Eco-Epidemiology: Ecology's Impact on Public Health
It’s dusk. The sun is just above the horizon and tree cover darkens the forest. Off the worn path, the only sounds we hear are the crunching leaves and twigs under our feet and small animals scurrying as we approach. I use a walking stick to keep from tripping on something buried in the litter and catch spider webs in front of me. I'm wandering through the forest, checking and setting Sherman traps to catch white-footed mice, for my practicum this summer.
I am conducting fieldwork in eco-epidemiology—an emerging field that explores how biological and environmental factors influence human disease—in collaboration with professor Maria Diuk-Wasser from Columbia’s Ecology, Evolution, and Environmental Biology Department. Our group seeks to understand the interplay between tick-borne diseases in the Northeast and biological and environmental drivers. Particularly, checking mice to predict an emerging pathogen outbreak and how climate may be affecting populations of the disease vector, Ixodes scapularis, the blacklegged tick.
My day starts at dawn when we head out to the forest to collect the Sherman traps we set the night before. The forest is much different during the day. The smell of dew and oak reminds me of Girl Scout camp, and birds singing around us are reminiscent of a David Attenborough documentary.
We check the traps hoping to catch Peromyscus leucopus, the primary reservoir of Lyme disease and other tick-borne pathogens in the Northeast, otherwise known as the white-footed mouse. Occasionally chipmunks, voles, shrews, and flying squirrels find their way into the traps. Some days we find our traps emptied, moved, and sometimes taken apart. Raccoons are the likely culprit—too big to fit in the trap but crafty enough to go trap-by-trap scavenging the oatmeal bait.
We tag the mice and other small mammals to track to their disease status throughout this field season, compare them to previous years, and follow them next season. Then we collect their external parasites, including ticks, fleas, and mites, along with a fecal sample, to determine their internal parasite load. By examining the parasite diversity of mice, we can gain a better understanding of how pathogens and parasites interact.
Before releasing the animals, we also collect blood and tissue samples for genetic information about the specimen and screen for infection. By comparing tick infection to mouse infection, we can learn how efficiently the ticks are transmitting diseases across locations. Insights from these studies can be used by disease ecologists or public health professionals to advance our understanding of tick-borne disease surveillance and prevention.
Most important for our main study, however, are the ticks that will be processed back at the lab and tested for infection of Borrelia burgdorferi and Babesia microti, the disease-causing agents of Lyme disease and babesiosis, respectively. Lyme disease is a complex bacterial infection that can be treated, but can have serious symptoms if left untreated including arthritis and neurological disorders. If you have ever wondered why the band Bikini Kill broke up, it was because frontwoman Kathleen Hanna was suffering from severe Lyme symptoms. Babesiosis is an emerging disease in the United States and is potentially fatal to the elderly and immuno-compromised.
Recent research suggests that B. microti may depend on B. burgdorferi to invade new host populations and maintain enzootic transmission. If this is the case, then we can use our findings from the field to inform mathematical models to predict when and where the infections may emerge. With this insight, public health departments can develop targeted prevention programs, and healthcare facilities can be notified to screen blood samples to prevent blood-borne transmission.
After empting the traps and collecting our biological samples, we drag for ticks, pulling pieces of corduroy cloth along the forest floor. Ticks looking for their next blood meal cling onto the fabric as if it were a potential host. We collect the ticks and process them for pathogens back in the lab. Disease transmission to humans typically occurs from nymphs, the younger and smaller life-stage, because their size leaves them undetected, allowing time to transmit the pathogen. At first glance, they are difficult to spot on the cloth, but a tick becomes noticeable when it shimmers from reflected sunlight as they stumble across the grooved fabric.
Each dragging site across Connecticut and Rhode Island has a weather station to compare the presence and density of ticks to climatic variables. The range of habitat for the blacklegged tick may be expanding and increasing variation in density due to climate change. Our group plans to use this data to inform models of human risk of tick-borne diseases.
Vector-borne diseases are on the rise as the range of habitat and survival for vectors and reservoirs are altered due to climate change. Tick-borne diseases in particular are becoming more prevalent in the United States along with the emergence of new pathogens. The best way to monitor human risk is to understand the vector population. Unfortunately, most public health departments do not have the resources to conduct a robust or unified vector surveillance program. This makes interdisciplinary collaborations with investigators such as Dr. Diuk-Wasser critically important for preventing new cases of tick-borne illnesses.
We often think of emerging diseases as exotic infections that occur on the other side of the world. But they are also occurring in our own backyards. Once restricted to a small area of the east coast, Lyme disease risk is increasing in the north eastern and north central United States, and the range of I. scapularis is expanding, at least in part due to changing weather conditions. To limit the rate of future cases of Lyme in addition to other vector-borne and zoonotic diseases in the U.S., it is critically important to consider both the biological and environmental influences of complex public health issues in light of our changing climate.
By Sara Zufan, Environmental Health Sciences, MPH '17
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