Biology Reference
In-Depth Information
control of tick-borne diseases. The majority of tick species in the United States are
classified as hard-bodied ticks (Ixodid). These ticks have four distinct life stages: egg,
larva, nymph, and adult. For the three latter stages, ticks will quest, i.e., seek a single
vertebrate host for a bloodmeal that will last several days. If successful in obtaining
a complete bloodmeal as a larva or nymph, these ticks will then remain dormant for
months to years before emerging as the next life stage to quest again. All adult female
ticks and some adult male ticks require a bloodmeal before reproduction and death.
Every tick species has a different set of preferred hosts for each life stage, different
strategies to find hosts, different lengths of time between life stages and different
competence for transmissible pathogens. For example,
Ixodes scapularis
, the black-
legged tick, prefers rodents for a larval meal, medium-size mammals for a nymphal
meal, and deer for the adult meal. This life history pattern creates unique temporal
scale patterns that are also dependent on off-host survival factors such as weather and
predation.
A number of types of models have been used to look at tick-borne diseases includ-
ing differential equation-based models [
47
-
49
], age-structured difference equation
models [
50
-
57
], GIS models [
58
-
60
], and others [
61
,
62
]. While each of these mod-
els are helpful to investigate the dynamics of tick and tick-borne diseases, they all use
a population approach with generalized mass-action interactions between ticks and
their hosts, ignoring many of the unique features of the tick life history. An agent-
based model provides the ideal tool to test the validity of population-level models
for ticks. The two preliminary models, TICKSIM and RMSF-SIM, presented here
are described in more detail in Gaff [
29
] and Tillinghast and Gaff [
43
], respectively.
TICKSIM models ehrlichiosis while RMSF-SIM models Rocky Mountain spotted
fever.
4.4.1
The Model
One challenge of ABM is the lack of standardization in published descriptions of
ABM, but Grimm and coauthors have attempted to solve this with a careful set of
instructions first established in 2006 [
63
,
64
] and revised in 2010 [
65
]. This provides
a standard framework for reporting described as the “Overview, Design concepts, and
Details (ODD)” for a given model. This example follows the ODD protocol which
consists of seven elements. The first three elements provide an overview, the fourth
element explains general concepts underlying the model's design, and the remain-
ing three elements provide details. In the
overview
, one states the purpose of the
exploration and the basic spatial and agent-behavior assumptions. The
design con-
cepts
explore complexities within a model, including basic assumptions underlying
the design, emergence of global traits from individual behaviors, adaptive traits that
have been assigned, and changes in states that occur by sensing surrounding loca-
tions or agents. The
details
are primarily parameter and initial values for the ABM.
While many of the details may seem trivial, these details along with the rest of the
ODD protocol provide the recipe for reproducibility of results.
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