Concerning malaria elimination, “WHO grants this certification when a country has proven, beyond reasonable doubt, that the chain of local transmission of all human (emphasis added) malaria parasites has been interrupted nationwide for at least the past 3 consecutive years.” This target is challenging enough, but becomes more complicated when we consider that zoonotic transmission of malaria among monkeys and humans has been documented in Brazil and Southeast Asia. We cannot expect monkeys to sleep under bednets, so creative and realistic solutions are needed.
The Malaria Eradication Research Agenda (malERA) recognizes this problem. Plasmodium knowlesi, originally found in macaque monkeys in Southeast Asia has been dubbed the fifth human malaria due to its spread to people as deforestation has disturbed the habitat of the monkeys. In particular malERA addresses the challenge of understanding the upward trend of this malaria infection in that region and the need for better understanding of transmission dynamics and proper diagnosis.
The danger of P. knowlesi is heightened by difficulties in diagnosing it and distinguishing it from other malaria species. “Recently, the prevalence of human infection with a simian malaria parasite, P. knowlesi, has become an important issue in a wide area of Southeast Asia. The identification of this parasite by microscopy is very difficult because it resembles the P. malariae parasite. However, the symptoms caused by P. malariae and P. knowlesi are very different, with only P. knowlesi causing severe and life-threatening malaria” (Komaki-Yasuda et al.)
Reports from Brazil highlight another ‘simian hotspot.’ While P. Knowlesi represents monkey infections reaching humans, the opposite may have happened to establish a reservoir in the New World. “P. vivax lineages appearing to originate from Melanesia that were putatively carried by the Australasian peoples who contributed genes to Native Americans. Importantly, mitochondrial lineages of the P. vivax-like species P. simium are shared by platyrrhine monkeys and humans in the Atlantic Forest ecosystem, but not across the Amazon, which most likely resulted from one or a few recent human-to-monkey transfers.” But looking even further back in natural history, Escalante and colleagues found, “compelling evidence that P. vivax is derived from a species that inhabited macaques in Southeast Asia.”
A recent study in this area found the worrying results that, “The low incidence of cases and the low frequency of asymptomatic malaria carriers investigated make it unlikely that the transmission chain in the region is based solely on human hosts, as cases are isolated one from another by hundreds of kilometers and frequently by long periods of time, reinforcing instead the hypothesis of zoonotic transmission.”
In Africa, Linda Duval and co-researchers, who found P. falciparum in blood samples from two chimpanzees belonging to two different subspecies, warn that, “If malignant malaria were eradicated from human populations, chimpanzees, in addition to gorillas, might serve as a reservoir for P. falciparum,”
It appears that the dynamics between monkeys, malaria and humans has a long history. Even once certified malaria-free countries face the threat of imported malaria from people crossing borders. Now we must recognize that the threat may already live within borders. So since existing malaria interventions to protect humans from malaria cannot be applied to monkeys, accelerated research on the genetics of the parasite and the mosquito is needed to prevent both primate groups from getting malaria.