A malaria parasite from gorillas has been found in an African monkey, suggesting it has jumped species and may be able to transfer to humans.
The finding has led some malaria experts to suggest that if transfer between monkeys and apes has occurred then monkey-to-human malaria transmission may already be happening. They have called for more research to quantify the risks.
"The evidence is sufficient to warrant further investigation into the possibility that these parasites may also jump to humans," said Beatrice Hahn, a professor of medicine at the University of Alabama at Birmingham, United States. "We need to screen humans who live in flying range of mosquitoes that also bite primates, to establish whether they are susceptible to the primate parasites."
Wild forest-living gorilla populations are known to harbour a parasite strain that is closely related to the human malaria parasite Plasmodium falciparum. And macaque monkeys in South-East Asia carry another malaria parasite, Plasmodium knowlesi — a potential threat to humans.
But this is the first time that a P. falciparum strain similar to the one that causes human malaria has been found in an African monkey — the spot-nosed guenon from Gabon (Cercopithecus nictitans).
The fact that "the genetic differences from the human strain are so slight" raises the possibility that monkey and ape malaria may be transmitted to humans, said François Renaud, a researcher at the French National Centre for Scientific Research, in Montpelier, and co-author of the study published in the Proceedings of the National Academy of Sciences (5 July).
As humans come into closer contact with apes and monkeys as a result of deforestation, commercial hunting and population growth, the opportunity for the parasites to be transmitted to humans will increase.
"One single successful cross-species transmission event has the potential to result in a major human pandemic," Hahn, who was not involved in the study, told SciDev.Net.
But David Conway, professor of biology at the London School of Hygiene & Tropical Medicine, United Kingdom, said the reservoir of malaria in African monkeys must be very small, given the low prevalence found in this study.
"Hopefully, monkey malaria will start to be recognised as an important area of research, but when examining the public health significance for humans, it is important to put the risk into context. Normal human malaria has a much higher prevalence, except in parts of South-East Asia where this has been reduced and the importance of malaria from monkeys has become more noticeable," Conway said.
Looking for human infections with monkey malaria is "like looking for a needle in a haystack", he said, adding that "there is every chance that human infections are occurring occasionally in the forest".
"In this particular case, the vector of malaria is the key determinant in determining any public-health risk," Conway said. "Identifying which species of mosquitoes transmits each parasite strain is a neglected area of research that needs additional funding."