Mandy Barron Graham Nugent Landcare 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 ID: 780473
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Slide1
The role of wild deer as a temporal vector of bovine tuberculosis (Tb)
Mandy Barron, Graham Nugent
Landcare
Research,
Lincoln
Slide2Goal 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
Slide3Tb 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)
Slide4Control possums → Tb in deer declines?
Apparently yes...
Deer Tb prevalence by year, Hauhungaroa
Range:
Possums poisoned
Possums aerially poisoned in 1994 and 2000
Possums not poisoned
Source: Nugent (2005)
Slide5But...
Plenty evidence possums scavenging and investigating deer carcasses
If carcasses infected – potential for possums to become infected
= “Tb spillback”
Possums
Deer
Photos: G Nugent
Slide6Deer as a temporal vector of Tb
Longevity of infected individuals:
c.f. time scale of Tb eradication program
Spillback risk period
Slide7Is it worth controlling deer?
Reduction in spillback risk period?
Reduction in no. spillback events?Cost-effective?
Photo: I
Yockney
Slide8A 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 possumsDeer to possum Tb transmission based on no. infected deer carcasses and carcass encounter rates
Possums
Deer
Slide9Case study: Hauhungaroa Range
Combined VCZs = 915 km
2
Slide10Possums: 3 aerial poisoning operations,
5 years apart
Deer:
NoneNon-selective – one-off foliage baiting
Selective – 5 yrs ground hunting of females, after 1st possum control
Selective – 5 yrs ground hunting of females, after 3rd
possum controlAll deer control scenarios included “background” hunting by recreational hunters which had a bias towards culling males
Control scenarios
Photo: LCR archives
Slide11Simulation results
No possum or deer control:
Deer population structure
Slide12Simulation results
“Standard” possum control strategy
:
Possums
Slide13Simulation results
“Standard” possum control strategy (and 30% by-kill of deer with initial possum control)
:
Deer
Slide14Simulation results
Selective deer control, targeting females, over 5 years
:
Deer
Slide15Reduction in deer to possum spillback through deer control?
Control scenario
Spillback
risk period
Prob. of spillbackCost of deer control ($/km
2)
No deer control7.10.06
0
Non-selective
5.3
0.02
1500
Female-targeted 1
6.2
0.03
3500
Female-targeted 26.20.033500
Slide16Conclusions
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
Slide17Caveats
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
Slide18Acknowledgements
Thanks to the Animal Health Board for funding this project (R-10731)