Food availability leads to more connected contact networks among peridomestic zoonotic reservoir hosts.

The North American deermouse (Peromyscus maniculatus) is a reservoir host for many zoonotic pathogens. Deermice have been well studied, but few studies have attempted to understand social interactions within the species despite these interactions being key to understanding disease transmission. We performed an experiment to determine if supplemental food or nesting material affected social interactions of deermice and tested if interactions increased with increasing population density. We constructed three simulated buildings that received one of three treatments: food, nesting material, or control. Mice were tagged with passive integrated transponder (PIT) tags, and their movement in and out of buildings was monitored with PIT tag readers. PIT tag readings were used to create contact networks, assuming a contact if two deermice were in the same building at the same time. We found that buildings with food led to contact networks that were approximately 10 times more connected than buildings with nesting material or control buildings. We also saw a significant effect of population density on the average number of contacts per individual. These results suggest that food supplementation which is common in peridomestic settings, can significantly increase contacts between reservoir hosts, potentially leading to increased transmission of zoonotic viruses within the reservoir host and from reservoir hosts to humans.

Species diversity concurrently dilutes and amplifies transmission in a zoonotic host-pathogen system through competing mechanisms.

In this era of unprecedented biodiversity loss and increased zoonotic disease emergence, it is imperative to understand the effects of biodiversity on zoonotic pathogen dynamics in wildlife. Whether increasing biodiversity should lead to a decrease or increase in infection prevalence, termed the dilution and amplification effects, respectively, has been hotly debated in disease ecology. Sin Nombre hantavirus, which has an ∼35% mortality rate when it spills over into humans, occurs at a lower prevalence in the reservoir host, the North American deermouse, in areas with higher small mammal diversity-a dilution effect. However, the mechanism driving this relationship is not understood. Using a mechanistic mathematical model of infection dynamics and a unique long-term, high-resolution, multisite dataset, it appears that the observed dilution effect is a result of increasing small-mammal diversity leading to decreased deermouse population density and, subsequently, prevalence (a result of density-dependent transmission). However, once density is taken into account, there is an increase in the transmission rate at sites with higher diversity-a component amplification effect. Therefore, dilution and amplification are occurring at the same time in the same host-pathogen system; there is a component amplification effect (increase in transmission rate), but overall a net dilution because the effect of diversity on reservoir host population density is stronger. These results suggest we should focus on how biodiversity affects individual mechanisms that drive prevalence and their relative strengths if we want to make generalizable predictions across host-pathogen systems.

INTERSPECIFIC COMPARISON OF HANTAVIRUS PREVALENCE IN PEROMYSCUS POPULATIONS FROM A FRAGMENTED AGRO-ECOSYSTEM IN INDIANA, USA.

Comparatively little is known about hantavirus prevalence within rodent populations from the Midwestern US, where two species of native mice, the prairie deer mouse ( Peromyscus maniculatus bairdii) and the white-footed mouse ( Peromyscus leucopus noveboracensis), are dominant members of rodent communities. We sampled both species in central Indiana and tested individuals for presence of hantavirus antibodies to determine whether seroprevalence (percent of individuals with antibodies reactive to Sin Nombre virus antigen) differed between species, or among different habitat types within fragmented agro-ecosystems. Prevalence of hantavirus antibodies varied significantly between species, with seroprevalence in prairie deer mice (21.0%) being nearly four times higher than in white-footed mice (5.5%). Seroprevalence was almost eight times higher within the interior of row-crop fields (37.7%) occupied solely by prairie deer mouse populations, relative to field edges (5.2%) or adjacent forest habitat (6.1%). In the fragmented Midwestern agro-ecosystem of this study, prairie deer mice appear to be the dominant hantavirus reservoir, with particularly high seroprevalence in populations within the interior of row-crop fields.

Toward a Mechanistic Understanding of Environmentally Forced Zoonotic Disease Emergence: Sin Nombre Hantavirus.

Understanding the environmental drivers of zoonotic reservoir and human interactions is crucial to understanding disease risk, but these drivers are poorly predicted. We propose a mechanistic understanding of human-reservoir interactions, using hantavirus pulmonary syndrome as a case study. Crucial processes underpinning the disease's incidence remain poorly studied, including the connectivity among natural and peridomestic deer mouse host activity, virus transmission, and human exposure. We found that disease cases were greatest in arid states and declined exponentially with increasing precipitation. Within arid environments, relatively rare climatic conditions (e.g., El Niño) are associated with increased rainfall and reservoir abundance, producing more frequent virus transmission and host dispersal. We suggest that deer mice increase their occupancy of peridomestic structures during spring-summer, amplifying intraspecific transmission and human infection risk. Disease incidence in arid states may increase with predicted climatic changes. Mechanistic approaches incorporating reservoir behavior, reservoir-human interactions, and pathogen spillover could enhance our understanding of global hantavirus ecology, with applications to other directly transmitted zoonoses.

Increased detection of Sin Nombre hantavirus RNA in antibody-positive deer mice from Montana, USA: evidence of male bias in RNA viremia.

Hantaviruses are widespread emergent zoonotic agents that cause unapparent or limited disease in their rodent hosts, yet cause acute, often fatal pulmonary or renal infections in humans. Previous laboratory experiments with rodent reservoir hosts indicate that hantaviruses can be cleared from host blood early in the infection cycle, while sequestered long term in various host organs. Field studies of North American deer mice (Peromyscus maniculatus), the natural reservoir of Sin Nombre hantavirus, have shown that viral RNA can be transiently detected well past the early acute infection stage, but only in the minority of infected mice. Here, using a non-degenerate RT-PCR assay optimized for SNV strains known to circulate in Montana, USA, we show that viral RNA can be repeatedly detected on a monthly basis in up to 75% of antibody positive deer mice for periods up to 3-6 months. More importantly, our data show that antibody positive male deer mice are more than twice as likely to have detectable SNV RNA in their blood as antibody positive females, suggesting that SNV-infected male deer mice are more likely to shed virus and for longer periods of time.

Transmission ecology of Sin Nombre hantavirus in naturally infected North American deermouse populations in outdoor enclosures.

Sin Nombre hantavirus (SNV), hosted by the North American deermouse (Peromyscus maniculatus), causes hantavirus pulmonary syndrome (HPS) in North America. Most transmission studies in the host were conducted under artificial conditions, or extrapolated information from mark-recapture data. Previous studies using experimentally infected deermice were unable to demonstrate SNV transmission. We explored SNV transmission in outdoor enclosures using naturally infected deermice. Deermice acquiring SNV in enclosures had detectable viral RNA in blood throughout the acute phase of infection and acquired significantly more new wounds (indicating aggressive encounters) than uninfected deermice. Naturally-infected wild deermice had a highly variable antibody response to infection, and levels of viral RNA sustained in blood varied as much as 100-fold, even in individuals infected with identical strains of virus. Deermice that infected other susceptible individuals tended to have a higher viral RNA load than those that did not infect other deermice. Our study is a first step in exploring the transmission ecology of SNV infection in deermice and provides new knowledge about the factors contributing to the increase of the prevalence of a zoonotic pathogen in its reservoir host and to changes in the risk of HPS to human populations. The techniques pioneered in this study have implications for a wide range of zoonotic disease studies.

Population density and seasonality effects on Sin Nombre virus transmission in North American deermice (Peromyscus maniculatus) in outdoor enclosures.

Surveys of wildlife host-pathogen systems often document clear seasonal variation in transmission; conclusions concerning the relationship between host population density and transmission vary. In the field, effects of seasonality and population density on natural disease cycles are challenging to measure independently, but laboratory experiments may poorly reflect what happens in nature. Outdoor manipulative experiments are an alternative that controls for some variables in a relatively natural environment. Using outdoor enclosures, we tested effects of North American deermouse (Peromyscus maniculatus) population density and season on transmission dynamics of Sin Nombre hantavirus. In early summer, mid-summer, late summer, and fall 2007-2008, predetermined numbers of infected and uninfected adult wild deermice were released into enclosures and trapped weekly or bi-weekly. We documented 18 transmission events and observed significant seasonal effects on transmission, wounding frequency, and host breeding condition. Apparent differences in transmission incidence or wounding frequency between high- and low-density treatments were not statistically significant. However, high host density was associated with a lower proportion of males with scrotal testes. Seasonality may have a stronger influence on disease transmission dynamics than host population density, and density effects cannot be considered independent of seasonality.

Prediction of Peromyscus maniculatus (deer mouse) population dynamics in Montana, USA, using satellite-driven vegetation productivity and weather data.

Deer mice (Peromyscus maniculatus) are the main reservoir host for Sin Nombre virus, the primary etiologic agent of hantavirus pulmonary syndrome in North America. Sequential changes in weather and plant productivity (trophic cascades) have been noted as likely catalysts of deer mouse population irruptions, and monitoring and modeling of these phenomena may allow for development of early-warning systems for disease risk. Relationships among weather variables, satellite-derived vegetation productivity, and deer mouse populations were examined for a grassland site east of the Continental Divide and a sage-steppe site west of the Continental Divide in Montana, USA. We acquired monthly deer mouse population data for mid-1994 through 2007 from long-term study sites maintained for monitoring changes in hantavirus reservoir populations, and we compared these with monthly bioclimatology data from the same period and gross primary productivity data from the Moderate Resolution Imaging Spectroradiometer sensor for 2000-06. We used the Random Forests statistical learning technique to fit a series of predictive models based on temperature, precipitation, and vegetation productivity variables. Although we attempted several iterations of models, including incorporating lag effects and classifying rodent density by seasonal thresholds, our results showed no ability to predict rodent populations using vegetation productivity or weather data. We concluded that trophic cascade connections to rodent population levels may be weaker than originally supposed, may be specific to only certain climatic regions, or may not be detectable using remotely sensed vegetation productivity measures, although weather patterns and vegetation dynamics were positively correlated.

Grazing Effects on Deer Mice with Implications to Human Exposure to Sin Nombre Virus.

We examined the effects of grazing on deer mouse (Peromyscus maniculatus) movements into buildings using passive integrated transponder (PIT) technology and small simulated buildings located on 0.6-ha treatment (grazing) and control (no grazing) plots. Twelve experimental 9-day trials were conducted over the course of the study. During these trials, mouse movements into buildings were monitored during three time periods (each 3 days in length). In the treatment plots these time periods corresponded to pre-grazing, grazing, and post grazing by horses. The number of individual deer mice entering buildings over time decreased in both the grazed and control plots during the 9 days of each experiment. The number of entrances per/individual among the pre-grazing, grazing and post grazing periods was different between control and treated plots for both males and females. The distribution of entrances/individual among the three periods differed between males and females in both grazed and control plots. The habitat modification caused by grazing appeared to reduce deer mouse activity (entrances/individual) in buildings but does not affect the number of mice entering buildings. Reducing vegetative cover by grazing or mowing may not affect the number of mice investigating small structures but grazing creates different activity patterns in the structures for neighboring deer mice.

Seasonal dispersal patterns of sylvan deer mice (Peromyscus maniculatus) within Montana rangelands.

We examined seasonal dispersal patterns and timing of new infections of Sin Nombre virus (SNV), as determined by recent acquisition of antibodies (seroconversion), in deer mice (Peromyscus maniculatus) at two Montana rangeland study sites over three years, 2004-2007. One study site was located in grassland habitat, and the other was located in shrub-steppe. In Montana, both of these habitats are commonly associated with peridomestic environments (in and around buildings). Peridomestic environments are where most reported human cases of hantavirus pulmonary syndrome (HPS) likely originate. Furthermore, deer mice dispersing from sylvan habitats colonize peridomestic environments. Thus, a thorough understanding of deer mouse dispersal is needed to help predict when humans are most at risk for exposure to SNV. We trapped mice at each study site twice a month, accumulating 85,200 trap nights of effort and capturing 6,185 individual deer mice a total of 22,654 times. We documented 980 dispersing individuals over 3 yr. We found positive correlations between the number of dispersing mice and number captured at each site, but there were no statistically significant seasonal differences in the number of dispersing mice. However, we did find a spring/summer bias in mice that seroconverted and dispersed, suggesting that recently infected deer mice are most likely to enter settings where humans may be exposed to SNV during spring and summer.

An Experimental Test of Factors Attracting Deer Mice into Buildings.

Deer mice (Peromyscus maniculatus) are the principal reservoir host of Sin Nombre virus (SNV). Deer mice use a wide variety of habitats including peridomestic settings in and around human dwellings, their presence in and around homes has been implicated as a risk factor for acquiring Hantavirus Pulmonary Syndrome. Deer mice are believed to enter buildings in order to gain access to a variety of resources including food, bedding material, and better thermal microclimates. However, no one has experimentally tested which factors influence mice use of buildings. We conducted experiments using small simulated buildings to determine the effects of two factors, i.e., food and bedding material, on mouse activity in these buildings. We also examined if these effects varied with time of year. We found that deer mice entered our buildings regardless of the presence or absence of food or bedding. However, the amount of activity in buildings was affected by what they contained. We found significantly higher indices of activity in buildings containing food compared to both empty buildings (control) and buildings containing bedding material. Time of year did not affect activity in buildings.

Seroprevalence against Sin Nombre virus in resident and dispersing deer mice.

Through dispersal, deer mice (Peromyscus maniculatus) enter peridomestic settings (e.g., outbuildings, barns, cabins) and expose humans and other deer mouse populations to Sin Nombre virus (SNV). In June 2004, research on deer mouse dispersal was initiated at 2 locations in Montana. During the course of the study, over 6000 deer mouse movements were recorded, and more than 1000 of these movements were classified as dispersal movements. More than 1700 individual deer mice were captured and tested for SNV, revealing an average SNV antibody prevalence of approximately 11%. Most of the dispersing and antibody-positive individuals were adult males. Among the few subadult dispersing mice discovered during the study, none were seropositive for SNV. Our results suggest that dispersal rates are higher in high abundance populations of deer mice and that during peak times of dispersal, human exposure to SNV, which commonly occurs in peridomestic settings, could increase.

Brush mouse (Peromyscus boylii) population dynamics and hantavirus infection during a warm, drought period in southern Arizona.

We monitored Limestone Canyon hantavirus (LSCV) antibody prevalence, host (brush mouse, Peromyscus boylii) abundance, and environmental variables (temperature and rainfall) in brush mice captured on three trapping webs in southern Arizona for 5 yr. Although seasonal patterns were subtle, we observed large multiyear variation in population abundance and antibody prevalence. Limestone Canyon hantavirus infection in brush mouse populations varied over time with prevalence ranging from 0% to 33%. At all trapping webs, evidence of infection disappeared completely for an extended period (up to 2 yr) and eventually reappeared, suggesting that dispersal may play a role in maintaining infection in brush mouse metapopulations. Weather during the study period was drier and warmer than average and these conditions, especially during spring through fall, may have contributed to low brush mouse population density and the local extinction of LSCV during the second year of the study. Nevertheless, population growth was associated with relatively warm, dry conditions during winter periods and a cool, wet spring and summer period in the fifth year of the study. After prolonged absence, LSCV infection was consistently detected only when brush mouse population abundance reached relatively high levels during that fifth year. Comparison of our results to similar studies suggests that stochastic events resulting in the loss or survival of a few infected mice in low-density host populations may result in local extinction of virus; reestablishment of infection may occur via immigration of infected individuals from adjacent populations, but may be successful only when populations are of sufficient density to support frequent rodent-to-rodent interactions and virus transmission.

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.

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.

Long-term dynamics of Sin Nombre viral RNA and antibody in deer mice in Montana.

Infections with hantaviruses in the natural host rodent may result in persistent, asymptomatic infections involving shedding of virus into the environment. Laboratory studies have partially characterized the acute and persistent infection by Sin Nombre virus (SNV) in its natural host, the deer mouse (Peromyscus maniculatus). However, these studies have posed questions that may best be addressed using longitudinal studies involving sequential sampling of individual wild-caught, naturally infected mice. Using enzyme immunoassay and polymerase chain reaction (PCR) analysis of monthly blood samples, we followed the infection status of deer mice in a mark-recapture study in Montana for 2 yr. Only six of 907 samples without IgG antibody to SNV contained detectable SNV RNA, suggesting that there is a very brief period of viremia before the host develops detectable antibody. The simultaneous presence of both antibody and viral RNA in blood was detected in consecutive monthly samples for as long as 3 mo. However, chronic infection was typified by alternating characteristics of PCR positivity and PCR negativity. Two possible interpretations of these results are that 1) viral RNA may be consistently present in the blood of chronically infected deer mouse, but that viral RNA is near the limits of PCR detectability or 2) SNV RNA sporadically appears in blood as a consequence of unknown physiological events. The occurrence of seasonal patterns in the proportion of samples that contains antibody and that also contained SNV RNA demonstrated a temporal association between recent infection (antibody acquisition) and presence of viral RNA in blood.

Removing deer mice from buildings and the risk for human exposure to Sin Nombre virus.

Trapping and removing deer mice from ranch buildings resulted in an increased number of mice, including Sin Nombre virus antibody-positive mice, entering ranch buildings. Mouse removal without mouse proofing will not reduce and may even increase human exposure to Sin Nombre hantavirus.