Sin Nombre Virus and Rodent Species Diversity: A Test of the Dilution and Amplification Hypotheses

Background

Species diversity is proposed to greatly impact the prevalence of pathogens. Two predominant hypotheses, the “Dilution Effect” and the “Amplification Effect”, predict divergent outcomes with respect to the impact of species diversity. The Dilution Effect predicts that pathogen prevalence will be negatively correlated with increased species diversity, while the Amplification Effect predicts that pathogen prevalence will be positively correlated with diversity. For many host-pathogen systems, the relationship between diversity and pathogen prevalence has not be empirically examined.

Methodology/Principal Findings

We tested the Dilution and Amplification Effect hypotheses by examining the prevalence of Sin Nombre virus (SNV) with respect to diversity of the nocturnal rodent community. SNV is directly transmitted primarily between deer mice (Peromyscus maniculatus). Using mark-recapture sampling in the Spring and Fall of 2003–2005, we measured SNV prevalence in deer mice at 16 landscape level sites (3.1 hectares each) that varied in rodent species diversity. We explored several mechanisms by which species diversity may affect SNV prevalence, including reduced host density, reduced host persistence, the presence of secondary reservoirs and community composition. We found a negative relationship between species diversity and SNV prevalence in deer mice, thereby supporting the Dilution Effect hypothesis. Deer mouse density and persistence were lower at sites with greater species diversity; however, only deer mouse persistence was positively correlated with SNV prevalence. Pinyon mice (P. truei) may serve as dilution agents, having a negative effect on prevalence, while kangaroo rats (Dipodomys ordii), may have a positive effect on the prevalence of SNV, perhaps through effects on deer mouse behavior.

Conclusions/Significance

While previous studies on host-pathogen systems have found patterns of diversity consistent with either the Dilution or Amplification Effects, the mechanisms by which species diversity influences prevalence have not been investigated. Our study indicates that changes in host persistence, coupled with interspecific interactions, are important mechanisms through which diversity may influence patterns of pathogens. Our results reveal the complexity of rodent community interactions with respect to SNV dynamics.     

Habitat variation influences movement rates and population structure of an intertidal fish

Deer mice (Peromyscus maniculatus) are the primary reservoir for Sin Nombre virus (SNV), a North American hantavirus that causes disease with high mortality in humans. Recent studies have proposed that habitat disturbance affects prevalence of SNV in deer mice; however, the outcomes proposed in these studies are in opposition to each other. Our objectives were to test these divergent hypotheses by: (1) measuring SNV infection in deer mice within a patchwork of disturbance; and (2) evaluating the relationships between SNV prevalence, population density and demography as possible mechanisms. In 2003 and 2004, we sampled 1,297 deer mice from 17 sites with varying levels of disturbance in the Great Basin Desert. Across sites and years, SNV prevalence varied from 0.0 to 38.9%. We found a negative relationship between SNV prevalence and disturbance. Although we found no direct relationship between SNV prevalence and deer mouse density, we found that density was highest on sites with the lowest levels of disturbance. The number of deer mice that survived across seasons (e.g., trans-seasonal survivors) differed across levels of disturbance and was greatest on our least disturbed study sites moderate on sites with intermediate levels of disturbance and zero on highly disturbed sites. On low-disturbance sites, a greater proportion of trans-seasonal survivors were SNV seropositive (28.80%) compared to the intermediate-disturbance sites (16.67). Collectively, our results indicate that habitat disturbance plays a predictive role in SNV prevalence, with highly disturbed sites having reduced long-term survival of deer mice, including survival of infected individuals.     

Temporal and Spatial Analysis of Sin Nombre Virus Quasispecies in Naturally Infected Rodents

Sin Nombre virus (SNV) is thought to establish a persistent infection in its natural reservoir, the deer mouse (Peromyscus maniculatus), despite a strong host immune response. SNV-specific neutralizing antibodies were routinely detected in deer mice which maintained virus RNA in the blood and lungs. To determine whether viral diversity played a role in SNV persistence and immune escape in deer mice, we measured the prevalence of virus quasispecies in infected rodents over time in a natural setting. Mark-recapture studies provided serial blood samples from naturally infected deer mice, which were sequentially analyzed for SNV diversity. Viral RNA was detected over a period of months in these rodents in the presence of circulating antibodies specific for SNV. Nucleotide and amino acid substitutions were observed in viral clones from all time points analyzed, including changes in the immunodominant domain of glycoprotein 1 and the 3' small segment noncoding region of the genome. Viral RNA was also detected in seven different organs of sacrificed deer mice. Analysis of organ-specific viral clones revealed major disparities in the level of viral diversity between organs, specifically between the spleen (high diversity) and the lung and liver (low diversity). These results demonstrate the ability of SNV to mutate and generate quasispecies in vivo, which may have implications for viral persistence and possible escape from the host immune system.     

The interplay of plant and animal disease in a changing landscape: the role of sudden aspen decline in moderating Sin Nombre virus prevalence in natural deer mouse populations

We examined how climate-mediated forest dieback regulates zoonotic disease prevalence using the relationship between sudden aspen decline (SAD) and Sin Nombre virus (SNV) as a model system. We compared understory plant community structure, small mammal community composition, and SNV prevalence on 12 study sites within aspen forests experiencing levels of SAD ranging from <10.0% crown fade to >95.0% crown fade. Our results show that sites with the highest levels of SAD had reduced canopy cover, stand density, and basal area, and these differences were reflected by reductions in understory vegetation cover. Conversely, sites with the highest levels of SAD had greater understory standing biomass, suggesting that vegetation on these sites was highly clustered. Changes in forest and understory vegetation structure likely resulted in shifts in small mammal community composition across the SAD gradient, as we found reduced species diversity and higher densities of deer mice, the primary host for SNV, on sites with the highest levels of SAD. Sites with the highest levels of SAD also had significantly greater SNV prevalence compared to sites with lower levels of SAD, which is likely a result of their abundance of deer mice. Collectively, results of our research provide strong evidence to show SAD has considerable impacts on vegetation community structure, small mammal density and biodiversity and the prevalence of SNV.     

How Host Population Dynamics Translate into Time-Lagged Prevalence: An Investigation of Sin Nombre Virus in Deer Mice

Human cases of hantavirus pulmonary syndrome caused by Sin Nombre virus are the endpoint of complex ecological cascade from weather conditions, population dynamics of deer mice, to prevalence of SNV in deer mice. Using population trajectories from the literature and mathematical modeling, we analyze the time lag between deer mouse population peaks and peaks in SNV antibody prevalence in deer mice. Because the virus is not transmitted vertically, rapid population growth can lead initially to reduced prevalence, but the resulting higher population size may later increase contact rates and generate increased prevalence. Incorporating these factors, the predicted time lag ranges from 0 to 18 months, and takes on larger values when host population size varies with a longer period or higher amplitude, when mean prevalence is low and when transmission is frequency-dependent. Population size variation due to variation in birth rates rather than death rates also increases the lag. Predicting future human outbreaks of hantavirus pulmonary syndrome may require taking these effects into account.     

A temporal dilution effect: hantavirus infection in deer mice and the intermittent presence of voles in Montana

The effect of intermittently occurring, non-reservoir host species on pathogen transmission and prevalence in a reservoir population is poorly understood. We investigated whether voles, Microtus spp., which occur intermittently, influenced estimated standing antibody prevalence (ESAP) to Sin Nombre hantavirus (SNV, Bunyaviridae: Hantavirus) among deer mice, Peromyscus maniculatus, whose populations are persistent. We used 14 years of data from central Montana to investigate whether ESAP among deer mice was related to vole presence or abundance while controlling for the relationship between deer mouse abundance and ESAP. We found a reduction in deer mouse ESAP associated with the presence of voles, independent of vole abundance. A number of studies have documented that geographic locations which support a higher host diversity can be associated with reductions in pathogen prevalence by a hypothesized dilution effect. We suggest a dilution effect may also occur in a temporal dimension at sites where host richness fluctuates. Preservation of host diversity and optimization of environmental conditions which promote occurrence of ephemeral species, such as voles, may result in a decreased ESAP to hantaviruses among reservoir hosts. Our results may extend to other zoonotic infectious diseases.     

Using MODIS satellite imagery to predict hantavirus risk

Aims Sin Nombre virus (SNV), a strain of hantavirus, causes hantavirus pulmonary syndrome (HPS) in humans, a deadly disease with high mortality rate (> 50%). The primary virus host is the deer mouse, and greater abundance of deer mice has been shown to increase the human risk of HPS. Our aim is to identify and compare vegetation indices and associated time lags for predicting hantavirus risk using remotely sensed imagery. Location Utah, USA. Methods A 5‐year time‐series of moderate‐resolution imaging spectroradiometer (MODIS) satellite imagery and corresponding field data was utilized to compare various vegetation indices that measure productivity with the goal of indirectly estimating mouse abundance and SNV prevalence. Relationships between the vegetation indices and deer mouse density, SNV prevalence and the number of infected deer mice at various time lags were examined to assess which indices and associated time lags might be valuable in predicting SNV outbreaks. Results The results reveal varying levels of positive correlation between the vegetation indices and deer mouse density as well as the number of infected deer mice. Among the vegetation indices, the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI) produced the highest correlations with deer mouse density and the number of infected deer mice using a time lag of 1.0 to 1.3 years for May and June imagery. Main conclusions This study demonstrates the potential for using MODIS time‐series satellite imagery in estimating deer mouse abundance and predicting hantavirus risk. The 1‐year time lag provides a great opportunity to apply satellite imagery to predict upcoming SNV outbreaks, allowing preventive strategies to be adopted. Analysis of different predictive indices and lags could also be valuable in identifying the time windows for data collection for practical uses in monitoring rodent abundance and subsequent disease risk to humans.     

COALESCENCE VERSUS COMPETITION: FIELD AND LABORATORY STUDIES OF INTRA‐ AND INTERSPECIFIC ENCOUNTERS AMONG COALESCING SEAWEEDS

Classical ecological theory predicts that whenever growing individuals share a common and limiting resource, such as substratum in mid‐intertidal and shallow subtidal habitats, preemptive competition will occur determining species abundance and distribution patterns. However, conspecificity of several ecologically dominant Rhodophyta may coalesce when grown in laboratory cultures. The extent at which intraspecific coalescence occurs in the field and whether the process may also happens during interspecific encounters remain to be determined. If intra‐ and interspecific coalescence effectively occurs, then coexistence through coalescence rises as an alternative to competition among red‐algal dominated intertidal and shallow subtidal communities. Populations of Mazzaella laminarioides and Nothogenia fastigiata living in mid‐intertidal, semi‐exposed rocky habitats in Central Chile are being used to test the above ideas. Intra‐ and interspecific encounters occur in the field throughout the year. Coalescence does occur among conspecific partners but it has not been detected in interspecific encounters. Rather, a thick interface of compressed cells, necrotic tissues and cyanobacterial nodules is formed between the two contacting partners. In addition, observations of laboratory cultures indicate that spore germination, germling survival and differentiation of erect axes in bispecific cultures may be reduced when compared to single‐species controls. Interspecific differences in growth and differentiation rates appear as the mechanisms explaining a lack of coalescence and negative effects during interspecific contacts. On the other hand, the existence of conspecific coalescence in the field suggests this process should be considered as a real alternative to intraspecific competition among coalescing Rhodophyta.     

Delayed density-dependent prevalence of Sin Nombre virus infection in deer mice (Peromyscus maniculatus) in central and western Montana

Understanding how transmission of zoonoses takes place within reservoir populations, such as Sin Nombre virus (SNV) among deer mice (Peromyscus maniculatus), is important in determining the risk of exposure to other hosts, including humans. In this study, we examined the relationship between deer mouse populations and the prevalence of antibodies to SNV, a system where the effect of host population abundance on transmission is debated. We examined the relationship between abundance of deer mice in late summer-early autumn and SNV antibody prevalence the following spring-early summer (termed delayed density-dependent [DDD] prevalence of infection) at both regional and local scales, using 12 live-trapping grids for 11-14 yr, across central and western Montana. When all trapping grids were combined (regional scale), there was a significant DDD relationship for individual months and when months within seasons were averaged. However, within individual grids (local scale), evidence of DDD prevalence of infection was observed consistently at only one location. These findings suggest that, although there is evidence of DDD prevalence of infection at regional scales, it is not always apparent at local scales, possibly because the regional pattern of DDD infection prevalence is driven by differences in abundance and prevalence among sites, rather than in autumn-spring delays. Transmission of SNV may be more complex than the original hypothesis of autumn-spring delayed density dependence suggests. This complexity is also supported by recent modeling studies. Empirical investigations are needed to determine the duration and determinants of time-lagged abundance and antibody prevalence. Our study suggests predicting local, human exposure risk to SNV in spring, based on deer mouse abundance in autumn, is unlikely to be a reliable public health tool, particularly at local scales.     

Deer mouse movements in peridomestic and sylvan settings in relation to Sin Nombre virus antibody prevalence

Prevalence of antibody to Sin Nombre virus (SNV) has been found to be nearly twice as high in deer mice (Peromyscus maniculatus) in peridomestic settings as in sylvan settings in two studies in Montana and one in New Mexico. We investigated whether this difference may be related to a difference in deer mouse movements in the two settings. We used radiotelemetry to determine home range size and length of movement for 22 sylvan (1991-1992) and 40 peridomestic deer mice (1995-1999). We also determined the percentage of locations inside versus outside of buildings for peridomestic mice. Though variable, average home range size for female deer mice was significantly smaller for peridomestic deer mice than for sylvan deer mice. The smaller home range in peridomestic settings may concentrate shed SNV, and protection from solar ultraviolet radiation inside buildings may increase environmental persistence of SNV. Both these factors could lead to increased SNV exposure of deer mice within peridomestic populations and result in higher antibody prevalence. Peridomestic deer mice moved between buildings and outside areas, which is evidence that SNV can be transmitted between peridomestic and sylvan populations.     

Epizootiology of Sin Nombre and El Moro Canyon hantaviruses, southeastern Colorado, 1995-2000

Sin Nombre virus (SNV) is an etiologic agent of hantavirus pulmonary syndrome. To better understand the natural history of this virus we studied population dynamics and temporal pattern of infection of its rodent hosts in southeastern Colorado (USA) from 1995 to 2000. We present evidence for the presence of two hantaviruses, SNV in deer mice (Peromyscus maniculatus) and El Moro Canyon virus in western harvest mice (Reithrodontomys megalotis), at our study sites. Sin Nombre virus appeared only sporadically in deer mouse populations; overall prevalence of antibody to SNV was 2.6%. El Moro Canyon virus was enzootic: seroconversions occurred throughout the year; antibody prevalence (11.9% overall) showed a delayed-density-dependent pattern, peaking as relative abundance of mice was declining. Males of both host species were more frequently infected than were females. An apparently lower mean survivorship (persistence at the trapping site) for SNV antibody-positive deer mice could indicate a detrimental effect of SNV on its host, but might also be explained by the fact that antibody-positive mice were older when first captured.     

Biogeographic and ecological regulation of disease: prevalence of Sin Nombre virus in island mice is related to island area, precipitation, and predator richness

The relative roles of top-down and bottom-up forces in affecting disease prevalence in wild hosts is important for understanding disease dynamics and human disease risk. We found that the prevalence of Sin Nombre virus (SNV), the agent of a severe disease in humans (hantavirus pulmonary syndrome), in island deer mice from the eight California Channel Islands was greater with increased precipitation (a measure of productivity), greater island area, and fewer species of rodent predators. In finding a strong signal of the ecological forces affecting SNV prevalence, our work highlights the need for future work to understand the relative importance of average rodent density, population fluctuations, behavior, and specialist predators as they affect SNV prevalence. In addition to illustrating the importance of both bottom-up and top-down limitation of disease prevalence, our results suggest that predator richness may have important bearing on the risk of exposure to animal-borne diseases that affect humans.     

Relationships of deer mouse movement, vegetative structure, and prevalence of infection with Sin Nombre virus

The effects of vegetative structure on movement of deer mice (Peromyscus maniculatus) were examined in two distinct vegetation associations, one near Hesperus and the other near Molina in western Colorado (USA) from June-October 1994 to October 1998. We monitored movement by live-trapping small mammals within Gambel's oak/mixed-grass (Hesperus) and sage brush/juniper (Molina) vegetation types. Vegetative structure differed between the sites with Molina having more cover provided by shrubs and Hesperus having more cover provided by forbs. Adult male deer mice moved greater distances at Hesperus than at Molina. Sub-adult males tended to move greater distances than did adult females. Relative abundances of deer mice tended to differ by season, but the average relative abundance of deer mice was greater at Molina. Long-term prevalence of infection with SNV was greater at Hesperus and was greatest in adult males at Hesperus (36.1%). Adult males at Molina exhibited a prevalence of infection with SNV of 25.0%. Infection with SNV was highly associated with scars or wounds for adult male, adult female, and juvenile male deer mice at Hesperus, but only for adult female deer mice at Molina. The presence of scars or wounds tended to be associated with greater age, but male deer mice at Hesperus were more likely to have wounds than female deer mice of the same age class. A similar pattern, excluding juveniles, was observed at Molina. Intraspecific interactions and environmentally elicited long-distance movements of deer mice may play a role in prevalence of infection with SNV in these animals.     

Demographic factors associated with prevalence of antibody to Sin Nombre virus in deer mice in the western United States

We used long-term data collected for up to 10 yr (1994-2004) at 23 trapping arrays (i.e., webs and grids) in Arizona, Colorado, Montana, and New Mexico to examine demographic factors known or suspected to be associated with risk of infection with Sin Nombre virus (SNV) in its natural host, the deer mouse (Peromyscus maniculatus). Gender, age (mass), wounds or scars, season, and local relative population densities were statistically associated with the period prevalence of antibody (used as a marker of infection) to SNV in host populations. Nevertheless, antibody prevalence and some of the risk factors associated with antibody prevalence, such as relative population density, gender bias, and prevalence of wounding, varied significantly among sites and even between nearby trapping arrays at a single site. This suggests that local microsite-specific differences play an important role in determining relative risk of infection by SNV in rodents and, consequently, in humans. Deer mouse relative population density varied among sites and was positively and statistically associated with infection prevalence, an association that researchers conducting shorter-term studies failed to demonstrate. Both wounding and antibody prevalence increased with mass class in both males and females; this increase was much more pronounced in males than in females and wounding was more frequent in adult males than in adult females. Prevalence of wounding was greatest among seropositive deer mice, regardless of mass class, but many deer mice without detectable wounds or scars eventually became infected. Many of these patterns, which will be useful in the development of predictive models of disease risk to humans, were only detected through the application of data collected over a long (10-yr) period and with abundant replication.     

COMPETITION AND THE EVOLUTION OF AGGRESSIVE BEHAVIOR IN TWO SPECIES OF TERRESTRIAL SALAMANDERS

The effects of competition on the evolution of interspecific interference mechanisms were studied by comparing the aggressive behavior of two terrestrial salamander species from two localities that differ in the intensity of interspecific competition. Plethodon jordani and P. glutinosus are closely related, ecologically similar species that are sympatric at intermediate elevations in the southern Appalachian Mountains. Previous removal and transplant experiments showed that interspecific competition is more intense in the northeastern Great Smoky Mountains, where the species are narrowly sympatric, than in the nearby Balsam Mountains, where sympatry is broader. In laboratory encounters, P. glutinosus from the Great Smoky Mountains were more aggressive to heterospecific and conspecific intruders than were P. glutinosus from the Balsam Mountains. For P. jordani, however, the variation in interspecific and intraspecific aggressive behavior among individuals within populations was as great as the variation between populations. Alpha-selection (i.e., improved competitive ability by the acquisition of interspecific interference mechanisms) has occurred in populations of P. glutinosus under conditions of intense interspecific competition. The evolution of aggressive behavior appears to have been influenced by the intensity of intraspecific competition as well.     

Changes in sin nombre virus antibody prevalence in deer mice across seasons: the interaction between habitat, sex, and infection in deer mice

We examined the impact of season and habitat on Sin Nombre virus (SNV) seroprevalence in deer mice (Peromyscus maniculatus) in Utah's Great Basin Desert from May 2002 through summer 2003. Low mouse captures in 2002 limited analysis for that year. In two seasons during 2003, mouse density and sagebrush cover were positively linked (spring: r = 0.8, P = 0.01; summer: r = 0.8, P = 0.04). In the spring, seroprevalence was negatively correlated with density (r = -0.9, P< 0.01); male and female antibody prevalence did not differ; and scarring was unrelated to antibody status. In the summer, density and antibody prevalence were unrelated; male seroprevalence was higher (chi(2) = 3.6, P = 0.05); and seropositive mice had more scars (t = 2.5, P = 0.02). We speculate nesting behavior could maintain SNV over the winter, whereas summer territoriality could be responsible for transmission.     

Sin Nombre hantavirus decreases survival of male deer mice

How pathogens affect their hosts is a key question in infectious disease ecology, and it can have important influences on the spread and persistence of the pathogen. Sin Nombre virus (SNV) is the etiological agent of hantavirus pulmonary syndrome (HPS) in humans. A better understanding of SNV in its reservoir host, the deer mouse, could lead to improved predictions of the circulation and persistence of the virus in the mouse reservoir, and could help identify the factors that lead to increased human risk of HPS. Using mark-recapture statistical modeling on longitudinal data collected over 15 years, we found a 13.4% decrease in the survival of male deer mice with antibodies to SNV compared to uninfected mice (both male and female). There was also an additive effect of breeding condition, with a 21.3% decrease in survival for infected mice in breeding condition compared to uninfected, non-breeding mice. The data identified that transmission was consistent with density-dependent transmission, implying that there may be a critical host density below which SNV cannot persist. The notion of a critical host density coupled with the previously overlooked disease-induced mortality reported here contribute to a better understanding of why SNV often goes extinct locally and only seems to persist at the metapopulation scale, and why human spillover is episodic and hard to predict.     

Expanding and invading plant species in sagebrush steppe affect multiple aspects of small-mammal ecology

Invasion and expansion of non-native and native plants have altered vegetation structure in many terrestrial ecosystems. Small mammals influence multiple ecosystem processes through their roles as ecosystem engineers, predators, and prey, and changes to vegetation structure can affect habitat use, community composition, and predator-prey interactions for this assemblage of wildlife. In the sagebrush (Artemisia spp.) shrublands of the western United States, invasion by non-native grasses and expansion of native conifer trees beyond their historical range has altered vegetation structure. These changes may potentially affect distributions and interactions of deer mice (Peromyscus maniculatus), which are generalist omnivores, and Columbia Plateau pocket mice (Perognathus parvus), more specialized granivores. To assess the extent to which altered habitat affects small-mammal density, survival, and home-range size, we examined these aspects of small-mammal ecology along a gradient of cheatgrass (Bromus tectorum) invasion and western juniper (Juniperus occidentalis) establishment in sagebrush shrublands in southwestern Idaho, USA. From 2017–2019, we used a spatially explicit mark-recapture design to examine attributes of small-mammal ecology along an invasion gradient. We did not find support for an effect of cheatgrass cover on density or survival of either species. Home-range size of deer mice was 2.3 times smaller in heavier cheatgrass cover (60%) compared to areas with little or no cheatgrass cover. Density of deer mice was highest (5 individuals/ha) in areas with 10% juniper cover and decreased with increasing juniper cover, whereas density of pocket mice was positively influenced by shrub cover. Survival of deer mice declined as juniper stem density increased. Conversely, survival of pocket mice increased with increasing juniper stem density. We found evidence for interspecific interactions between these 2 species, in the form of a density-dependent effect of deer mice on pocket mouse home-range size. Home-range size for pocket mice was 2 times smaller in areas with the highest estimated density of deer mice compared to areas with low densities of deer mice. Our data provides unique information about how small mammals in the sagebrush steppe are affected by expanding and invasive plant species and potential ways that habitat restoration efforts, in the form of conifer removal, may influence small mammals. Understanding the response of small mammals to conifer expansion or removal may shed light on the demographic and numerical responses of other wildlife associated with the sagebrush biome, including predators.     

Bayou virus detected in non‐oryzomyine rodent hosts: an assessment of habitat composition,reservoir community structure,and marsh rice rat social dynamics

In the United States, Bayou virus (BAYV) ranks second only to Sin Nombre virus (SNV) in terms of hantavirus pulmonary syndrome (HPS) incidents, having been confirmed in cases from Texas and Louisiana since its discovery in 1994. This study on BAYV infection among sympatric, non‐oryzomyine rodents (“spillover”) in Freeport, TX, is the first to link patterns of hantavirus interspecific spillover with the spatiotemporal ecology of the primary host (marsh rice rat, Oryzomys palustris). Mark‐recapture and/or harvest methods were employed from March 2002 through May 2004 in two macrohabitat types. Rodent blood samples were screened for the presence of IgG antibody to BAYV antigen by IFA after which Ab‐positive blood, saliva, and urine were analyzed for the presence of viral RNA by nested RT‐PCR. From 727 non‐oryzomyine captures, five seropositive (but not viral RNA positive) individuals were detected: one each of Baiomys taylori, Peromyscus leucopus, and Reithrodontomys fulvescens; and two Sigmodon hispidus. Spillover hosts were not associated with macrohabitat where O. palustris abundance, density, or seroprevalence was highest. Rather, spillover occurred in the macrohabitat indicative of greater overall disturbance (as indicated by grazing and exotic plant diversity) and overall biodiversity. Spillover occurred during periods of high seroprevalence detected elsewhere within the study region. Spillover locations differed significantly from all other capture locations in terms of percent water, shrub, and grass cover. Although greater habitat and mammal diversity of old‐fields may serve to reduce seroprevalence levels by tempering intraspecific contacts between rice rats, greater diversity also may create an ecologically opportunistic setting for BAYV spillover. Impacts of varying levels of disturbance and biodiversity on transmission dynamics represent a vastly uncharacterized component of the evolutionary ecology of hantaviruses.