the role of wild deer as a temporal vector of bovine tuberculosis (tb)
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The role of wild deer as a temporal vector of bovine tuberculosis (Tb). Mandy Barron, Graham Nugent Landcare Research, Lincoln. “ TBfree New Zealand”. Goal of NPMS for Tb: Eradication of Tb from wildlife hosts across large areas of NZ - PowerPoint PPT PresentationTRANSCRIPT
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)