Directly tracking disease-carrying mice from space would seem to be a tall order – and even without knowing the full capabilities of military satellites, I suspect the ability to do so is still a couple of years off yet. But researchers at the University of Utah have come up with an indirect way of tracking rodents by using satellite images to monitor surges in vegetation that boost mouse populations. Such a method could help forecast outbreaks of rodent-borne illnesses worldwide by allowing the creation of risk maps that show when and where outbreaks are likely to occur.
For the study, researchers tested the method on deer mice that carry hantavirus and proliferate when their food supply is abundant, but they say it could be applied to any animal that responds to vegetation. By combining satellite imagery with data from thousands of mice captured over three years in central Utah they found that the total number of trapped mice and the number of mice with a strain of hantavirus known as Sin Nombre virus both climbed after peaks in greenery.
Sin Numbre virus is carried by rodents, primarily deer mice, in the western U.S. and it causes a disease in humans known as hantavirus pulmonary syndrome. Hantavirus is contracted by inhaling dust containing mouse urine of feces and kills 42 percent of its victims.
Although previous studies have looked for links between satellite images and deer mouse populations, they used less trapping data that was collected in a single trapping season. For their study, the University of Utah researchers caught mice over six trapping seasons in the spring and autumn over three years to reveal how the population changed over time.
The researchers also used several different methods for estimating the amount of fresh vegetation in an area from satellite images. Their goal was to see which measures are the best predictors of mouse populations so that health officials could use the information to see where hantavirus outbreaks are likely to occur.
It was found that, although all vegetation indices significantly correlated with mouse population, the best predictors were two greenness indices known as the Normalized Difference Vegetation Index (NDVI) and the enhanced vegetation index (EVI). For both indices they found the strongest correlations were for mouse population booms one year and 1.3 years after surges in plant growth stemming from snow and rainfall.
"The point of this whole exercise is to develop disease-risk maps, which would show the distribution of infected hosts – in this case, deer mice – overlaid with human population density," explains Thomas Cova, an author of the study and associate professor of geography at the University of Utah. "Although the focus of this work is hantavirus in deer mice, it contributes to our broader understanding of how to monitor the spread of infectious diseases from space, which in the long run could save lives," he adds.
After vegetation growth spurts, the fraction of mice that carried hantavirus stayed relatively constant, but the absolute number of infected mice increased along with the mouse population. More infected mice mean more opportunities for humans to get sick, so mouse population size indicates risk to humans.
The study indicates people are at greatest risk of catching hantavirus a little over a year after peaks in plant growth and it pinpoints the best methods for measuring those peaks. The researchers say this should allow the creation of risk maps showing where and when outbreaks are likely to occur.
The University of Utah researchers say the results of the study are consistent with earlier findings showing an increase in hantavirus infections the year after the heavy rains of the 1991-1992 El Nino climate pattern that brought increased rains in the U.S. Southwest.