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A Hidden Cycle of Lyme Disease Bacteria in Michigan’s Forests By Sarah Hamer, Jean Tsao and Graham Hickling (623 Words) Southwestern Michigan, April 2011: Lyme disease has emerged from the forest shadows to become one of the most talked-about public health threats in the United States. Hundreds of news articles have explained how dramatic recovery of deer numbers in recent decades has helped trigger a slow-motion tsunami of invading blacklegged ticks that is bringing Borrelia burgdoferi – the spiral bacteria that causes Lyme disease – to county after county across the Midwest and Northeast. There is, however, an important new twist in this tale. Our field studies in Michigan’s southwestern Lower Peninsula indicate that B. burgdorferi is not necessarily being introduced for the first time to these counties by the invading ticks. Rather, we believe that strains of this bacterium are already present in some of Michigan’s forests, hidden in ‘cryptic’ disease cycles that involve a new cast of wildlife characters – rabbits, birds, and a species of tick that few of us have ever seen. With the help of local bird banding enthusiasts, we have spent the past four years examining 20,000 birds – and dozens of rabbits and other small mammals – at a study site near Kalamazoo that lies 60 miles east of the present distributional limit of the blacklegged tick invasion of southwestern Michigan. No blacklegged ticks were found on the birds or mammals living at our field site; however, we found 12,000 instances of other tick species on these wildlife. The most common was a tick named Ixodes dentatus, which makes its living feeding on birds and rabbits. Surprisingly, when we ran our diagnostic tests we found that 4 percent of these birdrabbit ticks were infected with B. burgdorferi, as were 20 percent of biopsy samples taken from directly from wild rabbits. What this reveals is that the agent of Lyme disease is already present in at least some Michigan forests that as yet have not been invaded by blacklegged ticks. These bacteria have remained hidden until now, because they cycle between birds and rabbits via I. dentatus – a tick species that does not bite humans! Genetic fingerprinting of the B.burgdorferi found at our study site reveals many differences from the Borrelia carried by the invading blacklegged ticks found to the west. The bird-rabbit Borrelia consist of many different strains – some unique to southwestern Michigan while others are known to be widespread across several Midwestern states. The presence of bacterial strains from other states is an important clue in this disease puzzle. Ticks move themselves only very short distances, instead being adapted to catch a free ride on a passing host. Since rabbits are not likely to be crossing state lines on a regular basis, finding these multi-state strains suggests to us that birds are very important in spreading certain Borrelia strains across large geographic areas. What do these several findings mean for Lyme disease risk in Michigan? Firstly, the ‘cryptic’ Borrelia spirochetes in our forests are likely capable of infecting and being spread by blacklegged ticks, so they may help accelerate the increase in human disease risk when blacklegged ticks first move into a new area. It is also possible that these Borrelia strains differ in the severity and symptoms of human disease that they cause – this is a knowledge gap that we aim to clarify though further research. And finally, these results give us a better appreciation for the key role birds play in moving ticks and tick-borne disease across the landscape – for example, perhaps down the great Midwestern flyways towards our southern states. The distribution map of Lyme disease in the U.S. has been repeatedly re-drawn since the disease was first discovered – from what we have learned in this study, we certainly anticipate that it will continue to be redrawn in coming decades. Dr. Sarah Hamer recently graduated with her Ph.D from Michigan State University’s Department of Forestry, Wildlife and Fisheries; she is currently completing her DVM in MSU’s College of Veterinary Medicine. Dr. Jean Tsao is an Assistant Professor in Wildlife Disease Ecology at MSU. Dr. Graham Hickling helped establish MSU’s Graduate Specialization in Wildlife Disease Ecology, and is now Director of the Center for Wildlife Health at the University of Tennessee. Further Reading: Hamer, S.A.; Hickling, G.J.; Sidge, J.L.; Rosen, M.E. and Tsao, J.I. (2011) Diverse Borrelia burgdorferi strains in a bird-tick cryptic cycle. Applied and Environmental Microbiology 77: 1999–2007.