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Epidemiological effects of badger culling and vaccination
Rosie WoodroffeZoological Society of London
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Two important facts about bovine TB
TB is a huge problem for both beef and dairy farmersBadgers are part of the problem
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Epidemiological effects of badger vaccination and culling
Introduction to disease dynamicsNonselective badger cullingBadger vaccinationCombined badger culling & vaccination
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Epidemiological effects of badger vaccination and culling
Introduction to disease dynamicsNonselective badger cullingBadger vaccinationCombined badger culling & vaccination
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Susceptible and infectious hosts
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susceptible
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susceptible infectious
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Immunity
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immune
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Population structure is important
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1981
0 1km
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1981
1982
1983
1984
1985
0 1km
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Culling
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Fewer infected hostsFewer susceptible hosts
Less frequent contact between infected and susceptible hosts
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What nonselective badger culling is meant to do
CULL
• Reduce numbers of infected animals• Reduce onward transmission of infection to other badgers• Reduce onward transmission to cattle
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Badger densities were reduced inside RBCT culling areas; but their territorial and ranging behaviour were also affected
Triplet D proactive
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As culls were repeated, the proportion of infected badgers increased
prev
alen
ce (r
elat
ive
tofir
st p
roac
tive)
error bars show 95% CI
RBCT culling led to a rapid drop in badger numbers… but numbers of infected badgers fell more slowly
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CULL
• Disrupts territorial system• Increases opportunities for contact between social groups• Increases opportunities for disease transmission• Increases number of cattle herds contacted by each badger
What badger culling actually does
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Badger culling has two opposing consequences
Fewer badgers - good
Each remaining badger more infectious – bad
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outside proactive
reactive culling
proactive culling
How does changing badger density influence TB risk to cattle?
% reduction in badger density
rela
tive
chan
ge in
cat
tle T
B in
cide
nce
20% 40% 60% 80% 100%
more cattle TB
less cattle TB
West Somerset
West Gloucestershire
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Approach Badger numbers Badger TB Cattle TB Annual cost/km2
Nonselective cull - large scale
much reduced increased less & more£1,800-£4,000
Nonselective cull - small scale
somewhat reduced increased more –
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Vaccination
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Vaccination Removes susceptibles by making them immune
No impact on those already infected
Nevertheless, helped eradicate smallpox and rinderpest, and to control many other diseases e.g. measles, rabies, human TB
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What badger vaccination is meant to do
• No effect on already-infected animals• Reduce onward transmission of infection
vaccinate
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vaccinate
What vaccination is meant to do
• No effect on already-infected animals• Reduce onward transmission of infection
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vaccinate
What vaccination is meant to do
• No effect on already-infected animals• Reduce onward transmission of infection• Lowers prevalence over time as infected animals die off
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vaccinate
What vaccination is meant to do
• No effect on already-infected animals• Reduce onward transmission of infection• Lowers prevalence over time as infected animals die off• Population structure likely to enhance vaccine benefits
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Approach Badger numbers Badger TB Cattle TB Annual cost/km2
Nonselective cull - large scale
much reduced increased less & more £1,800-£4,000
Nonselective cull - small scale
somewhat reduced increased more –
Vaccination unchanged reduced (less) £1,500-£4,000
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Selective culling
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no social
perturbation
social perturbation
Catch and test 60-80% of badgers
Detect and cull 49% of infected badgers
Vaccinate test-negative badgers
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CSL (now AHVLA) 2009:
“if... [selective] culling produced no social perturbation then the reduction in the number of infected badgers, and the reduction in herd breakdowns, was greater than either culling or vaccination...
If... culling resulted in repeated perturbation of social groups each time a badger social group... had an animal culled, then there was a dramatic increase in the number of infected badgers and the number of herd breakdowns”
Estimates of the threshold numbers of badgers culled
needed to prompt increase in territory size
Bielby et al (in prep) – effects of 1986-98 small-scale culls on badger populations in 1998-2002
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Approach Badger numbers Badger TB Cattle TB Annual cost/km2
Nonselective cull - large scale
much reduced increased less & more£1,800-£4,000
Nonselective cull - small scale
somewhat reduced increased more –
Vaccination unchanged reduced (less) £1,500-£4,000
Selective cull somewhat reduced (increased) (increased) >£2,880
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ConclusionsNonselective culling, vaccination, and selective culling function by different mechanisms but in principal all have the potential to control wildlife disease
Population structure can have a major impact on disease transmission rates
Culling alters badger population structure in ways which accelerate transmission, undermining benefits for TB control
By contrast, badger population structure is likely to enhance the efficacy of vaccination
Badger vaccination is likely to be cheaper than culling, and is unlikely to cause harm; however its contribution to cattle TB control is not yet known