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

Abstract

Background

Although malaria cases have substantially decreased in 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 past few decades in Rio de Janeiro (RJ) state, including an outbreak during 2015–2016. The singular clinical and epidemiological aspects in several human cases, and collectively with molecular and genetic data, revealed that they were due to the non-human primate (NHP) parasite Plasmodium simium; however, the understanding of the autochthonous malarial epidemiology in Southeast Brazil can only be acquired by assessing the circulation of NHP Plasmodium in the foci and determining its hosts.

Methodology

A large sampling effort was carried out in the Atlantic forest of RJ and its bordering states (Minas Gerais, São Paulo, Espírito Santo) for collecting and examining free-living NHPs. Blood and/or viscera were analyzed for Plasmodium infections via molecular and microscopic techniques.

Principal findings

In total, 146 NHPs of six species, from 30 counties in four states, were tested, of which majority were collected from RJ. Howler monkeys ( Alouatta clamitans) were the only species found infected. In RJ, 26% of these monkeys tested positive, of which 17% were found to be infected with P. simium. Importantly, specific single nucleotide polymorphisms–the only available genetic markers that differentiate P. simium from P. vivax–were detected in all P. simium infected A. clamitans despite their geographical origin of malarial foci. Interestingly, 71% of P. simium infected NHPs were from the coastal slope of a mountain chain (Serra do Mar), where majority of the human cases were found. Plasmodium brasilianum/malariae was initially detected in 14% and 25% free-living howler monkeys in RJ and in the Espírito Santo (ES) state, respectively. Moreover, the malarial pigment was detected in the spleen fragments of 50% of a subsample comprising dead howler monkeys in both RJ and ES. All NHPs were negative for Plasmodium falciparum.

Conclusions/Significance

Our data indicate that howler monkeys act as the main reservoir for the Atlantic forest human malarial parasites in RJ and other sites in Southeast Brazil and reinforce its zoonotic characteristics.

Author summary

The present work comprises an unprecedented capture effort and large-scale field survey of Plasmodium species in non-human primates (NHPs) in RJ, a state recording three-decade history of autochthonous human cases of benign tertian malaria lacking epidemiological clarification of their origin. This is the first study to describe the infection rates by Plasmodium spp. in free-living NHPs in RJ, thereby matching the spatial distribution of P. simium in NHP with that of the local human cases of benign tertian malaria occurring due to this parasite. This study confirmed howler monkeys as the only reservoir of this zoonotic malarial parasite in RJ and reported that specific single nucleotide polymorphisms were present in all P. simium infected howler monkeys, despite their geographical origin of malarial foci. Moreover, this is the first study to record P. brasilianum/malariae in free-living NHPs from RJ, and to illustrate their widespread distribution in this state. Collectively, these findings help us in evaluating the simian malaria prevalence in the Atlantic Forests and in assessing the zoonotic characteristics of autochthonous human malaria in Rio de Janeiro, thus providing assistance in shaping surveillance and control.