Background: Although malaria transmission was eradicated from southeast Brazil, a significant increase in the number of Plasmodium vivax-like autochthonous human cases has been reported in remote areas of the Atlantic Forest in the last decades in Rio de Janeiro (RJ) state, including an outbreak in 2015-2016. The singular clinical and epidemiological aspects of several human cases combined with molecular and genetic data revealed that they were due to the non-human primate (NHP) parasite P. simium. The full understanding of the epidemiology of the autochthonous malaria in southeastern Brazil depends, however, upon the knowledge on the circulation of NHP Plasmodium in the foci and the determination of its reservoirs. Methodology: A large sampling effort was carried out in the Atlantic forest of RJ and its bordering states (Minas Gerais, Sao Paulo, Espirito Santo) for capture and examination of free-living NHPs. Blood and/or viscera were analyzed for Plasmodia infections through molecular and microscopic techniques. Principal findings: In total, 146 NHPs of six species, from 30 counties in four states were tested. Howler monkeys (A. guariba clamitans) were the only NHP species found infected. In RJ, 26% of howlers were positive, among them 17% were found to be infected with P. simium. Importantly, specific single nucleotide polymorphisms were detected in all P. simium infected howlers regardless geographical origin of malaria foci. Interestingly, 71% of P. simium infected NHP were from the coastal slope of a mountain chain (Serra do Mar), where most human cases have been occurring. P. brasilianum/malariae was detected for the first time in 14% free-living howlers in RJ as well as in 25% of those from the Espirito Santo state. Moreover, malarial pigment was detected in spleen fragments of 50% of a subsample composed of howler monkeys found dead in both RJ and ES. All NHPs were negative for P. falciparum. Conclusions/Significance: Our data indicate the howler monkeys as the main reservoir of the Atlantic forest human malaria in RJ and other sites in Southeast Brazil and reinforce its zoonotic nature.

Howler monkeys are the reservoir of malaria parasites causing zoonotic infections in the Atlantic forest of Rio de Janeiro