A form of malaria parasite that has spread from howler monkeys to humans in Brazil has been identified by researchers, raising concerns for eradication of the disease in Brazil and beyond.

Malaria was thought to have been eradicated from southern and south-eastern Brazil 50 years ago, but more than 1,000 cases reported since 2006 from the Atlantic Forest region, in Rio de Janeiro state, including two outbreaks in 2015 and 2016, led researchers to investigate.

The initial assumption was that the new cases were caused by the Plasmodium vivax malaria parasite, one of the main global species. Most of the people affected were visitors, who might have been infected elsewhere.

But analysis of DNA samples has now established that some of the infections did not come from a human malaria parasite. They were discovered to be a zoonotic malaria caused by Plasmodium simium, a parasite that normally lives only in monkeys.

Zoonotic diseases are those transmitted from animals to humans, and this is only the second discovery of a zoonotic parasite. The previous one was found in macaque monkeys and is responsible for a high proportion of the human malaria cases in south-east Asia.

The researchers, who have published their findings in the Lancet Global Health journal, say they do not believe malaria is being transmitted from one human to another via the bites of mosquitos carrying the simium parasite. But there are concerns for the global effort to stamp out the disease.

“There is no evidence that zoonotic malaria can be transmitted from human to human via mosquitoes,” said Dr Patrícia Brasil, of the Instituto Nacional de Infectologia Evandro Chagas in Rio de Janeiro, who is one of the authors. “In addition there is no current threat to people in the city of Rio de Janeiro, or in other non-forest areas of the Rio de Janeiro state, where transmission of the disease does not exist.

“However, its unique mode of transmission via monkeys and the fact that it occurs in areas of high forest coverage mean that zoonotic malaria poses a unique problem for malaria control efforts and may complicate the drive towards eventual elimination of the disease. Although benign and treatable, visitors should follow measures to avoid insect bites when going into the forest.”

The scientists focused on 49 cases of malaria reported in 2015-16, mostly middle-aged men who lived in urban areas of Rio de Janeiro state and who had visited the forest for leisure or work-related activities. They successfully sequenced the DNA in 33 cases and found they were all the monkey parasite. Their conclusion is that many past cases will have been misdiagnosed as caused by the human vivax parasite.

In a linked comment in the journal, Matthew J Grigg, of the Menzies School of Health Research in Darwin, Australia and Georges Snounou, of the Sorbonne University in Paris, France, say it is imperative that studies establish whether this is the simium parasite or whether it might be a reservoir of vivax in Brazilian monkeys, which “would pose a substantial threat to malaria elimination throughout the continent and possibly beyond”.

