Valentine et al. Parasites Vectors (2019) 12:463 https://doi.org/10.1186/s13071-019-3732-0 Parasites & Vectors REVIEW Open Access Sylvatic cycles of arboviruses in non-human primates Matthew John Valentine1*† , Courtney Cuin Murdock2,3,4,5 and Patrick John Kelly6† Abstract Arboviruses infecting people primarily exist in urban transmission cycles involving urban mosquitoes in densely pop- ulated tropical regions. For dengue, chikungunya, Zika and yellow fever viruses, sylvatic (forest) transmission cycles also exist in some regions and involve non-human primates and forest-dwelling mosquitoes. Here we review the investigation methods and available data on sylvatic cycles involving non-human primates and dengue, chikungunya, Zika and yellow fever viruses in Africa, dengue viruses in Asia and yellow fever virus in the Americas. We also present current putative data that Mayaro, o’nyong’nyong, Oropouche, Spondweni and Lumbo viruses exist in sylvatic cycles. Keywords: Sylvatic, Primates, Enzootic, Arboviruses, Mosquitoes, Chikungunya, Dengue, Zika, Yellow fever Background NHPs precede and lead to short-lived outbreaks in peo- Many medically important and emergent arboviruses ple [3, 6]. (arthropod-borne viruses) originated in non-human pri- Urban transmission cycles of arboviruses in densely mates (NHPs), which typically show no clinical signs of populated tropical regions can result in explosive epi- infection but become viraemic and help maintain the demics and pandemics, although there can also be low virus in nature [1]. In the natural forest habitats of the levels of transmission only sufcient to maintain the NHPs, arboreal mosquitoes transmit arboviruses from viruses in the population. Some arboviruses, like den- infected to naïve animals in what is termed a sylvatic gue (DENV), chikungunya (CHIKV) and Zika (ZIKV), transmission cycle (NHP-mosquito-NHP-mosquito, have become fully adapted to urban cycles and no longer etc.). People can become infected if they encroach on for- require NHPs, forest mosquitoes and a sylvatic cycle for est habitat (either through deforestation, hunting, agri- their maintenance [7]. However, sylvatic cycles could still culture, or urbanization) and are fed upon by mosquitoes have important implications for human infections. Tey carrying arboviruses, or if infected forest mosquitoes may act as refugia for arboviruses which enable re-emer- move into areas of human habitation to obtain a blood gence once human epidemics have passed and immunity meal. When humans infected in the forest enter urban in the population (herd immunity) has waned. Further, environments, arbovirus infections can rapidly spread they might provide selective environments where new amongst people transmitted by highly anthrophilic, strains of arboviruses can develop with increased (or urban mosquitoes. Te sylvatic transmission cycle is decreased) virulence for people. Also, such novel strains then said to have ‘spilled over’ into an urban transmission may overcome immunity developed in response to vac- cycle [2–5]. Tis is well documented in the case of the cines designed for existing urban strains, so called ‘vac- yellow fever virus (YFV), where amplifcations in infected cine redundancy’ [8]. Because of the potential signifcance of sylvatic cycles, leading researchers [3, 9–11] have emphasised the *Correspondence: [email protected] importance of future investigations into their roles in the †Matthew John Valentine and Patrick John Kelly contributed equally to this work epidemiology of devastating diseases like dengue, chi- 1 One Health Centre for Zoonoses and Tropical Veterinary Medicine, Ross kungunya, yellow fever and Zika. Below we review the University School of Veterinary Medicine, Island Main Road, West Farm, methods used to investigate sylvatic cycles and available Basseterre, Saint Kitts and Nevis Full list of author information is available at the end of the article epidemiological data on sylvatic cycles of arboviruses in © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Valentine et al. Parasites Vectors (2019) 12:463 Page 2 of 18 NHPs, primarily focusing on the most recent and largest (vectors) that feed on them in a forest ecosystem (Table 1) arbovirus outbreaks in people (Fig. 1). [3, 7, 10–15]. Determining the infection status of the NHP Investigation of sylvatic cycles As stated by Kuno & Chang [15], “Te three commonly For the purposes of this review, we regard a sylvatic cycle used data for identifying vertebrate reservoirs for arbo- to be present if there is evidence of biological transmis- viruses have been (i) virus isolation from suspected sion (i.e. viral isolation, serological testing, blood-meal animals, (ii) relatively high antibody prevalence in the analysis and experimental infection) of an arbovirus animals captured in the feld and (iii) demonstration between a NHP (the vertebrate host) and mosquitoes of viraemia (of higher virus titre and duration) in the Fig. 1 Timeline of investigations of sylvatic cycles in NHPs and sylvatic mosquitoes around the world since the 1950’s. Countries capitalised in bold indicate arboviral isolation in both sylvatic mosquitoes and NHPs. Countries in italics indicate arboviral isolation in NHPs only Table 1 Criteria and techniques used to establish the presence of arboviral sylvatic cycles Criteria Techniques employed to demonstrate the criteria Sufcient populations of immuno-naïve (susceptible) NHPs Antibody detection Sufcient viraemia in the NHP to infect feeding mosquitoes Viral isolation from blood Viral genome detection from blood Antibody detection Sufcient populations of spatially and temporally coincident competent (capable of Mosquito capture and identifcation viral transmission) infected mosquitoes that feed on NHPs Virus detection in mosquitoes Blood-meal analysis of fed mosquitoes Observations of NHPs being fed upon by forest mosquitoes Viral transmission experiments (between NHPs and mosquitoes) Valentine et al. Parasites Vectors (2019) 12:463 Page 3 of 18 suspected animals typically obtained under laboratory prevent captured mosquitoes from desiccation which conditions” [15]. renders them unidentifable. Animal-baited mosquito net traps are cheap and easy to use but mosquitoes read- Viral isolation from suspected animals ily escape and the enclosure can alter mosquito behav- Isolation of virus from forest NHPs is strong evidence iour [27]. Handheld sweep nets and aspirators are labour for a sylvatic cycle, however arboviral viremias are only intensive and not species-specifc but aspiration is more short lived, from one to seven days [16–19]. Tus, fnding successful at capturing blood-fed individuals [27]. Hosts virus, or identifying fragments of its genome in the case on which mosquitoes feed can be discovered by direct of PCR, in naturally infected animals during feld studies observation [23] or by genotyping the blood meal found is serendipitous [20–24]. in engorged mosquitoes, so called blood meal analysis [32, 33]. Relatively high antibody prevalence in the animals captured Demonstration of virus in remote forest-dwelling mos- in the feld quitoes that cohabit with and feed upon NHPs is con- Although antibody detection has been used to implicate sidered reliable evidence for a sylvatic cycle [28–30], NHPs in sylvatic cycles in more recent studies [10, 25, especially when combined with concurrent serological 26], stand-alone serology of NHPs, particularly by ELISA, surveys or viral detection in NHPs. Viral presence can be is weak evidence of sylvatic transmission because anti- determined by inoculation of cell cultures with homog- bodies only indicate previous exposure (potentially years) enised monospecifc pools of 30–50 mosquitoes and of the NHP to the virus and make no assessment of viral identifcation of virus in the supernatant by IFA, CFT, kinetics or transmission. neutralisation tests or PCR on extracted RNA [28–30]. Confrming the above criteria (Table 1) and establish- Demonstration of viraemia (of higher virus titre ing with certainty that sylvatic cycles are present is often and duration) in the suspected animals typically obtained difcult in practice. Some studies, principally the early under laboratory conditions ones, established the existence of sylvatic cycles using the Te fnal evidence for a sylvatic cycle involves the experi- above criteria [20–24, 29, 30]. Other studies have relied mental demonstration of a viraemia of sufcient titre to more on investigating viral presence in either sylvatic enable mosquitoes to become infected with subsequent mosquitoes [28] or NHPs [10]. transmission to other NHPs [14, 15]. Tese data are typi- cally obtained under laboratory conditions. Laboratory tests used to investigate sylvatic cycles Laboratory methods used to detect arboviral infections Determining the infection status of forest‑dwelling in NHPs are virus isolation, nucleic acid amplifcation by mosquitoes PCR, and serological techniques [34, 35], while detection Tere are many diferent methods