Epidemiological effects of badger culling and vaccination

Rosie WoodroffeZoological Society of London

Two important facts about bovine TB

TB is a huge problem for both beef and dairy farmersBadgers are part of the problem

Epidemiological effects of badger vaccination and culling

Introduction to disease dynamicsNonselective badger cullingBadger vaccinationCombined badger culling & vaccination

Epidemiological effects of badger vaccination and culling

Introduction to disease dynamicsNonselective badger cullingBadger vaccinationCombined badger culling & vaccination

Susceptible and infectious hosts

susceptible

susceptible infectious

Immunity

immune

Population structure is important

1981

0 1km

1981

1982

1983

1984

1985

0 1km

Culling

Fewer infected hostsFewer 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

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

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

Badger culling has two opposing consequences

Fewer badgers - good

Each remaining badger more infectious – bad

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

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

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

• No effect on already-infected animals• Reduce onward transmission of infection

vaccinate

vaccinate

What vaccination is meant to do

• No effect on already-infected animals• Reduce onward transmission of infection

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

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

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 culling

no social

perturbation

social perturbation

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”

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

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

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