Ecology of zoonotic diseases
Figuring out the what, where and when of disease outbreaks
By Nadine Lymn, ESA director of public affairs
Plague, Lyme disease, Hantavirus, West Nile Virus—these bacteria and viruses are zoonotic diseases that can be transmitted to people from animals like ticks, mosquitoes and rodents and were the subject of a recent Ecological Society of America (ESA) congressional briefing.
Disease transmission is an environmental issue–understanding the ecological dynamics at play is crucial. Ecology can help sleuth out the source of new diseases and help predict where and when new outbreaks are likely to occur. That was a key message Robert Parmenter and Gregory Glass had for the congressional, federal agency and scientific society staff attending ESA’s briefing on April 23, 2013.
Parmenter directs the Scientific Services Division of the US Department of Agriculture’s Valles Caldera National Preserve in New Mexico. He has years of experience with zoonoses, especially plague and Hantavirus. Glass is a professor at Johns Hopkins School of Public Health and directs the Global Biological Threat Reduction Program of the Southern Research Institute. His work has included a focus on Lyme disease and Anthrax, along with hantaviruses. Between the two of them, Parmenter and Glass painted a vivid picture of the dynamics of these diseases and how collaborations between ecological and medical research can solve disease mysteries, such as that of the first hantavirus outbreak in the Four Corners region of the US.
In May of 1993, people were coming down with flu-like symptoms that rapidly filled their lungs with fluid, killing some of them. Others just as quickly recovered. A sense of panic gripped the area and scientists from the Centers for Disease Control arrived and, within 19 days, identified the disease as Hantavirus Pulmonary Syndrome for which there is neither a vaccine nor a cure. But where had it come from and why now?
Enter the ecologists who began field surveys and discovered that the source for the disease was the deer mouse, whose aerosolized urine and feces can infect humans with Hantavirus if inhaled. An unusually wet winter had led to a boom in the deer mouse 
population, which resulted in many of them descending from higher elevation forested areas to lower elevation areas where people tend to live, increasing the likelihood of mouse-human encounters. Since 1993, there have been other Hantavirus outbreaks, but now, with a much better understanding of the ecological factors in play, people can be notified ahead of time that conditions are right for increased chances of exposure and take appropriate precautionary steps.
It turns out that different varieties of Hantavirus are present in a variety of rodents across North and South America. Rats living in big cities like Baltimore or Washington, DC’s? Hantavirus is present. Hantavirus can move from rodent to rodent or from rodent to human—it doesn’t need an intermediary. But other zoonotic diseases, like plague, do.
Plague was known as the Black Death in the Middle Ages when it killed an estimated 30-60 percent of Europe’s human population. Once it infects a victim’s bloodstream, plague causes the skin to die and turn black. Parmenter showed truly hideous images of what that looks like. But if you thought plague is a disease that disappeared centuries ago, think again. It is very much present in the western half of the United States. It arrived in North America in 1899, when the Nippon Maru docked in San Francisco and plague-infected rats disembarked. Unlike Hantavirus, plague’s transmission to humans is more complicated. It needs a vector to convey it from rodents to humans. That vector is the flea, a blood-sucking parasite that sometimes abandons its wild rodent host to move to other mammals, including humans.
“I think we should change New Mexico’s state motto to Land of the Flea, Home of the Plague,” said Parmenter. Nearly 80 percent of all US cases of plague occur in the Southwest and Parmenter lives smack dab in the hot zone. Rock squirrel fleas are the primary source but handling infected prairie dogs, cats, rabbits and other animals is another way people can contract plaque. As with hantavirus, weather plays an important role in plague outbreaks by influencing the populations of mammal hosts as well as the breeding success of fleas. The good news is that New Mexico’s health department has successfully stopped each individual human case of plague before it could progress. Vigilance and early treatment with antibiotics can effectively halt plague. The intriguing ecological mystery of plague in the US, said Parmenter, is that it does not occur East of the 100th Meridian.
Temperature, humidity, stream flows and other data collected by federal agencies, give ecologists crucial information they can use to evaluate environmental conditions and help forecast likely disease outbreaks. Agencies such as the National Aeronautics and Space Administration, National Oceanic and Atmospheric Administration and US Geological Survey use a variety of satellites as well as on-the-ground instruments to collect environmental data for their own purposes, said Glass. But, together with what ecologists know about species that play a key role in a disease, agencies provide valuable data that make it possible to understand what is going on with a pathogen and where and when people are most likely to encounter it.
“Are there some clear predictors of disease outbreaks?” one briefing attendee asked. Temperature and precipitation are often key, said Glass, but must be viewed within context. In a tropical region, an unusually wet season can actually lead to fewer mosquito vectors because rivers and other waters move too swiftly for the insects to breed, so a disease like malaria would be expected to go down. But in a dry, desert-like region, an unusually wet season could create breeding pools that lead to an increase of malaria.
“Will climate change affect disease?” another person wanted to know. Definitely, was the answer. In the US, a disease like Lyme is projected to spread northward as the climate grows warmer, said Glass. A disease could also move out of a range it’s been in previously, if climate change makes a given region inhospitable to that pathogen.
Here in the Washington, DC area, the black-legged ticks—vectors of Lyme disease and the Asian Tiger mosquito, vector of West Nile, will soon be re-appearing. But at least we don’t need to worry about plague.
Photos: Asian Tiger mosquito, James Gathany, CDC; Deer Mouse, Sally King, NPS; Parmenter and Glass during ESA briefing, Nadine Lymn, ESA.