Monkey malaria could represent a new human strain
Monkey malaria may be more widespread among humans than previously thought and could represent a new strain of the disease more dangerous to humans, says a new study published in the UK-based medical journal, the Lancet (2004;363:1017-24).
The study's authors, Professor Balbir Singh from the Faculty of Medicine and Health Sciences at the Universiti Malaysia Sarawak and colleagues, found that the monkey malaria parasite, Plasmodium knowelsi, accounted for 58% of malaria cases in Kapit division in Sarawak, Malaysia. These cases had previously been misidentified as Plasmodium malariae, one of the four human parasites.
Blood samples taken between March 2000 and November 2002 from 208 patients with what was thought to be P. malariae were tested using genetic sequencing and 120 of these turned out to be P. knowelsi. The misdiagnosis is thought to have occurred due to similarities in appearance on thick blood-films between the two strains and the fact that laboratory technicians are only trained to recognize the four species of human parasites, Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and P malariae.
Nick White, Professor of Tropical Medicine at Mahidol University in Thailand and Oxford University in England, said in a commentary accompanying the study that repeated misdiagnosis of monkey malaria could explain occasional reports of malaria in people exploring or travelling through uninhabited jungle areas or nature reserves.
"There have been rare reports of natural human infections with monkey parasites but nothing on the scale reported by Balbir Singh and colleagues," said White in the commentary.
Bernard Nahlen, Senior Scientific Adviser at WHO's Roll Back Malaria programme in Geneva, added that there are many non-human strains of malaria, which can infect birds, reptiles and other mammals.
"The finding here that a monkey-strain has been found frequently in humans is the interesting point," said Nahlen.
The study raises the question of whether monkey malaria was already or would become capable of human to human transmission.
Whereas P malariae multiplies every three days in the blood and infections are never severe, P. knowlesi multiplies daily and is potentially dangerous.
However, John Barnwell, Chief of the Research and Development Laboratories Unit of the Malaria Branch Division of Parasitic Diseases at the US Centers for Disease Control and Prevention, pointed out that because antimalarials have not been used on monkeys, their parasites are still drug-sensititive and are easily treated with chloroquine. What is important is the potential for the emergence of a new human disease, said Barnwell.
Scientists have established that human-to-human transmission of monkey malaria is possible under laboratory conditions but so far have not found cases of natural transmission.
"The very high numbers of infections in this small area and close timescale could set up the potential to have natural human-to-human transmission happening now or in the near future," Barnwell said, referring to the study.
"This is how new diseases emerge all the time and the potential to establish a new human malaria is there," said Barnwell, adding that genetic data suggests that P vivax, the second major human malaria strain which first infected humans 40 000 to 60 000 years ago in south-east Asia, was derived from the local monkey malaria populations.
Barnwell said it would be interesting to monitor whether other species of monkey malaria such as Plasmodium cynomolgi which are present in monkeys in the Sarawak region of Borneo could be infecting humans too.
Dr Kevin Palmer, Regional Malaria Adviser at WHO's Regional Office for South-East Asia in Manila, agreed that more research was needed to establish the full implications of these findings.
"At this point it is clearly not a public health problem but if it turns out that P knowlesi has or is in the process of adapting to human transmission, we may be facing a future problem … a fifth species of human malaria," said Palmer.