Site: Sevilleta LTER
Photo of a rodent sampling location at the Sevilleta LTER
Mike Friggens

In the spring of 1993, a flu-like disease appeared in young healthy adults in the Four Corners region of the southwestern United States, with an early mortality rate of 70%. The U.S. Centers for Disease Control identified the cause as a previously-unknown hantavirus, a group of viruses carried by rodents and known to infect humans in Asia and Europe, but not previously identified in North America. Now called Sin Nombre virus (SNV), this species is one of over 20 hantavirus species identified in the New World since the 1993 Four Corners outbreak. The close association of known hantaviruses and rodent populations meant that understanding the emergence of this unknown disease required detailed knowledge of the ecological context associated with rodent population dynamics and human contact.

SEV scientists at the University of New Mexico (UNM), including biologists and medical scientists working in the laboratory and field and in the rodent collections of UNM’s Museum of Southwestern Biology, initiated a long-term effort to understand the immediate cause of the hantavirus outbreak and the ecology of this infectious disease in North America. Working with state and federal scientists, the team sampled rodent populations around the homes and workplaces of each patient, finding 30% of rodents trapped were infected with SNV and that rodent numbers were higher than average where the patients spent most of their time. The prediction that rodent numbers were higher than usual during 1993 was confirmed by ongoing sampling of rodent density, initiated in 1989 with the establishment of the Sevilleta LTER, in the same climatic region as the hantavirus outbreak. Analysis of long-term data from SEV further revealed that annual precipitation strongly influenced rodent density in the following year because of its effect on vegetation growth in this water limited region. Additional work by the research team revealed that the abundance of SNV-positive rodents was highly correlated with previous-year rodent density.

This research was one of the early studies to document the direct link between conditions in the physical environment and human health as mediated by ecological processes that influence the transmission dynamics of disease agents (the Ecology of Infectious Diseases is now a thriving field, as evidenced by an ongoing NSF panel of that name). The above-average precipitation in 1992 was driven by the El Nino Southern Oscillation, a fluctuation in sea surface temperature off the west coast of South America that drives above-average precipitation in the southwestern U.S. The hypothesis that El Nino precipitation anomalies were associated with the rodent population increases that led to the SNV outbreak were validated during the 1997-1998 El Nino. Finally, the rapid understanding of the mechanisms behind the hantavirus outbreak in 1993 was only possible because of the availability of rodent population data for comparison with precipitation data at SEV.

Graph for
Annual precipitation (top panel), Normalized Difference Vegetation Index (NDVI), an index of vegetation greenness associated with vegetation productivity (middle panel) and rodent density (rodents/km2) in grasslands at the Sevilleta LTER before and after the hantavirus outbreak in the Four Corners region. Arrows between panels show the linkages between precipitation and vegetation productivity in the same year and rodent density in the following year.
Yates et al. 2002