The nested PCR assay and molecular characterization confirmed that the patient was infected with P. knowlesi. Since the incubation period of P. knowlesi in the liver is 9 to 12 days [2], this indicates that he most probably acquired his infection while jungle trekking in the Bario Highlands of Sarawak, Malaysian Borneo.
Plasmodium knowlesi parasites morphologically resemble those of P. malariae [1, 2, 10] and PCR assays can correctly differentiate between the two species [3]. However, PCR assays are not a rapid method of detection and are not a viable option for routine diagnosis. Rapid diagnostic tests are becoming an increasingly popular method for diagnosis of malaria [11], particularly by doctors on call and medical laboratory technologists in non-malaria endemic countries, who are not experienced in identifying malaria parasites by microscopy. It is possible that the low parasitaemia may have been a contributing factor to the inability of the rapid diagnostic test to detect knowlesi malaria in this Swedish patient, since these tests are not as sensitive as microscopy [12]. In a recent study of monoclonal antibodies against Plasmodium lactate dehydrogenase that have been developed for use in a rapid diagnostic test, it was shown that certain antibodies that were thought to be specific for P. falciparum and P. vivax, also bind to P. knowlesi [13]. However, it remains to be determined whether this, and commercially available rapid diagnostic tests, are able to detect P. knowlesi infections by examining a larger number of patient samples.
Early identification and adequate anti-malarial treatment of suspected cases are essential since P. knowlesi has a 24-hr erythrocytic cycle, and high parasitaemias with fatal outcome in humans can occur [4]. Human P. knowlesi infections resolve rapidly following treatment with chloroquine [3] and the current report indicates that mefloquine is also an effective anti-malarial for knowlesi malaria. However, in severely ill knowlesi malaria patients with high parasitaemias, management and treatment as for severe falciparum malaria has been recommended due to the rapid rate at which P. knowlesi can multiply [4].
Previous reports of human knowlesi malaria infections were of people that have either lived or worked in the forest and forest fringe areas of south-east Asia [10]. There has been a recent report of a Finnish traveller, who returned from Peninsular Malaysia with knowlesi malaria [14]. Taken together with this report of P. knowlesi infection in a Swedish traveller returning from Malaysian Borneo, this indicates that knowlesi malaria infections in travellers are probably not rare events, particularly since south-east Asia has become a popular tourist destination.
In conclusion, this report shows that P. knowlesi can infect short-term visitors to areas in south-east Asia, where knowlesi transmission occurs, and mefloquine was an effective anti-malarial. The P. knowlesi infection in the Swedish traveller was not detected by a rapid diagnostic test and evaluation of rapid diagnostic tests with further samples are necessary since early identification and appropriate anti-malarial treatment of suspected cases are essential, due to the rapid growth and potentially pathogenic nature of knowlesi malaria. Physicians should, therefore, be aware that knowlesi infection is an important differential diagnosis in febrile travellers with a recent travel history to forested areas in Southeast Asia, including short-term travellers, who tested negative with rapid diagnostic tests.