Monkey Malaria Infects Increasing Numbers of People in Southeast Asia
A species of malaria parasites previously confined to monkeys is triggering virulent disease in increasing numbers of people in Borneo, leading one malaria expert to warn it may be emerging as a “fifth human malaria.”
Currently four species of plasmodium infect humans: plasmodium vivax, plasmodium falciparum, plasmodium malariae and plasmodium ovale. Fifty years ago, NIH scientists in Malaysia established that monkey malaria, plasmodium knowlesi, carried by an Anopheles species confined to the upper canopy, was rare in humans, if it occurred at all.
In 2004, a PCR study in the Kapit division of Sarawak state in Borneo revealed that all infections that had been diagnoses as malariae were actually cases of knowlesi (and other non-malariae species). This news passed without much notice, since at that time, P. knowlesi infections were considered rare, and mild in humans, similar to the p. malariae infections they were misdiagnosed as, which are never life-threatening and do not require prompt, immediate treatment.
Then, in late 2004 and early 2005, four people died of what was diagnosed as p. malariae in Sarawak. That’s suspicious since malariae isn’t a killer.
Researchers now know that in fact plasmodium knowlesi killed these patients.
Far from being mild, it turns out that these plasmodium knowlesi parasites replicate faster than any known primate malaria. Without immediate treatment, knowlesi can kill. All four of the recorded deaths occurred in hospitals, among people between the ages of 40 and 70, after treatment with antimalarials that came too late.
Far from being rare, knowlesi infections in humans appear to be widespread in parts of Borneo as well as in peninsular Malaysia, and parts of Thailand and China. At one district hospital in Borneo, researchers found that 71 percent of malaria cases had been caused by knowlesi.
Whether plasmodium knowlesi is crossing over from long- and pig-tailed macaques into humans, or is simply opportunistically infecting humans in monkey territory is unknown. The mosquitoes that carry knowlesi do bite both humans and monkeys, but they are generally confined to the upper canopy. Humans make themselves vulnerable to these mosquitoes as they increasingly encroach upon forest areas. That’s been happening for years.
The big question is whether knowlesi can establish a chain of transmission in humans without a monkey in the middle. As of yet, there is no evidence of human-to-human transmission. However, the forms of the parasite infective to mosquitoes (gametocytes) do develop in humans.
The other big question is: why now? The most dramatic change in the ecology of malaria in the region recently has been the great success of the malaria control programs, which have reduced infections by 90 percent in some places. “It has been established for nearly 80 years that one species of malaria parasite will suppress the growth of another species,” writes the NIAID’s Thomas McCutchan in the journal Future Microbiology. “This idea could provide a simple answer for the apparent proliferation of P. knowlesi infections in Borneo.”
In other words, get rid of vivax and falciparum, and knowlesi could rush in to fill the gaps. With the vast reservoir of malaria parasites in primates (not to mention birds, lizards, and the rest), that’s a frightening thought, indeed.
For more:
McCutchan TF et al. Is a monkey malaria from Borneo an emerging human disease?. Future microbiology (2008) vol. 3 pp. 115-8
Cox-Singh et al. Plasmodium knowlesi malaria in humans is widely distributed and potentially life threatening. Clin Infect Dis (2008) vol. 46 (2) pp. 165-71
