Humans have frequently enabled pathogens to overcome physical barriers to dispersal (1) . 26
The European conquest of the Americas brought Old World diseases to the New World, 27 movement of troops during World War II propagated dengue viruses across the Asia-Pacific 28 region (2), and air travel has provided an international transmission network for emerging 29 infectious diseases (EIDs) such as the 2019 (ongoing) SARS-CoV-2 pandemic (3), the 2002-30
2003 SARS-CoV-1 outbreak (4) and pandemic influenza (5). Today, the majority of 31 pathogens that infect humans are broadly distributed across geographic regions-globalized 32
by human movement and population expansion, particularly during the past century (1) . 33
Animal pathogens have likewise spread globally through anthropogenic channels. The 34
globalization of agriculture has expanded the geographic range of many livestock diseases 35
with major economic repercussions, which continue to disproportionately affect the 36 developing world (6). Domestic and wild animals translocated by humans have introduced 37 their pathogens to new ecosystems, threatening biodiversity conservation-an anthropogenic 38 impact termed "pathogen pollution" (7). In some cases, these invasive animal infections have 39 2 maintained transmission post-emergence in local wildlife, establishing persistent reservoirs 40 that subsequently reseed transmission and thwart control efforts in the original animal host 41
population. Examples include African Swine Fever virus in Eastern Europe, where a novel 42 enzootic cycle of the invasive livestock pathogen in wild boars has prevented disease 43 eradication (8,9); and rabies virus in Africa, where human-mediated dispersal of domestic 44 dogs established wild carnivore reservoirs that now contribute to rabies persistence in both 45
wildlife and human communities (10). 46
Clearly, the global spread of zoonoses poses a unique and critical threat to human 47
health. Novel enzootic cycles occur when zoonoses are introduced to new regions, infect 48
local wildlife (spillback), and persist in local animal host populations (enzootic 49 establishment). Figure 1 provides a diagram of these processes and Table 1 provides 50
definitions of all terms in this paper). Now, more than ever, global conditions are ideal for the 51 generation of novel enzootic cycles. In an increasingly connected world, international trade 52
and travel provide pathways for pathogen introductions, while the recent surge in the 53 emergence and re-emergence of animal pathogens has increased the number of zoonoses 54 poised to exploit those pathways (11). Human population expansion into natural habitat is 55
intensifying contact between humans and animals, creating more opportunities for imported 56 zoonoses to spill back into naïve wildlife populations (12). The probability that these 57
introduced infections persist in animal populations is increasing as human development 58
pushes wildlife into crowded habitat patches and climate change alters transmission 59 conditions (7). 60
In this era of globalization, zoonoses are increasingly being recognized as global 61
threats. The emergence of SARS-CoV-2 in Wuhan, China has since affected 210 countries 62 and territories, causing nearly 2 million cases and 125,000 deaths worldwide as of April 15 th 63 2020 (13). The pandemic has prompted an extraordinary global response-many countries 64
have imposed nationwide lockdowns and closed their borders, nonessential international 65 travel has largely been suspended (14) to build infrastructure for global health security (17); and the WHO has declared recent 70 outbreaks of Ebola, H1N1, and Zika as public health emergencies of international concern 71
(PHEICs) (18). Nevertheless, this dialogue on the globalization of infectious disease 72 continues to conflate zoonoses and human-specific pathogens, often overlooking what makes 73 the spread of zoonoses so uniquely dangerous-the potential for enzootic reservoirs to 74 establish in previously naïve regions. Some zoonoses such as Ebola virus and SARS-CoV-1 75
have remained within the human population after introductions to new regions. Conversely, 76
introductions of yellow fever virus (YFV) in South America, Yersinia pestis (plague) in the 77
Americas ( An emerging body of literature is beginning to discuss the risk that human-to-animal 88 transmission will seed persistent enzootic reservoirs (22-25), but overall, disease emergence 89 and spillover from wildlife continues to dominate the conversation on zoonotic transmission. 90
Our understanding of enzootic establishment is limited by the difficulty in quantifying a 91 process that is both highly stochastic and the product of interactions between multiple 92 systems. In this review, we provide a conceptual framework to begin disentangling this 93 ecological complexity, applying concepts from disease ecology, invasion biology, and 94
population ecology. Cross-species pathogen emergence has previously been compared to 95 species invasions (26) and population ecology used to understand post-introduction 96 persistence (27) in the context of zoonotic spillover in human populations and host shifts 97 within wildlife communities. Adapting this interdisciplinary theory on pathogen emergence 98
to enzootic establishment, we review potential ecological mechanisms affecting the 99 probability that translocated zoonoses emerge in novel enzootic cycles. We discuss the 100 impact of each mechanism on the process of enzootic establishment: (1) local wildlife 101 becomes exposed, (2) the zoonosis successfully infects the novel hosts, and (3) transmission 102
persists indefinitely (28,29). We ground our discussion in the neotropics, where four 103 arboviruses of zoonotic origin-YFV, DENV, CHIKV, and ZIKV-have separately been 104
introduced into the human population. We additionally discuss the utility of modeling 105
approaches, which we illustrate by building a simulation model for our neotropics case study.
Our aim is to delineate the ecological processes that shape the outcome of zoonotic 107
translocations as a first step towards developing a framework for predicting and preventing 108 novel enzootic cycles, and therefore we do not discuss other key factors such as immunity, 109
phylogeny, and evolution. preventing the other three arboviruses, as well as any new introductions, from also 139 establishing persistent enzootic reservoirs will be critical for forestalling further human 140 morbidity and mortality from zoonotic transmission in the American tropics. However, the 141 majority of the work on these imported arboviruses has focused on retrospective analysis of 142 the conditions that enabled their introductions-particularly the global invasion of mosquito 143
vectors Ae. aegypti and Ae. albopictus-and the transmission burden in the human population 144
(38-41). Only a few recent papers have begun to discuss the threat of enzootic establishment 145
(22-25,42). We add to this discussion by considering how each ecological mechanism 146 identified in our review may affect the trajectory of DENV, CHIKV, and ZIKV in South 147
America, using YFV as a frame of reference. 148 149 Wildlife exposure 150 Once a translocated zoonosis has established in a new human system, there is an immediate 151 risk that the pathogen spills back into local wildlife populations. The probability of spillback 152 first depends on the rate at which wildlife is exposed, which can be captured by propagule 153
pressure-a concept from invasion biology that represents the number, and temporal and 154 spatial distribution of nonnative individuals introduced to a new system, and a key 155 determinant of invasion success (43,44). 156
Propagule pressure hinges on introduction pathways between a source and recipient 157
population. The propagule pressure of a translocated zoonosis on local wildlife will vary 158 based on the availability of transmission pathways between the human (source) and wildlife 159
(recipient) populations. Borrowing from landscape and movement ecology, Borremans et al.
(45) identified permeability-the likelihood that source and recipient hosts, along with the 161 pathogen, enter an ecosystem boundary region-as the ecological basis of pathways available 162
for cross-species pathogen emergence across ecosystem boundaries. With respect to the 163 human-wildlife boundary, permeability for translocated zoonoses will increase with wildlife 164 tolerance of (or preference for) anthropogenically modified landscapes, and human 165
communities' proximity to the edge of a species' habitat and incursions into natural habitat 166 for resource extraction. 167
Host boundary permeability creates opportunities for contacts between infected 168 source and recipient hosts, increasing propagule pressure on the recipient host population. 169
Transmission route determines the type(s) of contact, and thus degree of permeability, 170 required for the translocated zoonosis to cross the human-wildlife interface (45,46). Zoonotic 171
introductions can occur via direct contacts such as bushmeat hunting or the wildlife trade-172 common in the developing world-or via indirect mechanisms such as environmental 173
contamination-e.g., if infected bats leave saliva on forest fruits consumed by humans or 174
shed excreta in the human environment (61,62,66). For directly transmitted zoonoses, 175
propagule pressure on local enzootic systems will require sufficient permeability for humans 176 and wildlife to come into close contact. Conversely, zoonoses that can survive outside their 177
hosts will be less constrained by host boundary permeability. Domestic animals often 178 intersect with both humans and wildlife and thus, have the potential to bridge transmission 179 between the two populations. Vectors can likewise function as bridge hosts; as long as a 180 competent vector is present, vector-borne zoonoses only require some degree of spatial and 181 temporal overlap to transmit between humans and wildlife. 182
Transmission burden in the human population inhabiting the new region-a 183 combination of time since introduction and the number of subsequent human cases-will 184 5 determine the volume of pathogen propagules available to exploit transmission pathways to 185 local wildlife (48). Consistent circulation and a high number of cases in the human 186 population may result in more opportunities to spill back into enzootic transmission cycles, 187 producing greater propagule pressure. The precise propagule pressure that led YFV to invade 188
New World non-human primate populations is unknown, but phylogenetic analyses suggest 189 multiple spillback introductions (49,50). Evidence that YFV reached its current widespread 190 distribution in South America through long-distance, human-mediated dispersal implies that 191 many spillback introductions occurred across a broad spatio-temporal landscape. enzootic establishment, the translocated zoonosis must be able to infect the exposed animals.
Infecting novel host species in a novel environment can be described as a niche shift. The 231 6 concept of an ecological niche has many nuanced definitions in ecology, but generally 232
represents the set of abiotic and biotic conditions that allow a species to occupy a particular 233 space within an ecosystem (62). A pathogen niche is defined by its hosts, vectors, 234 ecophysiological requirements, and the many ways in which these parts interact (63,64). Like 235 other species, a translocated zoonosis will have a realized niche-existing transmission 236
cycles-and a fundamental niche-the range of systems the zoonosis could theoretically 237
invade if given the opportunity (63). The probability that the zoonosis undergoes a niche shift 238
to infect novel hosts in its new range depends on the degree of overlap between its 239 fundamental niche and novel environment (62). 240
Host specificity influences the breadth of the pathogen's fundamental niche. 241
Generalists are defined by broad fundamental host ranges (65), which intuitively, will 242
intersect with a wider range of enzootic systems, facilitating shifts to novel hosts (66-68). 243
Alternatively, zoonoses can evolve to invade environments outside of their original 244 fundamental niches (62). Some pathogen types inherently have higher potential for 245 fundamental niche shifts than others. In particular, YFV, DENV, CHIKV, and ZIKV are all 246
single-stranded RNA viruses-a group of pathogens previously shown to be the most likely 247
to shift host species, predisposed to cross-species emergence by high mutation rates (28 The outcome of spillback events depends on the potential for transmission between 306 individuals in the novel animal host population. If animal transmission is limited, spillback 307 might result in an isolated wildlife case, or alternatively, trigger an outbreak that threatens 308 conservation efforts but eventually dies out (94,95). However, above a critical transmission 309 threshold, the translocated zoonosis will persist indefinitely-the final step of successful 310 enzootic establishment. In disease ecology and epidemiology, that critical transmission 311 threshold is represented by the effective reproduction number (R)-the average number of 312 secondary cases generated by a single infected individual in a population of susceptible and 313 non-susceptible hosts. While R > 1, each infected individual will, on average, produce at least 314 one secondary infection, allowing the pathogen to persist. The effective reproduction number 315 is a function of pathogen attributes such as transmissibility and duration of infectiousness, as 316
well as the average rate of contact between infected and susceptible individuals (and vectors 317
if vector-borne) in a given population. 318
The average contact rate between infected and susceptible hosts is largely shaped by 319 population ecology (96). Dynamic demographic rates and structuring of host populations 320 determine the abundance, distribution, and movement of hosts available to translocated 321 zoonoses in enzootic systems. Wildlife populations with high carrying capacities will have 322 larger baseline populations of susceptible hosts. High birth rates replenish the supply of 323 susceptible hosts, inhibiting mortality and conferred immunity from depleting the susceptible 324 population and driving the effective reproduction below the threshold of persistence (97).
Spatial structuring of susceptible hosts can either limit or enhance the potential for pathogen 326
persistence. Pathogen dispersal between patches can promote persistence across a 327 metapopulation by buffering against local depletion of susceptible individuals. On the other 328 8 hand, spatial structure can limit contacts between patches, lowering the effective reproduction 329 number and driving an epidemic to extinction (96,98). 330
While some New World monkeys are large species with low birth rates, one of the 331 clade's defining features is its diverse set of remarkably small-bodied, short-lived primates. 332
For example, the pygmy marmoset (Callithrix pygmaea) is a small-bodied New World 333 primate with bimodal annual birth peaks, high twin birth rates, and a natural lifespan of about 334 10 years (99). These smaller-bodied primates also tend to require smaller home ranges and 335 live in higher densities, allowing for greater sympatric species richness. A meta-analysis of 336
New World primate assemblages found that on average, forest sites contained six sympatric 337
species, but this number could reach as high as 14 species, peaking near the equator (100). 338
New World enzootic mosquito species occupy the canopy, preying primarily on primates. 339
These vectors do not appear to demonstrate strong host preferences and thus, likely bridge 340
transmission between sympatric groups of primate species. This spatial structure-vector-341 mediated pathogen dispersal between primate groups (intra-and inter-species)-may have 342
facilitated the enzootic establishment of YFV. It has been hypothesized that enzootic YFV 343
persists within primate metapopulations occupying continuous forest in wandering 344 epizootics, in which transmission continually shifts between subpopulations. We suspect that 345 the demographic rates and structuring of primate host and mosquito vector populations in the 346
New World would, as with YFV, facilitate the persistence of DENV, CHIKV, and ZIKV 347
should they spill back successfully.
The ecology of enzootic establishment in the Anthropocene previously non-endemic regions, significantly expanding its range in the New World. 374 375 9 A case for modeling 376 It is challenging to approach the ecological complexity of enzootic establishment through 377 field and experimental studies alone. Mathematical models can however be a useful tool that 378 allows us to integrate existing data and ecological theory to elucidate system dynamics, 379
particularly when data are sparse, as is often the case with enzootic systems (22,104,105) . 380
Many modeling approaches can be applied to a range of questions that allow us to better 381 understand the risk that translocated zoonoses will emerge in novel enzootic cycles. For 382 example, species distribution models, or ecological niche models-statistical approaches that 383 leverage associations between presence-absence information and environmental variables to 384
infer habitat suitability-can be used to identify at-risk systems where high permeability of 385 humans, vectors, and translocated zoonoses create ideal conditions for enzootic establishment 386
(46,106-108). Metapopulation modeling is often an important tool to demonstrate conditions 387 of pathogen persistence in wildlife populations given that they are typically fragmented (96). 388
Next-generation matrix methods are a useful tool to quantify the effective reproduction 389
number of translocated zoonoses in novel enzootic systems to understand the probability of 390 successful invasion (109). Explicit simulations that capture significant amounts of the 391 complexity of systems have been effectively used to compare the impact of interventions in 392 the human and wildlife population (110). We would also argue that simple models can be 393 very useful in helping us understand the system-specific dynamics that influence the 394 ecological processes underlying enzootic establishment. increasing EIP to 10 days, as we might observe with ZIKV, decreased the probability of 433 persistence, whereas shortening EIP to 2 days, as we might observe with CHIKV or at warm 434 temperatures, increased the probability of persistence (Figure 2) 
The goal of this study is to stimulate research on the emergence of novel enzootic cycles and 452 begin to disentangle the underlying ecology complexity. We have argued here that the 453 establishment of novel enzootic cycles is a pressing threat with the capacity to dramatically 454 alter disease dynamics. The International Task Force for Disease Eradication identifies the 455 existence of an animal reservoir as a barrier to eradicating a disease because enzootic 456 transmission often feeds human epidemics (119). In some cases, enzootic cycles have even 457
contributed to the evolution of pandemic pathogens; for example, pigs have functioned as 458 "mixing vessels" for the evolution of pandemic swine influenza (120). We delineated 459 potential ecological mechanisms at each stage of enzootic establishment, grounding our 460 discussion in the neotropics, where the danger of enzootic establishment is evident in the 461 history of YFV and an ongoing threat given the endemic circulation of DENV, CHIKV, and 462 ZIKV in the human population. Enzootic YFV, which has triggered devastating human 463 epidemics across the neotropics, has expanded its geographic range since its initial 464 establishment (121). There is a real danger that DENV, CHIKV, and ZIKV will also establish 465 persistent enzootic reservoirs in the New World, similarly inhibiting efforts to prevent future 466 human epidemics. Given that enzootic cycles are nearly impossible to control or eradicate, 467
avoiding enzootic establishment will be critical to mitigate the current arbovirus public health 468 emergency in the New World. Moreover, enzootic cycles can thwart efforts to eradicate 469 pathogens in the human population, as has occurred with the carriage of Guinea worm by 470 dogs (122) . In particular, the recent discovery of natural ZIKV (34-36) infection in non-471 human primates in Brazil calls for renewed urgency to understand the potential for enzootic 472
persistence. 473
Spillback events in which humans introduce pathogens into wildlife populations may 474 become common occurrences, as urbanization and anthropogenic pressure on wildlife 475
populations increase opportunities for human-wildlife contact. As a result, understanding the 476 risk that arboviruses will persist in sylvatic cycles after spillback events should be established 477 as a research priority, particularly in the era of SARS-CoV-2. There is significant concern 478 that SARS-CoV-2 could spill back into susceptible wildlife within its expanded geographic 479 range and establish novel enzootic reservoirs, becoming endemic outside of China. The high 480
burden and global distribution of human SARS-CoV-2 transmission places propagule 481
pressure on a wide range of enzootic systems. Bats-the putative origin of SARS-CoV-2 482
(123)-are the second most diverse mammalian group, inhabit every continent except 483
Antarctica, and harbor a large diversity of coronaviruses (124). Many bat species migrate 484 long distances (>1,000 km) (125), increasing their probability of exposure to SARS-CoV-2 485
and capacity to subsequently spread the virus. Although previous work has demonstrated that 486
human-adapted CoV often cannot infect bat cells (126), the cell-surface receptor implicated 487
in SARS-CoV-2 invasion of host cells-angiotensin converting enzyme 2 (ACE2)-has been 488 found to be highly diverse across bat species and in some cases, conducive to human SARS-489
CoV-2 infection (127). ACE2 also appears to be conserved across mammal species, 490
suggesting that the virus may have the potential to establish in a broad range of other 491 mammalian host species (128-130). Furthermore, there is a possibility that human SARS-492
CoV-2 could infect initially unsuitable host species through mutation events (131). 493
To mitigate the risk that SARS-CoV-2 establishes novel enzootic reservoirs and 494
becomes endemic outside of China, it is critical to identify susceptible wildlife species and 495 populations and implement policies that limit their exposure to the virus. The US government 496 has, at present, suspended all bat research to prevent humans from infecting and seeding an 497 enzootic reservoir of SARS-CoV-2 in North American bats (132). However, bat research is 498 still active in many other parts of the world, as are other forms of bat-human contact, though 499 many countries have banned bat bushmeat in response to the pandemic (133). Additional 500 policies may be needed to minimize human contact with other potentially susceptible wildlife 501
populations-species with ACE2 receptors predicted to associate with the SARS-CoV-2 502 spike receptor binding site (RBD), and populations that occupy anthropogenic landscapes 503 (particularly in regions experiencing a high burden of human transmission) and characterized 504 by demographic rates and structuring predicted to facilitate sustained epidemics and enzootic 505
persistence. It is also critical to monitor via surveillance whether these potentially susceptible 506 animals become exposed and begin to demonstrate capacity for between-host transmission. In 507 particular, researchers should monitor domestic animals, which can bridge transmission 508 between human and wildlife populations (45). Notably, efficient SARS-CoV-2 replication 509 and confirmed cases have been reported in cats (129,134) and ACE2 receptors of domestic 510
cattle have been predicted to associate with the SARS-CoV-2 RBD (135). 511
The risk of enzootic persistence depends on a multitude of factors, many of which are 512 unknown or poorly characterized, such as infectivity, number of spillback events, and the 513 transmission conditions needed for persistence. Here we provide a conceptual framework of 514 ecological factors to begin addressing the challenge of predicting this risk. Considering 515 ecological mechanisms is a first step towards developing targeted intervention strategies. We 516 additionally argue for the utility of modeling in detangling ecological complexity, providing a 517 simulation model of arboviral transmission in New World primates and mosquitoes as an 518
example. Our results indicate that although long extrinsic incubation periods (EIPs) can 519 reduce the probability of enzootic persistence, the long mosquito lifespans that are 520 characteristic of tropical New World sylvatic species may negate this effect-suggesting that 521
differences in EIP that we may have expected to be important in determining the translocated 522
arboviruses' relative risk of enzootic establishment are unlikely to be relevant. Overall, our 523 work is important in highlighting the need to be vigilant of imported zoonoses and 524
emphasizing the importance of robust programs to mitigate the risk of spillback events that 525 lead to enzootic persistence. 526 Table 1 . Definitions of terms used in this paper. 527 528 Table 2 populations (enzootic establishment). 553
• Understanding when and why translocated zoonoses establish novel enzootic cycles 554 requires disentangling ecologically complex and stochastic interactions between the 555 zoonosis, the human population, and the natural ecosystem. 556
• Permeability of the human-wildlife interface for humans, wildlife, and the translocated zoonosis creates pathways for propagule pressure.
Transmission route determines the type(s) of contact, and thus degree of permeability, required for a translocated zoonosis to cross the humanwildlife interface.
Habitat specificity: generalists are more tolerant of human-modified landscapes, increasing wildlife permeability.
Natural resource extraction and land conversion increases human permeability.
Several New World primate hostsincluding Aotus spp., the most susceptible to YFV-are habitat generalists that often occupy the forest edge.
Deforestation, urbanization, and bushmeat hunting encroach on wildlife habitat in South America.
Vector-borne transmission may have initially constrained YFV, but now, abundant mosquitoes likely increase human-mediated dispersal and may facilitate wildlife exposure to urban transmission without direct contact with humans.
Transmission burden in the human population determines the volume of pathogen propagules available to exploit pathways to enzootic systems.
Heightened arboviral burden in the human population likely increases propagule pressure of DENV, ZIKV, and CHIKV on New World enzootic systems.
Enzootic transmission of a translocated zoonosis persists indefinitely while R > 1.
Demographic rates such as the carrying capacity and birth rate determine the abundance of susceptible hosts.
a translocated zoonoses infects exposed animals, undergoing a niche shift.
Species diversity creates the host environment available to translocated zoonoses, and facilitates or inhibits invasion depending on the context. When additional species are susceptible and infectious to the translocated zoonosis, increasing species diversity is expected to amplify the probability of enzootic infection. However, when additional species are not competent hosts, there can be a dilution effect.
Competitive exclusion can constrain host environments created by biodiversity. Native zoonoses already circulating in an enzootic system can compete for susceptible hosts in an enzootic system, preventing infections of translocated zoonoses.
New World primates and mosquitoes are remarkably diverse. Given evidence that these species are competent arboviral hosts, this biodiversity could amplify the probability of enzootic infections of YFV, DENV, CHIKV, and ZIKV.
Conserved cellular receptors and transmission dynamics suggest that YFV, DENV, CHIKV, and ZIKV are generalists. The high mutation rates of single-stranded RNA viruses may additionally trigger fundamental niche shifts.
There is some evidence of cross-protection between the arboviruses, but cocirculation in urban transmission cycles does not support this hypothesis.
Overlap between the fundamental niche of the zoonosis and the novel environment determines the probability that the zoonosis undergoes a niche shift to infect novel hosts in its new range.
Pathogen host specificity influences the breadth of the fundamental niche. Generalists are more likely to have the ability to infect and transmit between novel hosts.
Host population ecology shapes the average contact rate between infected and susceptible hosts-a key determinant of the effective reproduction number.
Population structure determines opportunities to contact susceptible hosts, and in metapopulations, can create pathogen dispersal patterns that either buffer against or promote local extinction.
Vector-mediated pathogen dispersal between primate groups (intra-and interspecies) may facilitate the persistence of New World enzootic YFV-a persistence pattern that has been described as wandering epizootics.
The New World has a diverse set of remarkably small-bodied, short-lived primates that live in relatively high densities. 
Our approach builds on Althouse et al. (1) , which explored how the force of infection and 3
primate birthrate affect the probability that ZIKV will establish a sylvatic cycle in the Americas. 4
Expanding their analysis, we explored the effects of the mosquito extrinsic incubation period 5
(EIP) and lifespan, and primate birthrate, and considered DENV, CHIKV, and YFV in addition 6
to ZIKV. We simulate the introduction of a single infected primate into metapopulations of 7 susceptible primate hosts and mosquito vectors, applying the Gillespie stochastic simulation 8 algorithm with the Binomial Tau Leap approximation to the following transition rates for 9 mosquitoes: 10 11 12
The equations divide primate and mosquito metapopulations into primate species 1,...i and 13 mosquito species 1,...j. The model assumes host preference, with mosquito species j biting the 14 corresponding primate species j more often than other primate species-characterized by the on-15 diagonal rates within a contact matrix. Primate-mosquito species pairs are coupled by off-16
diagonal cross-biting rates, calculated as a fixed fraction (10%) of the on-diagonal within-pair 17 biting rates. Given that vertical transmission in the mosquito population is thought to be 18 negligible in enzootic cycles (2), the mosquito transovarial transmission rate is set to zero.
Thus, all primates and mosquitoes are born susceptible to arbovirus infection at rates and 20
infected at rates (t), proportional to the number of mosquito bites given or received per day and 21 the probability of successful transmission. We assume that birthrate = 1/lifespan-a conservative 22 estimate given that primates' peak reproductive years occur before the age of mortality (3) and 23
mosquitoes have the capacity to complete multiple gonotrophic cycles within a lifetime (4). 24
Thus, birthrates vary as we explore three mosquito lifespans (7, 14, and 21 days). Transmission 25 probabilities vary seasonally in Equation 14 and 15 due to changes in environmental conditions 26 such as rainfall and temperature (5). After infection, mosquitoes enter the exposed compartment, 27
progressing to the infectious period at one of three fixed rates ! ! (1/2, 1/7, or 1/10 days). 28