Ecology of Lyme 1

1 collaborator

Michael Zito (Author)

WHAT IS IT?

The ecology of Lyme disease is complex. This model looks at a small piece of this system: That of whether or not a non-carrier larval tick will become a carrier as a result of taking a blood meal from its first host.

The dilution effect is the idea that biodiversity decreases the risk to humans of contracting Lyme disease. Lyme is caused by the pathogenic bacterium Borellia burgdorferi. The pathogen is transmitted from host to host by the black-legged tick (genus Ixodes). Black legged ticks take a number of blood meals during their life cycle. Larval ticks are not carriers when they emerge from their eggs. Whether they become carriers as nymphs depends on the competency of their first host in transmitting the bacterium and the survival rate of the larval ticks.

There are two types of agents in this model: Black-legged ticksand a variety of host species.

Black legged ticks are the disease vectors and take a blood meal from a warm-blooded host. The host can be a competent host or a dilution host.
Host Species Densities Some hosts are natural reservoirs for B. burgdorferi. These competent hosts readily transmit the pathogen to ticks and ticks have a high survival rate on these hosts. Othe hosts are dilution hosts. Dilution hosts have a lower probabilty of transmitting B. burgdorferi and often kill many larval ticks through their grooming behavior.

HOW IT WORKS

The model setup randomly distributes the agents described above within the world. The larval ticks remain stationary, questing for a blood meal. The host species move randomly through the world at different rates relative to their size.

At each tick:

If a tick and one or more hosts are on the same patch and the tick has not fed, the tick will take a blood meal from a randomly selected host on the same patch.
The probability that the tick feeds, survives and becomes infected is determined by the species specific body burden, probability of molting, and reservoir competence. The parameters used for these probabilities are taken from the references 2 and 6.
If the tick survives it molts into the nymph stage (becomes larger).
If the tick becomes infected it changes color from black to red.
If a larva does not attempt feeding it remains small.

The simulation ends after 1000 ticks

HOW TO USE IT

Input parameters:

numLarvalTicks: Sets the number of larval ticks in the world
num{Hosts}: Sets the number of each species of hosts in the world

Outputs:

A graph of infected nymphs, uninfected nymphs, and total of all nymphs over time
The total number of surviving nymphs (D)N)
The total number of surviving nymphs that are infected (DIN)
The percentage of surviving nymphs that are infected (DIN)

THINGS TO TRY

Change the number of each species in various combinations to investigate their effect on DON, DIN, and NIP. Eliminate species by setting their density to zero. Double or halve the densities. Run the predefined Behavior Space experiments (from the Tools menu). Load the output into a spreadsheet and analyze the data. Create your own Behavior Space experiments of othe species combinations.

EXTENDING THE MODEL

Add chooser and input for time for ending model at some time t or until all fed Add acorns to simulate masting events Add environmental patch sizes to simulate the effect of habitat fragmentation on differential species survival Add deer that deposit adults that lay eggs Add the full life cycle of ticks from egg to larva to nymph to adult

CREDITS AND REFERENCES

Allan B.F., Keesing F., and Ostfeld R. S. 2003 Effect of forest fragmentation on Lyme Disease Risk. Conservation Biology 17:267–272. CDC. 2015. Lyme Disease. The Centers for Disease Control and Prevention, 4 March 2015. Retrieved from http://www.cdc.gov/lyme/ on July 7, 2015.
Keesing, F., J. Brunner, S. Duerr, M. Killilea, K. LoGiudice, K. Schmidt, H. Vuong, and R.S. Ostfeld. 2009. Hosts as ecological traps for the vector of Lyme disease. Proc R Soc B 276, 3911–3919.
Kremen C. and R.S. Ostfeld. 2005. A call to ecologists: measuring, analyzing, and managing ecosystem services. Front Ecol Environ 3 (10), 540–548.
Krohne, D.T. and G.A. Hoch. 1999. Demography of Peromyscus leucopus populations on habitat patches: the role of dispersal. Canadian Journal of Zoology. 77: 1247–1253.
LoGiudice, K., R.S. Ostfeld, K.A. Schmidt, and F. Keesing. 2003. The ecology of infectious disease: effects of host diversity and community composition on Lyme disease risk. Proc. Natl Acad. Sci. USA 100, 567–571.
Richard S. Ostfeld, Taal Levi,Felicia Keesing, Kelly Oggenfuss, and Charles D. Canham Tick-borne disease risk in a forest food web Ecology, 99(7), 2018, pp. 1562–1573

Comments and Questions

Michael Zito

The parameters used in this model are from

https://www.researchgate.net/profile/Felicia_Keesing/publication/325026242_Tick-borne_disease_risk_in_a_forest_food_web/links/5b3a2b404585150d23ef5ab8/Tick-borne-disease-risk-in-a-forest-food-web.pdf

Posted about 6 years ago

Click to Run Model

;;----- Agent types
breed [blticks bltick]
breed [mice mouse]
breed [squirrels squirrel]
breed [possums possum]
breed [chipmunks chipmunk]

blticks-own [infected? fed?]                                     ;; larval ticks feed once and may become carriers of Lyme as a result
mice-own [body-burden reservoir-competence host-quality]         ;; each species can carry Lyme and have a probability of transmitting Lyme
squirrels-own [body-burden reservoir-competence host-quality]
possums-own [body-burden reservoir-competence host-quality]
chipmunks-own [body-burden reservoir-competence host-quality]

to setup

  ca

  ;; color the patches
  ask patches [set pcolor 63]

  ;; intialize breeds and randomly distribute them in the world
  create-blticks larval-ticks [
    set shape "spider"
    set color black
    set size 0.5
    set infected? FALSE
    set fed? FALSE
    setxy random-xcor random-ycor
  ]

  create-mice mouse-density [
    set shape "mouse side"
    set color gray
    set size 2.5
    set body-burden 28
    set reservoir-competence 0.91
    set host-quality 0.493
    setxy random-xcor random-ycor
  ]

    create-squirrels squirrel-density [
    set shape "squirrel"
    set color gray
    set size 3
    set body-burden 142
    set reservoir-competence 0.02
    set host-quality 0.172
    setxy random-xcor random-ycor
  ]

    create-chipmunks chipmunk-density [
    set shape "chipmunk"
    set color brown
    set size 2.5
    set body-burden 36
    set reservoir-competence 0.57
    set host-quality 0.243
    setxy random-xcor random-ycor
  ]

    create-possums possum-density [
    set shape "possum"
    set color gray + 3
    set size 3
    set body-burden 254
    set reservoir-competence 0.03
    set host-quality 0.035
    setxy random-xcor random-ycor
  ]


  reset-ticks
end 

to go

  ;; stop when all ticks have fed
  ;if all? blticks [ fed? ] [ stop ]
  if ticks = 1000 [stop]
  ;; have each breed move at a different speed
  move

  ;; ticks will feed on a host if one is on the same patch
  ask blticks [feed]

  tick
end 

to move

  ;; all hosts move randomly around the world
  ask squirrels [
    left random 50
    right random 50
    repeat 8 [fd 0.1]
  ]

  ask mice [
    left random 50
    right random 50
    repeat 1 [fd 0.1]
  ]

   ask possums [
    left random 50
    right random 50
    repeat 6 [fd 0.1]
  ]

   ask chipmunks [
    left random 50
    right random 50
    repeat 2 [fd 0.1]
  ]
end 

to feed

    ;; define local variables
    let pInfected  0
    let pFed 0
    let burden 0

    ;;find a host, exclude other ticks
    let host one-of other turtles-here with [member? self blticks = FALSE ]

    ;; set local variables to the host's properties
    ;; for each feeding event if larva has not aready fed
    if host != nobody and not fed?[
      ask host [
        set pFed host-quality
        set pInfected reservoir-competence
        set burden body-burden
      ]

    ;; ticks feed once, molt, and become larger
    ;; survival is determined based on the host quality
    ;; infection status is determined based on the host's reservoir competence
      if random 254 <= burden [                       ;; is host below its body burden?
        ifelse random-float 1.0 <= pFed               ;; does host feed larva?
        [
          set fed? TRUE                               ;; feed
          set size size * 2                           ;; molt
            if random-float 1.0 <= pInfected          ;; did B.burgdorferi transfer?
            [
              set infected? TRUE                      ;; if so change infection status
              set color red                           ;; and color
            ]
        ]
        [ die ]                                       ;; else larva dies
      ]
    ]
end 

to-report DON

  report count blticks with [fed?]
end 

to-report DIN

  report count blticks with [infected?]
end 

to-report NIP

  report 100 * DIN / DON
end 

to reset

  ;;  resets model parameters to intial satate

  set larval-ticks 1000
  set mouse-density 40
  set squirrel-density 8
  set possum-density 1
  set chipmunk-density 20
end

There are 4 versions of this model.

Uploaded by	When	Description	Download
Michael Zito	about 6 years ago	minor edits	Download this version
Michael Zito	about 6 years ago	updated info tab references	Download this version
Michael Zito	about 6 years ago	Removed extraneous variable	Download this version
Michael Zito	about 6 years ago	Initial upload	Download this version

Attached files

File	Type	Description	Last updated
Ecology of Lyme 1.png	preview	Preview for 'Ecology of Lyme 1'	about 6 years ago, by Michael Zito	Download

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