Diluting Disease with Felicia Keesing and Rick Ostfeld
Episode 32 // Released January 16, 2020
In this episode of Big Biology, Felicia Keesing and Rick Ostfeld explain the dilution effect.
Many diseases occur more often in biologically simple communities or on cultivated land--regions that are not biologically diverse
If there are lots of species in a region that are poor hosts for a disease or bad neighbors to its vector, the potential pool of susceptible organisms is diluted. This protects individuals that are vulnerable to the disease.
Felicia and Rick coined the term studying Lyme disease.
SUMMARY
How is declining biodiversity affecting the occurrence and spread of Lyme disease? Is there a way to reduce the transmission of tick-borne diseases using ecological approaches?
On this episode of Big Biology we talk with Felicia Keesing and Rick Ostfeld, two disease ecologists working at the Cary Institute of Ecosystem Studies in Millbrook, New York. Felicia is a professor at Bard College, and Rick is a staff scientist at the Cary Institute. They study the ecology of tick-borne illnesses including a remarkable phenomenon called the dilution effect.
In front of a live audience, we discussed the dilution effect, a term Felicia and Rick coined 20 years ago that is based on their study of ticks, mice and the causative agent of Lyme disease, a bacterium called Borrelia burgdorferi. White-footed mice, which are common in the forests of the eastern and central U.S., are especially good at carrying Borrelia and are often responsible for passing it on to ticks. Felicia and Rick observed that biodiverse ecosystems tend to have fewer infected ticks and hence lower rates of Lyme infection. In other words, high host diversity dilutes the risk of disease.
Read about their Tick Project, focused on testing environmental interventions to prevent Lyme and other tick-borne diseases.
RESOURCES
Here are links to a few of the resources we talked about on the episode or used to prepare for the interview:
BOOKS:
“Lyme Disease: The Ecology Of A Complex System”
This controversial book is a comprehensive, synthetic review of research on the ecology of Lyme disease in North America. It describes how humans get sick, why some years and places are so risky and others not. It challenges dogma - for instance, that risk is closely tied to the abundance of deer - and replaces it with a new understanding that embraces the complexity of species and their interactions. It describes why the place where Lyme disease emerged - coastal New England - set researchers on mistaken pathways. It shows how tiny acorns have enormous impacts on our probability of getting sick, why biodiversity is good for our health, why living next to a small woodlot is dangerous, and why Lyme disease is an excellent model system for understanding many other human and animal diseases. Intended for an audience of professional and student ecologists, epidemiologists, and other health scientists, it is written in an informal style accessible also to non-scientists interested in human health and conservation.
“Infectious Disease Ecology: Effects Of Ecosystems On Disease And Of Disease On Ecosystems”
Gathering thirteen essays by forty leading experts who convened at the Cary Conference at the Institute of Ecosystem Studies in 2005, this book develops an integrated framework for understanding where these diseases come from, what ecological factors influence their impacts, and how they in turn influence ecosystem dynamics. It marks the first comprehensive and in-depth exploration of the rich and complex linkages between ecology and disease, and provides conceptual underpinnings to understand and ameliorate epidemics. It also sheds light on the roles that diseases play in ecosystems, bringing vital new insights to landscape management issues in particular. While the ecological context is a key piece of the puzzle, effective control and understanding of diseases requires the interaction of professionals in medicine, epidemiology, veterinary medicine, forestry, agriculture, and ecology. The essential resource on the subject, Infectious Disease Ecology seeks to bridge these fields with an ecological approach that focuses on systems thinking and complex interactions.
PAPERS:
Frontiers in research on biodiversity and disease. (Ecology Letters, 2015):
Global losses of biodiversity have galvanised efforts to understand how changes to communities affect ecological processes, including transmission of infectious pathogens. Here, we review recent research on diversity–disease relationships and identify future priorities. Growing evidence from experimental, observational and modelling studies indicates that biodiversity changes alter infection for a range of pathogens and through diverse mechanisms. Drawing upon lessons from the community ecology of free-living organisms, we illustrate how recent advances from biodiversity research generally can provide necessary theoretical foundations, inform experimental designs, and guide future research at the interface between infectious disease risk and changing ecological communities. Dilution effects are expected when ecological communities are nested and interactions between the pathogen and the most competent host group(s) persist or increase as biodiversity declines. To move beyond polarising debates about the generality of diversity effects and develop a predictive framework, we emphasise the need to identify how the effects of diversity vary with temporal and spatial scale, to explore how realistic patterns of community assembly affect transmission, and to use experimental studies to consider mechanisms beyond simple changes in host richness, including shifts in trophic structure, functional diversity and symbiont composition.
Recent controversy over whether biodiversity reduces disease risk (dilution effect) has focused on the ecology of Lyme disease, a tick-borne zoonosis. A criticism of the dilution effect is that increasing host species richness might amplify disease risk, assuming that total host abundance, and therefore feeding opportunities for ticks, increase with species richness. In contrast, a dilution effect is expected when poor quality hosts for ticks and pathogens (dilution hosts) divert tick blood meals away from competent hosts. Even if host densities are additive, the relationship between host density and tick encounters can be nonlinear if the number of ticks that encounter a host is a saturating function of host density, which occurs if ticks aggregate on the remaining hosts rather than failing to find a host before death. Both dilution and amplification are theoretical possibilities, and assessing which is more prevalent required detailed analyses of empirical systems. We used field data to explore the degree of tick redistribution onto fewer individuals with variation in intraspecific host density and novel data-driven models for tick dynamics to determine how changes in vertebrate community composition influence the density of nymphs infected with the Lyme bacterium. To be conservative, we allowed total host density to increase additively with species richness. Our long-term field studies found that larval and nymphal ticks redistribute onto fewer individuals as host densities decline, that a large proportion of nymphs and adults find hosts, and that mice and chipmunks feed a large proportion of nymphs. White-footed mice, eastern chipmunks, short-tailed shrews, and masked shrews were important amplification hosts that greatly increased the density of infected nymphs. Gray squirrels and Virginia opossums were important dilution hosts. Removing these two species increased the maximum number of larvae attached to amplification hosts by 57%. Raccoons and birds were minor dilution hosts under some conditions. Even under the assumption of additive community assembly, some species are likely to reduce the density of infected nymphs as diversity increases. If the assumption of additivity is relaxed, then species that reduce the density of small mammals through predation or competition might substantially reduce disease risk.
LINK:
Read about their Tick Project, focused on testing environmental interventions to prevent Lyme and other tick-borne diseases.
IMAGES:
Recording the episode!
Our wonderful audience.
MEET THE GUESTS
Names: Felicia Keesing and Rick Ostfeld
Institutions: Bard College and the Cary Institute
Areas of Expertise: Disease Ecology
Lab Websites: The Keesing Lab, Rick Ostfeld
QUOTEABLE
“If you say what’s the strongest argument against the dilution effect, that’s like saying what’s the strongest argument against rainbows. It happens …The rich conversation is around the generality. Can we predict it? Can we guess whether it’s going to be in a system?”
Felicia Keesing
“Initially the dilution effect was an idea that we came upon in an inductive sense, so we were collecting information in a system. We were studying the way the system operates and we were realizing that places where there were lots of species of mammals and birds seemed to have reduced tick populations and reduced tick infection.”
Rick Ostfeld
EXTRAS
Check out our 30 second Meet The Scientist clip from 1/20/20!
Also, check out Rick's jeans tucked into socks look in the photo above. He told us that's one of the easiest ways to avoid tick bites in New York. Remember to tuck in your pants next time you're walking in an area with Lyme.
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