Lyme Disease Lyme Disease Transmission Ecology Transmission Ecology

In VirginiaIn Virginia

Virginia Department of HealthVirginia Department of HealthDavid N. Gaines, Ph.D.

Public Health EntomologistDivision of Environmental Epidemiology

B. burgdorferi B. burgdorferi is transmitted primarily between ticks, rodents is transmitted primarily between ticks, rodents and other small mammals.and other small mammals.

Lyme disease is an illness caused by the bites of ticks infected Lyme disease is an illness caused by the bites of ticks infected with the bacteria species known as with the bacteria species known as Borrelia burgdorferiBorrelia burgdorferi..

Ticks may also transmit Ticks may also transmit B. burgdorferi B. burgdorferi to other mammals such to other mammals such as dogs or people where it may cause disease.as dogs or people where it may cause disease.

The Lyme Disease Agent

Borrelia burgdorferi Borrelia burgdorferi is one of several closely related (genetically is one of several closely related (genetically similar) species in the U.S.similar) species in the U.S.

In the U.S. mid-Atlantic Region, genetically related In the U.S. mid-Atlantic Region, genetically related Borrelia Borrelia species species include: include: B. bessettiiB. bessettii, , B. andersoniiB. andersonii, , B. americanaB. americana, , and and B. carolinensis.B. carolinensis.

Among the various closely related American Among the various closely related American BorreliaBorrelia species, only species, only B. burgdorferi B. burgdorferi (sensu stricto) is known to (sensu stricto) is known to cause Lyme disease.cause Lyme disease.

The Lyme Disease Agent

Lone Star Tick American Dog TickBlacklegged Tick

Tick Vectors of Disease in VirginiaAmong 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 Adult blacklegged ticks prefer to feed on deer and use deer as a mating ground.a mating ground.

Blacklegged nymphs have much broader feeding preferences Blacklegged nymphs have much broader feeding preferences than the adult ticks. than the adult ticks.

Nymphs have been collected from over 100 different Nymphs have been collected from over 100 different species of animals as well as people.species of animals as well as people.

Feeding Preferences of the Blacklegged Tick

Adult blacklegged ticks may occasionally be found feeding on Adult blacklegged ticks may occasionally be found feeding on other animals such as dogs, cats, foxes, or on people.other animals such as dogs, cats, foxes, or on people.

The main vectors of Lyme disease to people are The main vectors of Lyme disease to people are nymph stage nymph stage blacklegged ticks blacklegged ticks - active from mid-spring to mid-summer. - active from mid-spring to mid-summer.

Inch (fractions)

Adult blacklegged ticks Adult blacklegged ticks do notdo not commonly bite people, but sometimes commonly bite people, but sometimes do, and can transmit Lyme disease to people during the fall, winter do, and can transmit Lyme disease to people during the fall, winter and early spring. and early spring.

The The BlackleggedBlacklegged

Tick’s Tick’s Two-yearTwo-year Life CycleLife Cycle

Tick Feeding Time Required for B. burgdorferi Transmission

As the As the B. burgdorferi B. burgdorferi replicate, they move through the tick’s gut replicate, they move through the tick’s gut wall and migrate toward the tick’s salivary glands.wall and migrate toward the tick’s salivary glands.

Infected blacklegged larvae go through a 6-month dormant period Infected blacklegged larvae go through a 6-month dormant period and then become nymphs - and then become nymphs - B. burgdorferiB. burgdorferi in the tick’s gut, in the tick’s gut, decline in number and become dormant. decline in number and become dormant.

When infected nymphs emerge in the spring, and feed on a When infected nymphs emerge in the spring, and feed on a warm-warm-bloodedblooded animal, or person, the animal, or person, the B. burgdorferi B. burgdorferi in the tick’s gut in the tick’s gut re-activate and begin to replicate.re-activate and begin to replicate.

It takes about 24 hours before the tick’s salivary glands begin to It takes about 24 hours before the tick’s salivary glands begin to become infected. By become infected. By 3636 hours, some infected ticks will have hours, some infected ticks will have begun to transmit begun to transmit B. burgdorferi B. burgdorferi to the host in salivary fluids.to the host in salivary fluids.

Feeding Time for B. burgdorferi Transmission

CUMULATIVE PROBABILITY OF B. burgdorferi TRANSMISSION FOR EACH

HOUR ATACHED

0.0

0.2

0.4

0.6

0.8

1.0

0 24 48 72 96

HOURS ATTACHED

PR

OB

AB

ILIT

Y

WEIBULL MODEL

T = (1 ‑ exp[-λγ (t ‑ G)γ ] ) k

From DesVignes et al.

Cumulative Probability of B. burgdorferi Transmission for Each Hour Attached

36 hours

White-footed mice are White-footed mice are highlyhighly competent reservoirs of competent reservoirs of B. B. burgdorferi.burgdorferi.

Can be infected and remain Can be infected and remain infectious with infectious with B. burgdorferiB. burgdorferi for up to six months. for up to six months.

Play a major role in maintaining Play a major role in maintaining the infection cycle in the the infection cycle in the blacklegged tick population.blacklegged tick population.

Reservoir Host Species for Lyme

Many other animal species have low competence as Many other animal species have low competence as reservoirs, reservoirs, oror cannot be infected by cannot be infected by B. burgdorferi,B. burgdorferi, and and play play little or no role in its transmissionlittle or no role in its transmission. .

Other animal species known to act as competent Other animal species known to act as competent B. burgdorferiB. burgdorferi reservoirs include: reservoirs include:

Eastern chipmunks, meadow voles, short-tailed shrews, Eastern chipmunks, meadow voles, short-tailed shrews, masked shrews, robins (high to moderate competence)masked shrews, robins (high to moderate competence)

Eastern grey squirrels, raccoons, skunks, opossums, lizards, Eastern grey squirrels, raccoons, skunks, opossums, lizards, white tailed deer, etc. white tailed deer, etc.

Low or non-competent host species include:Low or non-competent host species include:

Competent and Non-Competent Host Species

Studies have shown that animal species diversity in forest Studies have shown that animal species diversity in forest environments influences the environments influences the B. burgdorferiB. burgdorferi infection infection rate in ticks. rate in ticks.

If a mixture of competent and non-competent host species are If a mixture of competent and non-competent host species are available for larval and nymph stage ticks to feed on, this can available for larval and nymph stage ticks to feed on, this can dilutedilute B. burgdorferiB. burgdorferi transmission. transmission.

Dilution EffectDilution Effect: Infected nymph stage ticks feed on non-: Infected nymph stage ticks feed on non-competent reservoir animals and their infection cannot be competent reservoir animals and their infection cannot be passed on to other ticks that feed on these same animals. 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 Animal species diversity is greatest in undisturbed forests and lowest in fragmented and disturbed forest environments. lowest in fragmented and disturbed forest environments.

There is a mixture of competent and non-competent host There is a mixture of competent and non-competent host species, and species, and B. burgdorferi B. burgdorferi is diluted. is diluted.

Undisturbed, Intact Forest

= competent reservoir species

The Effect of Forest Environment

Fragmented, Disturbed Forest

= competent reservoir species

The Effect of Forest EnvironmentIn fragmented and disturbed forest environments, white-In fragmented and disturbed forest environments, white-

footed mice often become the predominant forest species. footed mice often become the predominant forest species.

Competent reservoirs predominate and help amplify Competent reservoirs predominate and help amplify B. B. burgdorferi burgdorferi in the local tick population. in the local tick population.

Suburbanization often results in fragmented and disturbed Suburbanization often results in fragmented and disturbed forest environments that favor white-footed mouse forest environments that favor white-footed mouse populations. populations.

Man-made structures around the edges of suburban forests Man-made structures around the edges of suburban forests (e.g., sheds, wood piles, etc.) may provide added nesting (e.g., sheds, wood piles, etc.) may provide added nesting sites for the white-footed mouse population.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 White-tailed deer are important because:because:

Serve as the primary source Serve as the primary source of blood for egg production of blood for egg production by female blacklegged by female blacklegged ticks. ticks.

They serve as an important They serve as an important mating ground for adult mating ground for adult blacklegged ticks. blacklegged ticks.

The Role of White-tailed Deer

An engorged female tick that drops off of a deer may have An engorged female tick that drops off of a deer may have the potential to lay up to 3,000 eggs.the potential to lay up to 3,000 eggs.

Deer ears covered by ticks Deer ears covered by ticks

The Role of White-tailed Deer

Blacklegged tick population density is generally positively Blacklegged tick population density is generally positively correlated with deer population density.correlated with deer population density.

Higher tick population densities can increase the rate of Higher tick population densities can increase the rate of B. B. burgdorferiburgdorferi transmission by ticks. transmission by ticks.

Studies have shown that total elimination of the local white-Studies have shown that total elimination of the local white-tailed deer population results in a 99% reduction of the tailed deer population results in a 99% reduction of the local blacklegged tick population (local blacklegged tick population (It will also eliminate the It will also eliminate the lone star tick population).lone star tick population).

White-tailed Deer Habitats

White-tailed deer prefer forest edge environments.White-tailed deer prefer forest edge environments.

A square mile of solid forest will support a much smaller A square mile of solid forest will support a much smaller population of deer than a square mile of fragmented forest.population of deer than a square mile of fragmented forest.

Fragmented forests have more of the edge habitat and Fragmented forests have more of the edge habitat and forage that deer require.forage that deer require.

Feed on vegetation that grows in sun-lit areas along the Feed on vegetation that grows in sun-lit areas along the forest edge and use the forest for refuge.forest edge and use the forest for refuge.

Suburbanization can result in a pattern of fragmented forest Suburbanization can result in a pattern of fragmented forest and increase white-tailed deer habitat and populations.and increase white-tailed deer habitat and populations.

Suburbanized areas restrict deer hunting activities to small Suburbanized areas restrict deer hunting activities to small geographic areas. geographic areas.

Deer are smart - properties and neighborhoods that are off-Deer are smart - properties and neighborhoods that are off-limits to hunters become a refuge for deer during the limits to hunters become a refuge for deer during the hunting season. hunting season.

Effect of Suburbanization on Deer Habitat

Effect of Suburbanization

Deer have adapted well to suburban environments and can Deer have adapted well to suburban environments and can survive in relatively densely developed areas.survive in relatively densely developed areas.

Ornamental vegetation and gardens around suburban Ornamental vegetation and gardens around suburban homes can provide good forage for deer.homes can provide good forage for deer.

Engorged ticks that land in suitable habitats will survive until Engorged ticks that land in suitable habitats will survive until the next season.the next season.

Mated, fed, female ticks drop off deer as they travel through Mated, fed, female ticks drop off deer as they travel through backyards and suburban forest blocks. backyards and suburban forest blocks.

Lyme Disease in Your Own Backyard!

Fed larval ticks drop off of white-footed mice as they move Fed larval ticks drop off of white-footed mice as they move through backyards and forests. through backyards and forests.

Surviving females produce and lay eggs that become larvae. Surviving females produce and lay eggs that become larvae.

Surviving larvae become nymphs, and if infected, may bite Surviving larvae become nymphs, and if infected, may bite and infect people in their own back yards. and infect people in their own back yards.

Tick Drags 2012 - Fairfax County Department of Health / Johns Hopkins University, School of Public Health

Tick Drags 2011 - Old Dominion University, Dept. of Biology/ U. of Richmond

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 2013 - Loudoun County Lyme Commission / Johns Hopkins University, School of Public Health

20%

20%6%

0%

Blacklegged tick infection rates detected by recent surveillance and testing efforts in Virginia

46%

18% 4% 5%

15%

8%

3%

Incidence of Lyme disease by county in Virginia in 2012

Summary

The transmission ecology of Lyme disease involves the complex The transmission ecology of Lyme disease involves the complex interactions of blacklegged ticks, interactions of blacklegged ticks, B. burgdorferiB. burgdorferi, competent , competent animal hosts, deer, and forest environments.animal hosts, deer, and forest environments.

More tick surveys and testing will be needed to help predict local More tick surveys and testing will be needed to help predict local levels of Lyme disease risk in Virginia, and understand the local levels of Lyme disease risk in Virginia, and understand the local factors influencing transmission ecology.factors influencing transmission ecology.

Lyme disease is a growing problem in Virginia and endemic zones Lyme disease is a growing problem in Virginia and endemic zones are expanding southwestward and appear to be linked to are expanding southwestward and appear to be linked to suburbanization and development. suburbanization and development.

Acknowledgements

Julia Foutz & Jim Broyhill – VDH, Division of Environmental EpidemiologyJulia Foutz & Jim Broyhill – VDH, Division of Environmental Epidemiology

Local Epidemiologists – VDH, Division of Surveillance and InvestigationLocal Epidemiologists – VDH, Division of Surveillance and Investigation

Dr. David Goodfriend – Loudoun County Dept Of HealthDr. David Goodfriend – Loudoun County Dept Of Health

Jorge Arias, PhD & Joshua Smith – Fairfax County Dept. of Health Jorge Arias, PhD & Joshua Smith – Fairfax County Dept. of Health

Nelson Lafon – Virginia Dept. Game and Inland Fisheries Nelson Lafon – Virginia Dept. Game and Inland Fisheries

For further information on Lyme disease and other tick-borne For further information on Lyme disease and other tick-borne diseases in Virginia, visit: diseases in Virginia, visit:

www.vdh.virginia.gov/TickBrochurewww.vdh.virginia.gov/TickBrochure

Azizul Islam – VDH, Division of Surveillance and InvestigationAzizul Islam – VDH, Division of Surveillance and Investigation

Andy Lima – Clarke Mosquito Control, Andy Lima – Clarke Mosquito Control,

Jory Brinkerhoff, PhD – University of Richmond, Department of Biology Jory Brinkerhoff, PhD – University of Richmond, Department of Biology

Rexford Dwamena , PhD – VDH, Office of Minority Health and Health EquityRexford Dwamena , PhD – VDH, Office of Minority Health and Health Equity

Holly Gaff, PhD, & Chelsea Wright – Old Dominion University, Dept. of Holly Gaff, PhD, & Chelsea Wright – Old Dominion University, Dept. of BiologyBiology