Presentation on theme: "Lyme Disease Transmission Ecology Lyme Disease Transmission Ecology In Virginia Virginia Department of Health David N. Gaines, Ph.D. Public Health Entomologist."— Presentation transcript:
Lyme Disease Transmission Ecology Lyme Disease Transmission Ecology In Virginia Virginia Department of Health David N. Gaines, Ph.D. Public Health Entomologist Division of Environmental Epidemiology
B. burgdorferi is transmitted primarily between ticks, rodents and other small mammals. Lyme disease is an illness caused by the bites of ticks infected with the bacteria species known as Borrelia burgdorferi. 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 (sensu stricto) is known to cause Lyme disease. The Lyme Disease Agent
Lone Star TickAmerican Dog TickBlacklegged 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 larva egg Each tick has three blood meals during its the two–year life span.
Adult blacklegged ticks prefer to feed on deer and use deer as a mating ground. 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. Feeding Preferences of the Blacklegged Tick 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 Ticks The Blacklegged TicksTwo-year Life Cycle Life Cycle
Tick Feeding Time Required for B. burgdorferi Transmission As the B. burgdorferi replicate, they move through the ticks gut wall and migrate toward the ticks salivary glands. Infected blacklegged larvae go through a 6-month dormant period and then become nymphs - B. burgdorferi in the ticks 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 ticks gut re-activate and begin to replicate. It takes about 24 hours before the ticks salivary glands begin to become infected. By 36 hours, some infected ticks will have begun to transmit B. burgdorferi to the host in salivary fluids.
Feeding Time for B. burgdorferi Transmission T = (1 exp[- λ γ (t G) γ ] ) k From DesVignes et al. Cumulative Probability of B. burgdorferi Transmission for Each Hour Attached 36 hours
White-footed mice are highly competent reservoirs of B. burgdorferi. Can be infected and remain infectious with B. burgdorferi for up to six months. Play a major role in maintaining the infection cycle in the blacklegged tick population. Reservoir Host Species for Lyme
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. 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) Eastern grey squirrels, raccoons, skunks, opossums, lizards, white tailed deer, etc. Low or non-competent host species include: Competent and Non-Competent Host Species
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 dilute B. 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 on Lyme Transmission
Animal species diversity is greatest in undisturbed forests and lowest in fragmented and disturbed forest environments. There is a mixture of competent and non-competent host species, and B. burgdorferi is diluted. Undisturbed, Intact Forest = competent reservoir species The Effect of Forest Environment
Fragmented, Disturbed Forest = competent reservoir species The Effect of Forest Environment In fragmented and disturbed forest environments, white- footed mice often become the predominant forest species. Competent reservoirs predominate and help amplify B. burgdorferi in the local tick population.
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. Effect of Suburbanization on White- footed Mouse Habitat
The Role of White-tailed Deer White-tailed deer are important because: Serve as the primary source of blood for egg production by female blacklegged ticks. They serve as an important mating ground for adult blacklegged 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. Deer ears covered by ticks
The Role of White-tailed Deer 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. 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).
White-tailed Deer Habitats White-tailed deer prefer forest edge environments. 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. Feed on vegetation that grows in sun-lit areas along the forest edge and use the forest for refuge.
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 on Deer Habitat
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. Ornamental vegetation and gardens around suburban homes can provide good forage for deer.
Engorged ticks that land in suitable habitats will survive until the next season. Mated, fed, female ticks drop off deer as they travel through backyards and suburban forest blocks. Lyme Disease in Your Own Backyard! Fed larval ticks drop off of white-footed mice as they move through backyards and forests. 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.
Tick Drags Fairfax County Department of Health / Johns Hopkins University, School of Public Health Tick Drags Old Dominion University, Dept. of Biology/ U. of Richmond Tick Drags University of Richmond, Dept. of Biology Deer Hunt Collections Virginia Dept. of Health / University of Richmond, Dept. of Biology Tick Drags Loudoun County Lyme Commission / Johns Hopkins University, School of Public Health
Incidence of Lyme disease by county in Virginia in 2012
Summary 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. 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. Lyme disease is a growing problem in Virginia and endemic zones are expanding southwestward and appear to be linked to suburbanization and development. Lyme disease is a growing problem in Virginia and endemic zones are expanding southwestward and appear to be linked to suburbanization and development.
Acknowledgements Julia Foutz & Jim Broyhill – VDH, Division of Environmental Epidemiology Local Epidemiologists – VDH, Division of Surveillance and Investigation Dr. David Goodfriend – Loudoun County Dept Of Health Jorge Arias, PhD & Joshua Smith – Fairfax County Dept. of Health Nelson Lafon – Virginia Dept. Game and Inland Fisheries For further information on Lyme disease and other tick-borne diseases in Virginia, visit: Azizul Islam – VDH, Division of Surveillance and Investigation Andy Lima – Clarke Mosquito Control, Jory Brinkerhoff, PhD – University of Richmond, Department of Biology Rexford Dwamena, PhD – VDH, Office of Minority Health and Health Equity Holly Gaff, PhD, & Chelsea Wright – Old Dominion University, Dept. of Biology