Lucy Gilbert Environmental associations of ticks and disease
Transcript of Lucy Gilbert Environmental associations of ticks and disease
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Lucy Gilbert
Environmental associations of ticks and disease
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Ticks in Europe
1. Ixodes arboricola 2. Ixodes caledonicus 3. Ixodes frontalis 4. Ixodes lividus 5. Ixodes rothschildi 6. Ixodes unicavatus 7. Ixodes uriae 8. Ixodes acuminatus 9. Ixodes apronophorus 10. Ixodes canisuga 11. Ixodes ventalloi 12. Ixodes trianguliceps 13. Ixodes vespertilionis 14. Ixodes hexagonus 15. Ixodes ricinus 16. Dermacentor reticulatus 17. Rhipicephalus sanguineus 18. Hyalomma marginatum 19. Haemaphysalis punctata 20. Argas reflexus 21. Argas vespertilionis 22. Ornithodoros maritimus
Tree hole tick Northern bird tick Passerine tick Sand martin tick Puffin tick Cormorant tick Seabird tick Southern rodent tick Marsh tick Fox tick Rabbit tick Shrew tick Long-legged bat tick Hedgehog tick Sheep tick Ornate cow tick Brown dog tick Two-host tick Coastal red tick Pigeon tick Blyborough tick Marine argasid
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Approx. distribution of Ixodes ricinus
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Pathogens transmitted by Ixodid ticks : Viruses: Tick-Borne Encephalitis Virus complex, including
Louping ill virus Protozoa: Babesia spp cause babesiosis, red-water fever Bacteria: Rickettsiae causing spotted fever rickettsiosis, tick typhus Coxiella burnetii causes Q-fever Anaplasma and Ehrlichia sp e.g. Anaplasma phagocytophilum causes Human Granulocytotropic Anaplasmosis (HGA) and Tick-Borne Fever in sheep & cattle. Neoehrlichia mikurensis Bartonella - cat scratch fever Francisella tularensis causing tularaemia Borrelia burgdorferi s.l. complex - Lyme
borreliosis
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Life cycle of I. ricinus & Borrelia
Most Borrelia infections in humans are from Nymphs. Larvae are uninfected.
Nymphs become infected when they are larvae feeding on rodents & birds
More larvae and nymphs feeding on rodents/birds increase transmission potential
Deer do not transmit Borrelia, but are main hosts for adult ticks, so create large larval tick populations
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April May July Aug Sept Oct June
Nym
phal
tic
k ab
unda
nce
NYMPHS
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• Climate
• Habitat,
• Hosts, e.g. wildlife & livestock
What are the environmental factors associated with ticks and tick-borne diseases?
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Using altitude to study the effect of climate on tick abundance
X 9 hills
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altitude (masl)400 500 600 700 800 900
nym
phs p
er d
rag
0
1
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Tick abundance decreases dramatically at higher altitudes – due to the colder
climate
increase of 3oC Gilbert (2010) Oecologia 162: 217-225
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Does risk of tick bites depend on the weather on the day?
We counted the proportion of ticks that were questing at each temperature
Experiment: 30 nymphs per tube 6 – 15oC
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Ticks are more active at warmer temperatures – so risk is higher on warm days Note also that Scottish ticks are cold-adapted!
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Which Habitats are worse for ticks?
Heather/bog Grasslands Semi-natural woodland
Conifer plantation
Tick surveys at 77 sites across Scotland
But high variation due to different regions, altitudes, climates, hosts. We need an experiment!
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Effect of animal densities on tick abundance?
Variance due to different habitats and regions – need to test the effect of deer experimentally
- Test deer vs no deer in same habitat, same place, same time…
Deer index (dung counts)
Red deer
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Moorland deer farm Small plots
V high deer density
Moorland estate Small plots
“normal” deer density
Commercial Forestry
An effective way to control ticks can be through deer management
Deer exclosure experiment: fenced vs unfenced areas
Gilbert et al. (2012) Ecological Applications 22: 658–667
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SEASON (May/June) Estimate (slope) = 1.8
Deer density Estimate (slope) = 0.06
Broadleaved woodland Estimate (slope) = 3.2
Coniferous woodland Estimate (slope) = 1.0
Heather moorland Estimate (slope) = 0.4
HABITAT
Mean annual temperature Estimate (slope) = 0.2
Frost days in September Estimate (slope) = -1.8
CLIMATE
Tying the tick factors together: statistical analysis of 300+ site surveys
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Lyme disease
No longer notifiable
http://www.documents.hps.scot.nhs.uk/giz/10-year-tables/lyme.pdf
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What is the prevalence of Borrelia in ticks in Scotland?
What drives these differences in prevalence?
Vegetation surveys,
dung counts, GIS climate data.
Surveys of 25 woodlands
Average prevalence = 5.6% Range = 1-14%
James et al. (2013) Parasitol. 140: 237-246
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B. burgdorferi s.l. Estimate Upper CI Lower CI z-value p-value
Deciduous wood 0.0341 -0.0007 0.0873 1.952 0.051
Altitude -0.0002 -0.0003 -0.0001 -2.954 0.003
Julian day 0.0007 0.0003 0.0012 3.112 0.002
Relative humidity -0.0012 -0.0021 -0.0002 -2.393 0.017
Ground vegetation height -0.0010 -0.0019 -0.0001 -2.173 0.03
Red deer index 0.0861 0.0054 0.2385 2.211 0.027
Which factors are associated with Borrelia in questing nymphs?
James et al. (2013) Parasitol. 140: 237-246
climate season weather Habitat/hosts hosts Habitat/hosts
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25 woodlands
Including 6 from the previous survey to examine temporal
stability.
Average Borrelia prevalence in nymph ticks = 2.1% (range 0-12%)
This study found no effect of deer or forest type on Borrelia prevalence. Only an effect of climate (warmer = more Borrelia).
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Theoretical Mathematical Models predict that as deer densities increase, there will be a rise then a fall in Borrelia prevalence:
Ratio of incompetent : competent transmission hosts
Few deer many deer
Borr
elia
pre
vale
nce
So far no research has investigated this properly. We need surveys covering the full range of deer densities.
I aim to do this and welcome your help to identify sites!
Why these differences between the 2 studies?
The effect of deer may depend on deer density, and what other hosts are available
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A study on garden/woodland birds found that the most important birds for ticks and Borrelia =
James et al. (2011) Ibis 153: 293–302
All ground foragers
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Ticks, Louping Ill virus and Mountain Hares
Using mathematical models to predict when culling mountain hares might reduce LIV
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R0=1
1. With grouse and hares only.
The model predicts that, in the absence of other hosts such as deer, culling hares could potentially reduce LIV if the estate is “above the line”
Gilbert et al. (2001) Journal of Animal Ecology 70: 1053-1061.
X
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2. Add deer to the system (very low density: 2 km-2).
In the presence of deer, even culling hares right down to zero will not work Because deer maintain the tick population while grouse transmit the virus
Gilbert et al. (2001) Journal of Animal Ecology 70: 1053-1061.
X
X
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Most estates also have deer:
Serious management implication: culling mountain hares is unlikely to control ticks and LIV in the majority of cases.
It is also crucial to first establish whether ticks and LIV are THE main problem limiting grouse numbers.
If other reasons (weather/habitat/predators/worms) are more
important, then tick or LIV control measures are less likely to help.
Harrison et al. 2010. Journal of Applied Ecology 47: 926-930 .
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THANK YOU!
Alan Bowman, Ken Forbes, Marianne James
Roman Biek, Caroline Millins
Rachel Norman, Ros Porter
Joseph Tomkins
Jim McLeod, Jennifer Aungier, Dave Riach