Risk of exposure to Lyme disease in northern California
Caryl Waggett, Environmental Science, Allegheny College and Robert Lane, Insect Biology, University of California at Berkeley
Lyme disease
M ixe d Hardw ood
15
• Most prevalent vector-borne disease in the United States. Human
incidence in California is patchy, reflecting underlying variability in vector
density and infection prevalence
• Three-part study designed to identify high risk habitats, develop
monitoring program and conduct epidemiological assessment of risk factors
Ecological assessment
• Four dominant habitats (n=48) and ecotones (n=10) flagged for ticks
• Collected and analyzed >3,500 ticks bi-weekly over 3 years
• Acarolgic risk, the number of infected ticks/100 m2, varied by:
— Habitat, season, tick life stage
Lyme disease is the most prevalent in the Northeast and
Midwest, but high numbers of human cases have been
identified in coastal northern California.
Mean number of infected ticks per 100 m2
12
9
6
3
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Woodland Gras s
Mixed Hardwood
15
12
9
6
3
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Jun
Jul
Aug
Jun
Jul
Aug
Gras s lands
15
12
9
6
3
0
Jan
Feb
Mar
Apr
May
Woodland Grass
Chaparral
15
12
9
6
3
0
Jan
Feb
Mar
Apr
May
Ecotone
15
12
9
6
3
0
Jan
Feb
Mar
Hardwood forests pose greatest risk of exposure to infected nymphs
Ecotones and chaparral pose moderate risk to infected adults
Apr
May
Jun
Jul
Aug
Month
Adults
1-4%
infected
— Abiotic factors correlated with acarologic risk included:
Nymphs
5-15%
infected
Grassland
The Lyme disease spirochete, Bb, has a complicated
life cycle involving two vectors and many hosts that
affect risk of human exposure. The tick responsible for
transmitting Bb to humans has definitive habitat
preferences and seasonal distributions.
Chaparral
Aspect, slope, canopy, leaf duff, substrate, pH, relative humidity and temperature
Habitat Signatures Derived from
TM5 Reference Bands
Band1
Band3
Band2
125125
125
125
Band5
Band7
Band4
125
125
125
90.00
125
Model results for monitoring Lyme disease risk
80.00
100100
100
100
75 75
75
Band7
125
75
Band7
125
75
Band7
125
Band7
75
125
CA population to double from 1990-2020 to 60 million people
50 50
50
100
50
100
100
50
50
100
Rural northern CA currently affordable, accessible
Increased habitat fragmentation, development in high risk habitats
• Model developed using satellite imagery to characterize
high risk habitats identified from ecological assessments
25 25
0
25
0
0
Band 1
75
50
Band 2
50.00
• Overall spectral signatures appear unique, but high band variability
present within each habitat
40.00
30.00
50
50
20.00
25
0
75
50
Band 3
25
75
50
0
25
0
Band 4
25
25
75
Band 1
(bl-gr visible)
Band 5
Band 2
(green visible)
Habitats
25
MHw
0
25
WG
Band 4
(near infra-red)
Band 5
(mid infra-red)
Band 7
(mid infra-red)
TM 5 Sensor
Band 7
Mixed Hardwood
Woodland Grass
Woodland Grass
Open Grassland
Chaparral
Open Grassland
Chaparral
Water
Ch
Habitat-specific reference signatures
0
High band variability within each habitat
100
Hardwood forests, woodland grass, grass, chaparral, control = water
Kappa statistic * 100
Overall model (n=351)
72% accurate
66
Water (n=90)
100% accurate
100
Open grassland (n=56)
92% accurate
91
Hardwood forests (n=67)
61% accurate
53
Chaparral (n=54)
52% accurate
42
Woodland grass (n=84)
48% accurate
37
120
110
• 89 reference sites (1,792 pixels) from 5 habitats
% Accurate
Water
25
G
0
Band 3
(red visible spectrum)
0
0
50
TM5 Model
0.00
Hardwood
0
Drought- and winter-deciduous trees had lost leaves
75
75
• High risk habitats not uniquely identifiable using satellite imagery
60.00
10.00
25
• Landsat TM5 imagery (30m resolution) from Nov 2002
100
100
100
100
Sensor Data
• Rapid changes in land use patterns in rural California
70.00
Band 6 not shown
Monitoring Lyme disease risks
90
• Tasseled cap model had similar accuracy (K*100)=64
80
• Values from 6 non-thermal bands used to create habitatspecific reference signatures
70
• Mixed hardwood forests have highest acarologic risk. Inability of
model to accurately characterize this habitat suggests that TM5
imagery may not be sufficiently sensitive for long-term and rapid
monitoring of human risk of Lyme disease in northern California
60
50
• Each of 51 validation sites (351 pixels) were compared to
reference signatures
40
30
20
• Each pixel characterized to habitat using root mean
square (RMS) error with smallest difference
10
0
0
• Model re-run using Tasseled cap indices
Greenness, wetness, brightness
20
40
60
80
Band 4
Band 4
Band 4
False color TM5 imagery of field sites
Landsat TM5 Band 4 v Band 5 comparison
showing inability to distinguish any single habitat
• Models from Northeast and Midwest with higher accuracy
characterized extreme habitats (e.g., forests v. meadows, or
wooded lots v. lawns) as opposed to two similar forest types
• Hyperspectral or higher resolution imagery may be necessary to
adequately characterize different forested habitats in region
Epidemiological assessment of risk factors
• Cross-sectional survey of community at high risk for Lyme disease, 14-year follow-up of
serological survey and questionnaire conducted in 1988-89 (Lane et al. 1992)
• 43% cumulative prevalence of Lyme disease, 1.5% annual incidence
• 11 of 76 variables examined significantly associated with exposure to Lyme disease
pathogen, Borrelia burgdorferi (Bb)
40
40
30
30
Bb Non-Exposed
Winter
mean hrs/wk
mean hrs/wk
Bb Exposed
20
20
10
10
0
Length of residence, annual time at residence, time spent outdoors, seasonal use of property, habitat
diversity, property management, gardening, caring for animals, ownership of farm animals, stargazing,
and lack of protective clothing
• Activities, behaviors placed residents at high risk for many tick-borne diseases
Spring
Spring
Summer
Winter
0
Summer
1988
2002
Fall
Fall
Lyme disease, human granulocytic and monocytic ehrlichiosis, babesiosis, bartonellosis
Residents in rural northern California live adjacent to
habitats at high risk from infected nymphs and along
ecotones at moderate risk from infected adult ticks.
• Despite rural environment and behavioral patterns (e.g., time spent outdoors), the
majority of risk factors were peri-domestic in nature—similar to Northeast and Midwest
• Educational efforts conducted 14 years earlier have proved effective
All residents spend1988
an extraordinary
amount of time outdoors,
but residents
who have
1988
2002
2002
been exposed to Bb spend statistically more time outdoors than non-exposed
residents. Residents spend less time outdoors now than they did in 1988-89.
These studies were supported by generous grants from NIH and CDC to R.S.L.