Lyme Disease Transmission Ecology In Virginia Virginia Department of Health David N. Gaines, Ph.D. Public Health Entomologist Division of Environmental Epidemiology
The Lyme Disease Agent Lyme disease is an illness caused by the bites of ticks infected with the bacteria species known as Borrelia burgdorferi. B. burgdorferi is transmitted primarily between ticks, rodents and other small mammals. Ticks may also transmit B. burgdorferi to other mammals such as dogs or people where it may cause disease.
The Lyme Disease Agent Borrelia burgdorferi is one of several closely related (genetically similar) species in the U.S. In the U.S. mid-Atlantic Region, genetically related Borrelia species include: B. bessettii, B. andersonii, B. americana, and B. carolinensis. Among the various closely related American Borrelia species, only B. burgdorferi (sensustricto) is known to cause Lyme disease.
Lone Star Tick Blacklegged Tick American Dog Tick Tick Vectors of Disease in Virginia Among the 16 tick species found in Virginia, only three species commonly bite people. Only one species, the blacklegged tick transmits Lyme disease.
Blacklegged Tick Life-cycle Stages egg bloodmeal larva bloodmeal bloodmeal Each tick has three blood meals during its the two–year life span.
Feeding Preferences of the Blacklegged Tick Blacklegged nymphs have much broader feeding preferences than the adult ticks. Nymphs have been collected from over 100 different species of animals as well as people. Adult blacklegged ticks prefer to feed on deer and use deer as a mating ground. Adult blacklegged ticks may occasionally be found feeding on other animals such as dogs, cats, foxes, or on people.
The main vectors of Lyme disease to people are nymph stage blacklegged ticks - active from mid-spring to mid-summer. Inch (fractions) Adult blacklegged ticks do not commonly bite people, but sometimes do, and can transmit Lyme disease to people during the fall, winter and early spring.
The Blacklegged Tick’s Two-year Life Cycle
Tick Feeding Time Required for B. burgdorferiTransmission Infected blacklegged larvae go through a 6-month dormant period and then become nymphs - B. burgdorferi in the tick’s gut, decline in number and become dormant. When infected nymphs emerge in the spring, and feed on a warm-blooded animal, or person, the B. burgdorferi in the tick’s gut re-activate and begin to replicate. As the B. burgdorferireplicate, they move through the tick’s gut wall and migrate toward the tick’s salivary glands. It takes about 24 hours before the tick’s salivary glands begin to become infected. By 36 hours, some infected ticks will have begun to transmit B. burgdorferito the host in salivary fluids.
Feeding Time for B. burgdorferiTransmission Cumulative Probability of B. burgdorferiTransmission for Each Hour Attached T = (1 ‑ exp[-λγ(t ‑ G)γ ] ) × k 36 hours From DesVigneset al.
Reservoir Host Species for Lyme White-footed mice are highly competent reservoirs of B. burgdorferi. Play a major role in maintaining the infection cycle in the blacklegged tick population. Can be infected and remain infectious with B. burgdorferi for up to six months. Photo by Jim Schulz / Chicago Zoological Soc.
Competent and Non-Competent Host Species Other animal species known to act as competent B. burgdorferi reservoirs include: Eastern chipmunks, meadow voles, short-tailed shrews, masked shrews, robins (high to moderate competence) Many other animal species have low competence as reservoirs, or cannot be infected by B. burgdorferi, and play little or no role in its transmission. Low or non-competent host species include: Eastern grey squirrels, raccoons, skunks, opossums, lizards, white tailed deer, etc.
The Effect of Forest Environment on Lyme Transmission Studies have shown that animal species diversity in forest environments influences the B. burgdorferi infection rate in ticks. If a mixture of competent and non-competent host species are available for larval and nymph stage ticks to feed on, this can diluteB. burgdorferi transmission. Dilution Effect: Infected nymph stage ticks feed on non-competent reservoir animals and their infection cannot be passed on to other ticks that feed on these same animals.
The Effect of Forest Environment Animal species diversity is greatest in undisturbed forests and lowest in fragmented and disturbed forest environments. Undisturbed, Intact Forest There is a mixture of competent and non-competent host species, and B. burgdorferi is diluted. = competent reservoir species
The Effect of Forest Environment In fragmented and disturbed forest environments, white-footed mice often become the predominant forest species. Fragmented, Disturbed Forest Competent reservoirs predominate and help amplify B. burgdorferi in the local tick population. = competent reservoir species
Effect of Suburbanization on White-footed Mouse Habitat Suburbanization often results in fragmented and disturbed forest environments that favor white-footed mouse populations. Man-made structures around the edges of suburban forests (e.g., sheds, wood piles, etc.) may provide added nesting sites for the white-footed mouse population.
The Role of White-tailed Deer White-tailed deer are important because: They serve as an important mating ground for adult blacklegged ticks. Serve as the primary source of blood for egg production by female blacklegged ticks.
Deer ears covered by ticks The Role of White-tailed Deer An engorged female tick that drops off of a deer may have the potential to lay up to 3,000 eggs.
The Role of White-tailed Deer Studies have shown that total elimination of the local white-tailed deer population results in a 99% reduction of the local blacklegged tick population (It will also eliminate the lone star tick population). Blacklegged tick population density is generally positively correlated with deer population density. Higher tick population densities can increase the rate of B. burgdorferi transmission by ticks.
White-tailed Deer Habitats White-tailed deer prefer forest edge environments. Feed on vegetation that grows in sun-lit areas along the forest edge and use the forest for refuge. A square mile of solid forest will support a much smaller population of deer than a square mile of fragmented forest. Fragmented forests have more of the edge habitat and forage that deer require.
Effect of Suburbanization on Deer Habitat Suburbanization can result in a pattern of fragmented forest and increase white-tailed deer habitat and populations. Suburbanized areas restrict deer hunting activities to small geographic areas. Deer are smart - properties and neighborhoods that are off-limits to hunters become a refuge for deer during the hunting season.
Effect of Suburbanization Deer have adapted well to suburban environments and can survive in relatively densely developed areas. Ornamental vegetation and gardens around suburban homes can provide good forage for deer.
Lyme Disease in Your Own Backyard! Mated, fed, female ticks drop off deer as they travel through backyards and suburban forest blocks. Fed larval ticks drop off of white-footed mice as they move through backyards and forests. Engorged ticks that land in suitable habitats will survive until the next season. Surviving females produce and lay eggs that become larvae. Surviving larvae become nymphs, and if infected, may bite and infect people in their own back yards.
Blacklegged tick infection rates detected by recent surveillance and testing efforts in Virginia 46% 20% 15% 20% 6% 0% 18% 4% 5% 3% 8% Tick Drags 2012 - Fairfax County Department of Health / Johns Hopkins University, School of Public Health Tick Drags 2013 - Loudoun County Lyme Commission / Johns Hopkins University, School of Public Health Tick Drags 2011 - University of Richmond, Dept. of Biology Deer Hunt Collections 2011 - Virginia Dept. of Health / University of Richmond, Dept. of Biology Tick Drags 2011 - Old Dominion University, Dept. of Biology/ U. of Richmond
Summary Lyme disease is a growing problem in Virginia and endemic zones are expanding southwestward and appear to be linked to suburbanization and development. The transmission ecology of Lyme disease involves the complex interactions of blacklegged ticks, B. burgdorferi, competent animal hosts, deer, and forest environments. More tick surveys and testing will be needed to help predict local levels of Lyme disease risk in Virginia, and understand the local factors influencing transmission ecology.
Acknowledgements Nelson Lafon – Virginia Dept. Game and Inland Fisheries Julia Foutz & Jim Broyhill – VDH, Division of Environmental Epidemiology Rexford Dwamena , PhD – VDH, Office of Minority Health and Health Equity Local Epidemiologists – VDH, Division of Surveillance and Investigation Azizul Islam – VDH, Division of Surveillance and Investigation Jorge Arias, PhD & Joshua Smith – Fairfax County Dept. of Health Jory Brinkerhoff, PhD – University of Richmond, Department of Biology Dr. David Goodfriend – Loudoun County Dept Of Health Andy Lima – Clarke Mosquito Control, Holly Gaff, PhD, & Chelsea Wright – Old Dominion University, Dept. of Biology For further information on Lyme disease and other tick-borne diseases in Virginia, visit: www.vdh.virginia.gov/TickBrochure