Transcript Epidemiological effects of badger vaccination

Epidemiological effects of badger
culling and vaccination
Rosie Woodroffe
Zoological Society of London
Two important facts about bovine TB
TB is a huge problem for both beef and
dairy farmers
Badgers are part of the problem
Epidemiological effects of badger
vaccination and culling
Introduction to disease dynamics
Nonselective badger culling
Badger vaccination
Combined badger culling & vaccination
Epidemiological effects of badger
vaccination and culling
Introduction to disease dynamics
Nonselective badger culling
Badger vaccination
Combined badger culling & vaccination
Susceptible and infectious hosts
susceptible
susceptible
infectious
Immunity
immune
Population structure is important
1981
0
1km
1981
1982
0
1km
1983
1984
1985
Culling
Fewer infected hosts
Fewer susceptible hosts
Less frequent contact between
infected and susceptible hosts
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
Badger
densities were
reduced inside
RBCT culling
areas; but their
territorial and
ranging
behaviour were
also affected
Triplet D proactive
prevalence (relative to
first proactive)
RBCT culling led to a
rapid drop in badger
numbers… but numbers
of infected badgers fell
more slowly
error bars show 95% CI
As culls were
repeated, the
proportion of
infected badgers
increased
What badger culling actually does
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
Badger culling has two opposing consequences
Fewer badgers - good
Each remaining badger
more infectious – bad
relative change in cattle TB incidence
How does changing badger density influence TB
risk to cattle?
reactive culling
more cattle TB
outside proactive
West Somerset
% reduction in
badger density
20%
40%
60%
80%
100%
West
Gloucestershire
proactive culling
less cattle TB
Approach
Nonselective cull
- large scale
Nonselective cull
- small scale
Badger numbers
much reduced
Badger TB
Cattle TB
increased
£1,800less & more £4,000
somewhat reduced increased
more
Annual
cost/km2
–
Vaccination
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
What badger vaccination is meant to do
vaccinate
•
•
No effect on already-infected animals
Reduce onward transmission of infection
What vaccination is meant to do
vaccinate
•
•
No effect on already-infected animals
Reduce onward transmission of infection
What vaccination is meant to do
vaccinate
•
•
•
No effect on already-infected animals
Reduce onward transmission of infection
Lowers prevalence over time as infected animals die off
What vaccination is meant to do
vaccinate
•
•
•
•
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
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
(less)
£1,500£4,000
reduced
Selective culling
Catch and test 60-80%
of badgers
Detect and cull 49%
of infected badgers
Vaccinate
test-negative
badgers
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”
Bielby et al (in prep) – effects of
1986-98 small-scale culls on
badger populations in 1998-2002
Estimates of the threshold
numbers of badgers culled
needed to prompt increase in
territory size
Approach
Nonselective cull
- large scale
Badger numbers
much reduced
Badger TB
Cattle TB
Annual
cost/km2
increased
£1,800less & more £4,000
Nonselective cull
- small scale
somewhat reduced increased
more
–
Vaccination
unchanged
(less)
£1,500£4,000
Selective cull
somewhat reduced (increased)
(increased)
>£2,880
reduced
Conclusions
Nonselective 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