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Plasmodium knowlesi may be common cause of malaria in Malaysia

18 Jan 2008

Paul Chinnock

Source: Wellcome Trust (see original article)

Researchers in Malaysia found that more than one in four of the malaria patients they examined were infected with Plasmodium knowlesi, a malaria parasite of macaque monkeys. P. knowlesi seems therefore to be more widespread in Malaysia than previously thought. Infections are usually misdiagnosed as the normally uncomplicated human malaria caused by P. malariae.

The research, published in the journal Clinical Infectious Diseases, was conducted by Professors Janet Cox-Singh and Balbir Singh, with colleagues at the University Malaysia Sarawak and three State Departments of Health in Malaysia. The research team made use of DNA-based technology with more than 1,000 samples from malaria patients across Sarawak, Malaysian Borneo.

Malaria, which kills more than one million people each year, is caused when Plasmodium parasites are passed into the bloodstream from the salivary glands of mosquitoes. Some types, such as P. falciparum, found most commonly in Africa, can be deadly. P. malariae, found in tropical and sub-tropical regions across the globe, is often known as “benign malaria” as its symptoms are usually less serious than other types of malaria.

Until recently, P. knowlesi, was thought to infect only monkeys, in particular long-tailed macaques found in the rainforests of South East Asia. Natural infections of humans were thought to be rare until they were described in one area in Sarawak. Under the microscope, the early parasite stages of P. knowlesi look very similar to P. falciparum, the most severe form of human malaria, while the later parasite stages are indistinguishable from the more benign P. malariae. Misdiagnosis as P. falciparum is clinically less important, as P. falciparum infections are treated with a degree of urgency and P. knowlesi responds to the same treatment. However, misdiagnosis as the more benign, slower growing parasite P. malariae is a problem.

P. knowlesi is unique among the malaria parasites of humans and non-human primates in that reproduces every 24 hours, and one of the features of fatal P. knowlesi infections is the high number of infected red blood cells in these patients. Therefore, even a short delay in accurate diagnosis and treatment could lead to the rapid onset of complications, including liver and kidney failure, and death.

“I believe that if we look at malaria infections in South East Asia more carefully, we will find that this potentially fatal type of the disease is more widespread than is currently thought,” says Professor Cox-Singh. “Given the evident severity of the illness that it causes, I would recommend that doctors treating patients with a laboratory diagnosis of P. malariae remain alert to the possibility that they may be dealing with the potentially more aggressive P. knowlesi. This would be particularly important in patients who have spent time in the forest fringe areas of South East Asia where the non-human primate host exists.”