The role of wild deer as a temporal vector of bovine

tuberculosis (Tb)

Mandy Barron, Graham NugentLandcare Research, Lincoln

• Goal of NPMS for Tb: Eradication of Tb from wildlife hosts across large areas of NZ

• Mostly through effective control of possums (“maintenance” host of Tb)

• Tb declines quickly if possums numbers kept low

“TBfree New Zealand”

Possums

Tb in wild deer

• Epidemiology: young deer only become infected once independent and low disease mortality

Possums Deer

• Wild deer are “spillover” hosts for Tb

• Pathology: high frequency of infection in head (tonsils and lymph nodes)

Control possums → Tb in deer declines?

• Apparently yes... Deer Tb prevalence by year, Hauhungaroa Range:

Possums poisoned

EHR (T)

0

20

40

60

1993 1995 1997 1999 2001 20030%

10%

20%

30%

0

20

40

60

80

100

1993 1995 1997 1999 2001 20030%

10%

20%WHR (NT)

0

20

40

60

1993 1995 1997 1999 2001 20030%

10%

20%

30%

40%

50%

Possums aerially poisoned in 1994 and 2000

Possums not poisoned

Source: Nugent (2005)

But...• Plenty evidence

possums scavenging and investigating deer carcasses

• If carcasses infected – potential for possums to become infected = “Tb spillback”

Possums DeerPhotos: G Nugent

Deer as a temporal vector of TbLongevity of infected individuals:

c.f. time scale of Tb eradication program

0

0.005

0.01

0.015

0.02

0.025

0.03

0

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16 18 20

Poss

um T

b pr

eval

ence

Poss

um d

ensi

ty

Spillback risk period

Is it worth controlling deer?• Reduction in spillback risk period?• Reduction in no. spillback events?• Cost-effective?

Photo: I Yockney

A modelling approach

• Deer population subdivided into classes:Age (0-15 yrs), Sex (M/F), Infection (Tb+/Tb-)

• Possum-Tb model from Barlow (2000)• Possum to deer Tb transmission based on no.

infected possums• Deer to possum Tb transmission based on no.

infected deer carcasses and carcass encounter rates

Possums Deer

Case study: Hauhungaroa RangeCombined VCZs = 915 km2

• Possums: 3 aerial poisoning operations, 5 years apart

• Deer:1. None2. Non-selective – one-off foliage baiting3. Selective – 5 yrs ground hunting of females,

after 1st possum control4. Selective – 5 yrs ground hunting of females,

after 3rd possum control

All deer control scenarios included “background” hunting by recreational hunters which had a bias towards culling males

Control scenarios

Photo: LCR archives

Simulation results

0 0.05 0.1

123456789101112131415

Female

TB-

TB+

00.050.1

123456789101112131415

Male

• No possum or deer control: Deer population structure

Simulation results• “Standard” possum control strategy:

Possums

0

0.01

0.02

0.03

0.04

0.05

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TB p

reva

lenc

e

Den

sity

per

km

2

Year

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0123456789

101112131415

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TB p

reva

lenc

e

Den

sity

per

km

2

Year

Simulation results• “Standard” possum control strategy (and 30%

by-kill of deer with initial possum control): Deer

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0123456789

101112131415

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TB p

reva

lenc

e

Den

sity

per

km

2

Year

Simulation results• Selective deer control, targeting females, over

5 years: Deer

Reduction in deer to possum spillback through deer control?

Control scenario Spillback risk period

Prob. of spillback

Cost of deer control ($/km2)

No deer control 7.1 0.06 0

Non-selective 5.3 0.02 1500

Female-targeted 1 6.2 0.03 3500

Female-targeted 2 6.2 0.03 3500

Conclusions• Model predicts deer control can reduce the

spillback period and the number of spillback events

• Non-selective control was most cost-effective out of the control scenarios tested

• BUT gains were small for $ spent so large-scale deer control not recommended

Caveats

• Assuming worst-case scenarios about infection of possums via deer carcasses

• Acceptability or technical feasibility of deer control scenarios not investigated

• Culling and necropsy of deer for surveillance has benefits for “proof” of Tb eradication

Acknowledgements

• Thanks to the Animal Health Board for funding this project (R-10731)