Natural Plasmodium infections in Brazilian wild monkeys: Reservoirs for human infections?☆
Introduction
Two parasite species are responsible for simian malaria in the forests of continental America, namely Plasmodium brasilianum and Plasmodium simium. These parasite species naturally infect monkeys from the Cebidae and Atelidae families. P. brasilianum (Gonder and Berenberg-Gossler, 1908) has a wide geographic distribution and was found in the Amazon forest in Panamá, Venezuela, Peru and Brazil, as well as in the Atlantic forest in the Southern and Southeastern regions of Brazil (Clark, 1931, Porter et al., 1966, Serrano, 1967, Marinkelle and Grose, 1968, De Arruda, 1985, De Arruda et al., 1989, Deane, 1992, Lourenço-de-Oliveira and Deane, 1995). P. simium (Fonseca, 1951) has only been found in Alouatta (howler monkeys) from southern and southeast areas of the Atlantic forest in Brazil (Deane, 1992). However, its geographic distribution may be even wider, as its description to date is based only on microscopic analysis of Giemsa-stained thick and thin blood films, methods that are of limited sensitivity.
P. simium and P. brasilianum resemble human Plasmodium vivax and Plasmodium malariae, respectively. These similarities occur at the morphological, genetic and immunological level (Coatney, 1971, Coatney et al., 1971, Cochrane et al., 1985, Barnwell, 1986, De Arruda et al., 1989, Deane, 1992, Escalante et al., 1995, Gozalo et al., 1997, Fandeur et al., 2000, Volney et al., 2002, Duarte et al., 2006).
Phylogenetic studies based on 18S rDNA gene sequences, cytochrome b mtDNA and circumsporozoite protein of P. brasilianum/P. malariae and P. simium/P. vivax (including “classic” P. vivax VK210 and variant P. vivax VK247), strongly suggest that these species are not distinct (Lal et al., 1988, Goldman et al., 1993, Escalante et al., 1995, Escalante et al., 1998, Fandeur et al., 2000, Li et al., 2001, Leclerc et al., 2004); and that Plasmodium falciparum/Plasmodium reichenowi (the latter of which is a parasite of Pan troglodytes, an Old World ape) are very closely related (Lal and Goldman, 1991, Escalante et al., 1995, Escalante et al., 1998).
Such similarities gave rise to the hypothesis that monkeys may act as a reservoir for human malaria (Coatney, 1971, Lal et al., 1988, De Arruda et al., 1989, Deane, 1992, Gozalo et al., 1997, Fandeur et al., 2000, Li et al., 2001, Volney et al., 2002, Leclerc et al., 2004). The importance of potential non-human reservoirs was highlighted by recent findings in Southeast Asia indicating that host switches in malaria can occur if the ecological circumstances allow close contact between humans and non-human primates (Singh et al., 2004, Cox-Singh et al., 2008). In the context of the human–simian relationship and human–simian transmission in the neotropics, an understanding of the dynamics of the Plasmodium population is an important step towards clarification of their epidemiological implications for human disease, particularly in residual foci of human autochthonous malaria cases in extra-Amazonian endemic areas of Brazil (Curado et al., 1997, Curado et al., 2006, Duarte et al., 2006, Cerutti et al., 2007).
The detection of asexual parasites by microscopy in GIEMSA-stained thick and thin blood films remains the standard method for diagnosis of malaria, although PCR methods are considered to be more sensitive and specific, particularly in cases with low parasite counts, when mixed infections are present or when the subject of interest is the standardization of control programs (Greenwood, 2002, Coleman et al., 2006). In the context of epidemiological studies, PCR is a valuable instrument for evaluating reservoir conditions.
To our knowledge the only study that allows a direct comparison between PCR and microscopy in the detection of natural Plasmodium infections in wild monkeys was conducted by Fandeur et al. (2000) in French Guiana, who found a prevalence of P. brasilianum of 5.6% by microscopy (blood films) and 11.3% by PCR.
Further studies using both techniques are required to allow a more accurate estimate of the prevalence of malaria in the monkey population and consequently, to enable the role of the monkey population as a reservoir for human infections to be evaluated. Such studies would also provide important sources of material for phylogeny studies.
The present study used microscopy and PCR techniques to investigate the burden of natural Plasmodium infection in the wild monkey population of the three distinct ecosystems in Brazil: Cerrado (a savannah-like habitat), semideciduous Atlantic forest and the Atlantic forest, all of which have a history of residual human malaria transmission.
Section snippets
Study area and monkey blood samples
Blood samples were obtained from 448 monkeys from three distinct regions where human malaria had previously been reported and described (Duarte et al., 2006). The localities are shown in Fig. 1.
(1) Cerrado: Monkeys were captured in the canopy woods in flooded areas of the lake at the “Serra da Mesa” hydroelectric dam during wildlife rescue operations in 1997. The “Serra da Mesa” highlands area located along the Tocantins River, in the State of Goiás in the Cerrado in the West-Central region of
Results
(1) Cerrado: All monkey specimens were negative by microscopy and PCR (N = 51; 42 A. caraya, 4 C. apella and 5 C. jacchus) (Table 1).
(2) Atlantic forest: The PCR positivity rate for Alouatta guariba clamitans was 11.3% (8/71), with equal proportions for P. vivax/P. simium and P. malariae/P. brasilianum (5.6% or 4/71 each), and one specimen was positive for P. falciparum (1.4% or 1/71) (Table 1).
C. apella (N = 30) and Callithrix sp. (N = 39) were negative in the microscopy and PCR tests (Table 1).
All
Discussion
This study investigated the occurrence of Plasmodium infections in wild monkeys from three distinct ecological areas in Brazil: Cerrado, semideciduous Atlantic forest and Atlantic forest. The results obtained by microscopy revealed the presence of infection with very low parasite counts in Alouatta monkeys. Cebus and Callithrix samples were negative in our study, which is in accordance with previous studies showing a very low prevalence of malaria infection in these genera (De Arruda, 1985,
Acknowledgements
We would like to thank Ananias A. Escalante (School of Life Sciences, Arizona University, USA); Dr. Silvia Maria di Santi, Maria de Jesus Costa and Christina Rita de Camillo Toniolo (Superintendência de Controle de Endemias, São Paulo), Dr. Heitor F. de Andrade Junior (Instituto de Medicina Tropical de São Paulo, Universidade de São Paulo); Dr. Luiz Eloy Pereira (Instituto Adolfo Lutz, São Paulo); Maria do Carmo O. Brígido, Dr. Humberto S. Ferreira, João Bosco da C. Araújo, Luís A. Costa
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Supported by Fundação de Amparo à Pesquisa do Estado de São Paulo/FAPESP (Proc 99/02570-0, 02/03869-3) and Laboratório de Investigação Médica, HCFMUSP (LIM 49).