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. 2012;7(6):e37254.
doi: 10.1371/journal.pone.0037254. Epub 2012 Jun 29.

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

Karoun H Bagamian  1 , Richard J Douglass, Arlene Alvarado, Amy J Kuenzi, Brian R Amman, Lance A Waller, James N Mills
Affiliations

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

Karoun H Bagamian et al. PLoS One. 2012.

Abstract

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.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Diagram of enclosures, nest burrows, and experimental design for density experiments.
Each enclosure had 4 nest burrows as depicted in Enclosure 1 (lower left). The external trapping grid had 26 lines of traps in 4 rows; traps were spaced approximately 10 meters apart (farther at the corners; drawing not to scale). The first trap of each line was placed flush to the enclosure, with all subsequent traps spaced about 10-m apart. Although the external grid surrounded the entire enclosure array, only two sides are depicted. Figure applies to experiments 1–3. Experiment A differed in having 3 susceptible mice in all 6 enclosures (i.e. no high density treatment).
Figure 2
Figure 2. Incidence of Sin Nombre virus transmission in North American deermice Peromyscus maniculatus).
(a)incidence by season/experiment and (b) incidence by density treatment and experiment. The incidence of transmission (number of transmission events per 100 mouse-weeks of observation, expressed as a percentage (see , Appendix S1.4) is reported above the each bar for (a) each season (each experiment A, 1, 2, 3) and (b) per density treatment for experiments 1–3. Numbers of transmission events/mouse-weeks are reported within each bar.
Figure 3
Figure 3. Seasonal median number of new wounds per individual deermouse.
Thick horizontal line is the median; top and bottom of boxes represent the 25th and 75th percentiles; whiskers indicate ranges, excluding outliers. Outlier is indicated by black dot. Medians with the same letter above the box are not significantly different.
Figure 4
Figure 4. Proportion of adult, male deermice with scrotal testes inside and outside enclosures.
a) The proportion of scrotal adult males (of total adult males captured) 2 weeks post-release into the enclosures inside (by density treatment) and outside of enclosures during each experiment. Experiment A only had low-density treatment groups; experiments 1–3 had high- and low- density treatments. Numbers of scrotal/total for each experiment are denoted above bars. Bars with the same letter above them are not significantly different within each category (high, low, outside) between experiments. Statistically significant comparisons between categories (high vs. low vs. outside) for experiment 3 are indicated by asterisks. b) Proportion scrotal at 4 trapping sessions by density treatment and by location (inside vs. outside enclosures) during Experiment 2. Numbers of scrotal/total for each trap session are denoted above bars. Statistically significant differences between categories (high, low, outside) at a given trap session are indicated by asterisks.
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