was found to be infected with a fifth Plasmodium species, P. knowlesi which usually infects macaques. Over the past few years, hundreds of human cases have been found in Malaysia. The clinical disease caused by P. knowlesi appears less severe than P. falciparum infection, but more severe than infection with other malaria-causing species. Diagnosis is based both on PCR and microscopy. P. knowlesi is currently. considered as the fifth species causing malaria in humans.
In 2007, a Finnish traveler was infected in Peninsular Malaysia with Plasmodium knowlesi, a parasite that usually causes malaria in monkeys. P. knowlesi has established itself as the fifth Plasmodium species that can cause human malaria. The disease is potentially life-threatening in humans; clinicians and laboratory personnel should become more aware of this pathogen in travelers.