Biology Reference
In-Depth Information
population through replacement of a new agent for every death. Finally, in
RMSF-SIM, predator and prey agents emits a “scent” as they move through the
environment. This allows predators to navigate toward a prey while prey will move
away from predators.
The tick agents are the primary vector of the disease. TICKSIM is based on the
dynamics of
Ehrlichia chaffeensis
. For both models, there is no direct host-to-host
transmission of the disease so all disease in hosts is from the ticks. RMSF-SIMmodels
Rickettsia ricketsii
, which is able to be transmitted from an infected female tick to the
eggs she lays. Every successive generation that is born carrying the disease has their
respective egg-laying capability cut to one-third of total capacity for three generations.
For more information on disease models, see Chapter 6.
4.4.6
Input
For TICKSIM, each simulation is initiated in a uniform 25x25 patch grid with an
initial 100 hosts randomly spread across the grid. The probability that a given host
will start the simulation infected is 0.1. Additionally, 1000 ticks are randomly spread
across the grid. The probability that a given tick will be infected is also 0.1. The
simulations are assumed to begin on June 1, at which time larval stages would not be
present, and thus the initial ticks are split into adults and nymphs with approximately
100 adults and 900 nymphs. All other parameters are given in Gaff [
29
].
For RMSF-SIM, each simulation is initiated in a divided 40X30 patch grid of
one fourth forest and three fourths grassland with an initial 75 predators and 500 prey
spread randomly across the grid. Additionally 15,250 ticks (10,000 eggs, 5000 larvae,
250 nymphs) are randomly spread across the grid. The simulations are assumed to
begin on January 1, at which time adult stages will not be present.
4.4.7
Simulation Experiments
An example experiment of TICKSIMwas used to explore the possibility of the estab-
lishment of a new tick population into a completely naive area. From this simulation,
it can be shown that as few as two nymphs dropping off in a given area can estab-
lish a new population approximately 33% of the time. Simulations with RMSF-SIM
show that while the transovarially transmitted disease always remains in a population
initially, the mortality and reduced fecundity precludes the disease from remaining
for more than 20 years. The combined results of these models show that a tick-borne
disease that cannot be transmitted from a female to her eggs has a limited chance
of establishment initially. When the females lay infected eggs, the disease always
remains in the system initially.
TICKSIMwas also modified and run with various combination of disease dynam-
ics to explore the likelihood of a new disease entering the system with the new tick.
Four scenarios for tick dynamics were used: (1) no transovarial transmission and no
reduced fecundity from infection; (2) no transovarial transmission but with reduced
fecundity from infection; (3) transovarial transmission at 10% and no reduced fecun-
dity from infection; and (4) transovarial transmission at 10% and reduced fecundity
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