Transcript Document

Lyme Disease and Biodiversity
Caren Collins, Lily Hoffman-Andrews, Amanda McLoughlin, and John Soghigian
Advisor: Dr. Todd Livdahl
Modeling the System
The Infective Agent and its Vector:
In order to test the host dilution concept, a model was constructed using the software, STELLA. The model contained
parameters for nine species and Ixodes scapularis. (fig 1). Figure 2 is a close up of the interactions between tick and whitefooted mouse. The model was run with different community compositions, varying from a low biodiversity community to higher
biodiversity community, with nine species. Figure 3 shows a community, in which the tick has only the white-footed mouse as
prey. Figure 4 demonstrates a community that is composed of the ticks, mice, and squirrels. Figure 5 shows how a very low
density of mice infected with Lyme disease can rapidly spread the disease. Figure 6 shows a community with nine species, a
representation of host dilution.
Borrelia burgdorferi is the spirochete bacteria responsible for
Lyme Disease (Fig. 1). It resembles other spirochetes in that it is a
highly specialized, motile, two-membrane, spiral-shaped bacteria
which lives primarily as an extracellular pathogen. In 1982, Willy
Burgdorfer isolated Borrelia burgdorferi from Ixodes ticks, as well as
from patients with Lyme disease. Ticks are small arachnids that act as
ectoparasites, living on the blood of mammals, birds, and even reptiles
(Fig. 2). Ixodes scapularis is the vector in the Northeast and North
central United States. Ixodes pacificus is the vector in the Northwest
United States.
Species
Body Burden
White-footed mouse
The tick life cycle requires two years and has three feedings with
three stages. Adult ticks mate in the fall and early spring and the
females lay their eggs on the ground. These eggs hatch into larvae by
summer. The larvae then feed on small animals, particularly the
white-footed mouse, in the summer and early fall (first feeding). The
larvae are inactive until spring when they molt into nymphs. Nymphs
feed on small animals in the late spring and summer (second feeding)
and molt into adults in the fall. The adults then feed, usually on
white-tailed deer, and the cycle begins again. Ticks become infected
with Borrelia burgdorferi when they feed on infected animals. Ticks
can become infected at any of the three feedings and can then pass on
the bacteria to animals and humans during subsequent feedings.
C TF Sor ex
1390
20.61%
36
55
25
900
13.35%
White-tailed deer
239
4.6
0.25
59.75
0.89%
Raccoon
127
1.3
0.2
25.4
0.38%
Virginia opossum
254
2.6
1
254
3.77%
Stripped Skunk
66.8
9.7
0.05
3.34
0.05%
Short-tailed shrew
62.9
41.8
25
1572.5
23.32%
Sorex Shrew
55.5
51.2
25
1387.5
20.58%
142
14.7
8.1
1150.2
17.06%
U ninfected D ee r
U ninfected Sor ex
U ninfected D ee r D ea ths
So rex D R
In fected Dee r D ea ths
U ninfected Opo ss um
N/A
A map was made of small mammal biodiversity in the continental United States
(Figure 10), using Idrisi’s Land Change Modeler, to determine whether there is
a relationship at this scale between biodiversity and Lyme disease. Figure 11
and Figure 12 show average species richness and 2005 Lyme disease rates, by
state.
In fected Opo ss ums
U ninfected Opo ss um DR
Opos su mD R
In fected Sor ex DR
Figure 10 – Small mammal species richness in the United States.
Opos su mIn fec tion s
Opos su mBirth s
U ninfected Sor ex DR
N/A
Opos su mBody Bu rde n
In fected Sor ex
So rex In fection s
So rex Bi rths
D eer Infe ctio ns
~70,000
Opos su mC ompe tenc y R ate
Opos su mBR
In fected Dee r
D eer Bir ths
C TF Opo ss um
N/A
So rex Bo dy Bu rde n
So rex BR
D eer Bir th R ate
N/A
So rex C omp ete nc y R ate
D eer Co mp ete ncy Ra te
% Chance for Tick
50
Ixodes
C TF D ee r
Ticks/Hec/Species
92.1
Red & Grey Squirrel
Fig. 2 Ixodes scapularis
Density Per Hectare
27.8
Eastern chipmunk
Fig. 1 Borrelia burgdorferi
Reservoir Competence
GIS – Biodiversity and Lyme disease
rates
In fected Opo ss um DR
D eer De ath Ra te
D eer Bo dy Bur den
tic k b irth rate
C TF ST Shre w
C TFSqu irre l
In fected Tick s
Sq uir rel Compe ten cy Ra te
U ninfected Tick s
ST Sh rew C ompetenc y R ate
Sq uir rel Birth R ate
ST Sh rew Bo dy Bu rde n
ST Sh rew BR
tic k b irth s
U ninfected Sho rt Tail Shr ew
In fection
U ninfected Squ irre l
More De ath s
D eath s
Sq uir rel Infe ctio ns
Sq uir rel Birth s
U ninfected Sho rt
Ta il Shre w Birth s
In fected Squ irre ls
In fected ST Shr ew
D eath R ate
ST Sh rew In fec tion s
Sq uir rel Bod y Burd en
U ninfected Squ irre l D eaths
In fected Squ irre l D eaths
U ninfected ST Shr ew De aths
In fected ST Shr ew DR
Sq uir rel Dea th Ra te
Sh ort Ta il Sh rew D R
Grap h 1
C han ce to fi nd Mic e
Mous e C ompe ten cy R ate
C TF R ac co on
R acc oo n C ompetenc y R ate
C TF C hi pmu nk
Mice Birth R ate
C hipmun k C ompe ten cy Rate
C TF Sku nks
Sk un k C ompetenc y R ate
C hipmun k BR
Sk un k Body Bu rde n
Stripp ed Sku nk BR
U ninfected Mic e
R acc oo n Bi rth Ra te
In fected Mic e
U ninfected C hi pmunk s
In fected Chi pmunk s
U ninfected Strip pe d Skun k
U ninfected R ac co on
In fected Rac co on s
Mice Infe ctio ns
Mice Births
C hipmun k Birth s
Stripp ed Sku nk Bir ths
R acc oo n Bi rths
In fected Sku nk
C hipmun k In fec tio ns
Stripp ed Sku nk Infecti ons
R acc oo n In fec tion s
Mice De atbs
Mous e Body Bu rd en
U ninfected R ac co on De aths
In fected Rac co n D ea ths
Mous e D ea th R ate
Mice De aths
U ninfected C hi pmunk De ath s
C hipmun k D ea th R ate
In fected Chi pmunk De ath s
U ninfected Strip pe d Skun k D ea ths
Stripp ed Sku nk DR
In fected Sku nk DR
C hipmun k Bod y Burd en
R acc oo n D eath R ate
R acc oo n Bo dy Bu rde n
Figure 11
Figure 12
Fig. 4: STELLA Model
Fig. 5: Close up of the model: the tick sector and a
species sector
Figure 12
Fig. 3 Tick Life Cycle
A statistically significant negative correlation
between average biodiversity and Lyme disease
rates was found (Pearson’s r = -0.338, N = 48, p
= 0.019). The results of the non-linear regression
are shown in Figure 12 and Table 1. While these
results must be taken with some caveats, given
the very broad scale of the analysis, they provide
intriguing support for the host dilution
hypothesis, especially when taken along with our
model results.
Table 1
Host Dilution
The Host Dilution hypothesis predicts that in communities with high
species diversity, the effect of the primary reservoir of a disease, such
as the white-footed mouse in Lyme disease, can be diluted by the
presence of other, less competent hosts of the disease. These other
hosts allow for the feeding of the vector, in this case, the tick, but
rarely infecting the vector or becoming infected. In the Host Dilution
hypothesis, there are different roles that the each organism in the
community has. We define dilution hosts as those characterized by
relatively high body burdens with low reservoir competence and a high
population density. In the case of Lyme disease, species such as
squirrels act as dilution hosts in the population. Rescue hosts are those
that have relatively high body burdens with moderately high reservoir
competency and are capable of maintaining the disease in the
population when the most competent host density is low. Shrews
would be an example of a rescue host in this community.
Fig. 6: Simulation run with infected ticks and initially
uninfected mice
Fig. 8: Simulation run with infected mice and initially
uninfected ticks.
Fig. 7: Simulation run with squirrels and mice
References
Animal Diversity Web. Accessed December 18, 2008 at http://animaldiversity.ummz.umich.edu.
Clark Labs. 2006. Idrisi Land Change Modeler. Computer software. Clark Labs, Worcester, MA, USA.
LoGiudice K, Ostfeld RS, Schmidt KA, Keesing F. The ecology of infectious disease: Effects of host diversity and
community composition on Lyme disease risk. Proceedings of the National Academy of Sciences 100(2): 567571.
Ostfeld RS, Keesing F. 2000. Biodiversity and disease risk: The case of Lyme disease. Conservation Biology 14(3): 722728.
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Fig. 9: Simulation run with all nine species. An example of
biodiversity at work.