Flux of Ixodes scapularis on Host Peromyscus leucopus, and the Establishment of Lyme Disease Borreli

1
Flux of Ixodes scapularis on Host Peromyscus
leucopus, and the Establishment of Lyme Disease
(Borrelia burgdorferi) in a Central Illinois
Forest

• Ray Eckensten

2
What Is Lyme Disease?

• It is caused by a bacteria, Borrelia
  burgdorferi,which attacks the human nervous
  system
• Lyme disease is a vector-borne disease, carried
  by deer ticks (Ixodes scapularis)

3
What Is a Vector Borne Disease?

• A disease that is not transmitted from human to
  human, but by an animal
• The animal that transmits the disease is referred
  to as the vector
• Often there is a population of animals that
  sustains the bacterial source of the disease.
  That population is called the reservoir

4
Vector I. scapularis
5
Reservoir Peromyscus leucopus
6
B. Burgdorferi Transmission Cycle
7
Lyme Disease Distribution
8
The Epidemiology of Lyme Disease

• Lyme is a disease that is still on the move
• By studying I. scapularis habitat, deer movement
  patterns, and the distribution of Lyme disease,
  we can predict where Lyme will appear next, and
  what areas are most at risk

9
Rays Study Site

• The site selected was an area that had been
  recently infested with ticks (within the last 4
  years), but as yet had no evidence of B.
  burgdorferi
• A county park near the Illinois river in the
  north-central region of the state
• Forested White oak, pin oak and red maple
  dominant with abundant leaf litter

10
(No Transcript)
11
(No Transcript)
12
Rays Study

• What is the flux of ticks on and off of mice?
• Can we model how quickly Lyme can become endemic
  to an area that already has an established I.
  scapularis population?

13
What is Modeling?

• Computer reproduction of a natural system
  designed to predict the system-wide affects of
  changing variables within the system.
• There are two basic types of models
  deterministic and probabilistic

14
Deterministic vs. Probabilistic

• Deterministic models use processes that are fixed
  by the coefficients for each variable. These
  models always produce the same result for any
  given variable set.
• Probabilistic models are models in which outcomes
  of at least some of the processes are obtained by
  drawing samples randomly from standard
  statistical distributions (binomial, normal,
  etc.), or empirical distributions based on field
  data.

15
Rays Model

• My model needed to answer the question how long
  will it take Lyme, once introduced, to become
  endemic at my site?
• We define endemic here as the site permanently
  maintaining an infection rate of 70 among the
  reservoir
• Based on my fieldwork I knew the area of the
  forest, that there were 29.2 mice, 162 larval
  ticks, 24.2 tick nymphs per 100 m2 and that
  each mouse picked up an average of 4 larval ticks
  per night

16
Rays Model

• My model was based on a Markov Chain Model, a
  relatively early type of epidemiological modeling
  that focuses on infected number of individuals in
  the reservoir population, well suited to
  deterministic modeling of disease transmission.
  Not well suited for situations where one needs to
  know the severity of an infection.
• A program called SPSS was use to do the vast
  majority of the models calculations

17
Basic Markov Chain Modeling

• pi(tn1) Ti1p1(tn) Ti2p2(tn) ...Tikpk(tn)
• Where
• p of infected mice
• t time interval
• T flux of ticks
• in matrix form
• Stnr Tr Stn

18
Model Results

• My model predicts that when Lyme is introduced,
  it will spread quickly through the mouse
  population and the study site will become endemic
  in about 3 years.

19
Weaknesses of the Model

• It would have been useful to do the fieldwork for
  the population data more then once over the
  course of the year. Trapping once a month from
  April to October would have given us much more
  confidence in the population data.

20
You Can Always Make a Better Model

• Theres always more a sophisticated modeling
  method, and the number of variables you can
  include is limited only by your imagination.
• When Lyme does appear in my study site, my model
  will be tested. The actual results of the Lyme
  introduction can be used to improve my model, and
  it can be applied more accurately to the next
  Lyme susceptible site.

21

• Ray Eckenstein
• rle2003
• Thanks for listening

22
(No Transcript)
23
Predicting Lyme the Risk Map

• This map indicates habitat suitability for I.
  scapularis, as well as areas where the tick has
  already established itself
• Based on deer movement data, it is expected that
  the ticks will be moved along riparian corridors
  by migrating deer populations