BEHAVIORAL ECOLOGY OF ORYZOMYS PALUSTRIS (MARSH RICE RAT) IN COASTAL TEXAS: IMPLICATIONS FOR THE LIFE-HISTORY STRATEGY OF BAYOU VIRUS (BAYV)
Prior to the 1990s, Hantaviruses (Family: Bunyaviridae) and their manifest human syndromes were viewed as public health concerns primarily for Eurasia. However, the Sin Nombre virus (SNV) outbreak of 1993-94 in the southwestern U. S. brought Hanta to North America’s front door, fueling intense global efforts by mammalogists, ecologists, virologists, and epidemiologists to identify and characterize other potential, host-hantavirus associations. Since then, more than 21 pathogenic, hantaviral genotypes have been discovered that can induce human hantavirus pulmonary syndrome (HPS), with symptoms ranging from proteinuria to pulmonary edema to massive exsanguination, with up to 40% of clinical cases ending in death. Over time, researchers have linked specific large-scale, ecological disturbances (e.g., climate change) and habitat conversion to increases in rodent host abundances with concomitant increases in HPS cases.
However, the factors involved in maintenance of hantavirus seroprevalence levels in host populations at finer scales have proven elusive, as rodent phenotypic responses to environmental context and sympatric species can drive infection patterns, and these relationships are usually non-linear and often extremely complex.
The marsh rice rat (Oryzomys palustris Harlan, 1837) is the primary host for the hantavirus genotype Bayou (BAYV; discovered in 1994), the second-most common genotype in N. A. and so far, it is responsible for HPS cases in Louisiana and Texas. O. palustris has the capacity for life longevity and distance dispersing, and is
an integral marshland mammal, performing important roles in the riverine and coastal marsh, trophic webs along the Atlantic Coast and the northern Gulf of Mexico. Although macro- and microhabitat preferences of Texas O. palustris have been clarified, we know little of its intraspecific or heterospecific social behaviors or other life history traits that may either accelerate or decelerate BAYV dissemination in a rodent community, having resultant ramifications for human BAYV infections.
Therefore, the essence of this dissertation is to advance the general understanding of BAYV occurrence and perpetuity in a terrestrial, small mammal community in coastal Texas, through a multi-scaled evaluation of community biodiversity, habitat composition, and rodent population demography and movements, in order to garner a better understanding of the life history strategy of BAYV. Unique approaches to describe the dynamical relationships of O. palustris and BAYV involved: (1) a melding of socioecological theory and GIS, and (2) the application of several robust regression techniques and The Dilution Effect. Using small mammal mark-recapture and harvest methods, specific emphases have been placed on filling research gaps in the transmission paradigm regarding: (1) the roles of breeding versus non-breeding females as potential infection drivers/mediators between seropositive and socially dominant, adult male O. palustris; (2) a possible “dilution effect” in this system, via a closer examination of the relative contributions of rodent species evenness, species richness, and true density of hosts on BAYV seroprevalence; and (3) discerning and describing infection costs in non-primary (male juveniles and subadults, and females of all age classes) O. palustris hosts by examining morphology, physiology, and behavior.
Contributions to the scientific community are as follows: (1) the first hantavirus study to investigate and show support for habitat selection in receptive females as a contributing factor to the spatiotemporal arrangement of socially dominant, seropositive males; (2) the first hantavirus study to parse out diversity effects on seroprevalence into species evenness and species richness and by using this approach, I determined that in this system, it is species richness (and not species evenness) that is the most reliable predictor of reducing BAYV seroprevalence in the rodent community; (3) the first empirical study to rigorously evaluate hantavirus infection costs in female and immature rodents, and to uncover that it is the females in all age groups, non-pregnant and pregnant, and their embryos and fetuses, which are negatively impacted by BAYV infection; and (4) the only robust dataset known to provide insights into rare life history information for Texas O. palustris populations. For the global community, the value of this research lies in its illumination of potential evolutionary trade-offs in BAYV host selection, infection, and fitness, in which all are strongly influenced by habitat characteristics. With a clearer understanding of the life history strategy of BAYV and the behavioral ecology of Texas O. palustris, we are now better equipped to forecast potential BAYV outbreaks in human population centers along the Atlantic coastline and the northern Gulf of Mexico, areas crucial to the U. S. economy, but also some of the most fragile and abused habitats in North America.