risk of exposure to lyme disease in northern california mixed hardwood woodland grass chaparral...
TRANSCRIPT
Risk of exposure to Lyme disease in northern California
Mixed Hardwood
Woodland Grass
Chaparral
Grassland
Nymphs 5-15% infected
Adults 1-4% infected
Mixed Hardwood
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Woodland Grass
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Caryl Waggett, Environmental Science, Allegheny College and Robert Lane, Insect Biology, University of California at Berkeley
All residents spend an extraordinary amount of time outdoors, but residents who have 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.
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.
Lyme disease is the most prevalent in the Northeast and Midwest, but high numbers of human cases have been identified in coastal northern California.
Bb Exposedmean hrs/wkBb Non-Exposedmean hrs/wk
19882002
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.
Lyme disease
• 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
Hardwood forests pose greatest risk of exposure to infected nymphs Ecotones and chaparral pose moderate risk to infected adults
— Abiotic factors correlated with acarologic risk included:
Aspect, slope, canopy, leaf duff, substrate, pH, relative humidity and temperature
Monitoring Lyme disease risks
• Rapid changes in land use patterns in rural California
CA population to double from 1990-2020 to 60 million people
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
• Landsat TM5 imagery (30m resolution) from Nov 2002
Drought- and winter-deciduous trees had lost leaves
• 89 reference sites (1,792 pixels) from 5 habitats
Hardwood forests, woodland grass, grass, chaparral, control = water
• Values from 6 non-thermal bands used to create habitat-specific reference signatures
• Each of 51 validation sites (351 pixels) were compared to reference signatures
• Each pixel characterized to habitat using root mean square (RMS) error with smallest difference
• Model re-run using Tasseled cap indices
Greenness, wetness, brightness
Band 4
Model results for monitoring Lyme disease risk
• High risk habitats not uniquely identifiable using satellite imagery
• Overall spectral signatures appear unique, but high band variability present within each habitat
TM5 Model % Accurate 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
• Tasseled cap model had similar accuracy (K*100)=64
• 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
• 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)
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
Lyme disease, human granulocytic and monocytic ehrlichiosis, babesiosis, bartonellosis
• 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
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TM 5 Sensor
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Mixed Hardwood Woodland Grass Open Grassland Chaparral Water
Woodland Grass Open Grassland Chaparral WaterHardwood
High band variability within each habitat
False color TM5 imagery of field sites
Habitat-specific reference signatures
Landsat TM5 Band 4 v Band 5 comparison showing inability to distinguish any single habitat
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These studies were supported by generous grants from NIH and CDC to R.S.L.