Is a Monkey Malaria from Borneo an Emerging Human Disease?

Abstract

In 2004, a team of researchers from Malaysia surprised the medical community by reporting that a significant number of people on the island of Borneo were becoming infected with a malaria parasite of monkeys, Plasmodium knowlesi [1]. The world was still unaware of the extent of the problem until 2008 when the same research group gave us a clear view of the gravity of the situation [2]. They found that P. knowlesi infections accounted for 71% of the cases of malaria in one regional hospital. Of even more concern, the team has shown that this form of malaria is potentially life threatening. The medical community is now asking whether P. knowlesi is a ‘fifth human malaria’ with the potential to spread to other parts of Southeast Asia and beyond. Many species of malaria parasite that naturally infect monkeys will also develop in humans. As early as the 1930s, human infections by monkey malarias were studied under controlled conditions in a number of laboratories, including extensive studies carried out in the USA and Romania. These studies showed that although the infections were not initially life threatening, they became increasingly virulent when cycled through humans in the absence of an intermediate passage through mosquitoes [William C, Pers. Comm] [3]. The virulence associated with the Malaysian infections raises the possibility that a new and more virulent strain of P. knowlesi is involved as well as the possibility that, in the cases of more virulent infection, some means of parasite transmission other than that initiated by the bite of an infected mosquito is playing a role. First reported in the Lancet in 2004, Singh and collaborators described their investigation of 208 cases of malaria from the Kapit division of Malaysian Borneo [1]. Their interest had arisen because of the severity of the disease in patients that had been diagnosed as being infected with Plasmodium malariae, which causes a relatively benign form of human malaria. They found that over half of these cases were the result of infection with a monkey malaria, P. knowlesi. In 2008, the group published the results of a more extensive study on the distribution of human infection by P. knowlesi in Sarawak and Sabah in Borneo and Pahang peninsular Malaysia. They demonstrated that these infections are occurring in significant numbers in many of the district hospitals. Furthermore, their investigation of four deaths in the region indicated high levels of infection with P. knowlesi and hepatorenal dysfunction, which is often associated with high parasitemias. This suggested that infection with P. knowlesi is potentially life threatening. Singh, Cox-Singh and Conway now proceed with their ground-breaking research on several fronts, including epidemiological studies to unravel the complicated story of how this emerging disease is being transmitted and further investigation of fatalities in the region to determine the extent to which the threat presents to the people of their region.