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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.