Malaria Transmission From Monkeys to Humans, Study Finds

Janis C. Kelly

September 07, 2017

Southeastern Brazil has been free from endemic malaria for more than 50 years. Now, a malaria outbreak in the area has been traced to zoonotic transmission from monkeys in the Atlantic Forest region of Rio de Janeiro state, Brazilian researchers report in an article published online August 31 in the Lancet Global Health.

These Plasmodium simium infections, many of which were initially misidentified as the typical human parasite Plasmodium vivax, are the first identified monkey-to-human transmission for the species. P simium does not produce particularly severe forms of human malaria, but the existence of a monkey reservoir for a malaria parasite capable of infecting humans will likely complicate efforts to eliminate malaria in Brazil.

Of note, 27 of the 39 malaria cases examined by the researchers were associated with ecotourism (25 visitors, 2 residents).

"[T]he malaria outbreaks in 2015 and 2016 in the Atlantic Forest of southeastern Brazil were caused by P simium, previously considered to be a monkey-specific species of malaria parasite that is related to but distinct from P vivax, and which has never conclusively been shown to infect human beings before," write Patrícia Brasil, MD, from the Instituto Nacional de Infectologia Evandro Chagas, Rio de Janeiro, Brazil, and colleagues.

"Such zoonotic transmission of a malaria parasite from a monkey reservoir to human beings has immediate consequences for public health in this region, and for future attempts to control and eventually eliminate malaria in Brazil. Thorough screening of the local non-human primate and mosquito (Anopheline) populations in this area is required to evaluate the extent of this newly recognised zoonotic threat to public health."

Malaria was once prevalent throughout Brazil, but was eliminated from all except the northern Amazon region through extensive malaria control efforts. Between 2006 and 2014, 43 autochthonous cases of malaria were reported in the Atlantic Forest area in southern Brazil.

This study was undertaken when that number jumped dramatically, with 33 cases in 2015 and 16 in 2016. Most of the patients lived in urban areas of Rio de Janeiro state and had visited the Atlantic Forest for leisure or work.

Fever was the main symptom, none of the patients required hospitalization, and all made full recoveries after treatment with chloroquine and primaquine.

The researchers collected samples from 33 cases for mitochondrial DNA sequencing, which was successfully completed for 28 samples. All were found to be P simium, which indicates zoonotic transmission, the authors explain. Sequencing of the full mitochondrial genome in samples from 3 patients showed that P simium is closely related to P vivax from South America.

In a linked comment, Matthew J. Grigg, PhD, from the Menzies School of Health Research, Darwin, Australia, and Georges Snounou, PhD, from the Sorbonne University, Paris, France, note that human beings have previously been shown to be susceptible to experimental infection by 8 of 25 malaria parasite species found in nonhuman primates. Moreover, one of those, P knowlesi, is now known to be the main cause of malaria in in Malaysia.

That said, P knowlesi has not shown evidence of human-to-human transmission in Asia. Whether that will be true for P simium in Brazil remains unclear. "The epidemiological data presented by Brasil and colleagues are most consistent with zoonotic-only transmission, including an absence of sequential case-clustering, and predominantly urban-residing older males with a history of recent travel to forest areas," the editorialists write.

Nonetheless, "[a]n anthropozoonotic P vivax reservoir in Brazilian monkeys would pose a substantial threat to malaria elimination throughout the continent and possibly beyond, including potential onwards transmission from hypnozoite relapses," they continue.

Dr Brasil said in a press statement, "There is no evidence that zoonotic malaria can be transmitted from human to human via mosquitoes. 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 authors and commentators have disclosed no relevant financial relationships.

Lancet Global Health. Published online August 31, 2017. Article full text, Comment full text

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