The role of wild deer as a temporal vector of bovine tuberculosis (Tb)

1. The role of wild deer as a temporal vector of bovine tuberculosis (Tb) Mandy Barron, Graham Nugent Landcare Research, Lincoln

2. “TBfree New Zealand” • 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 Possums

3. Tb in wild deer • Pathology: high frequency of infection in head (tonsils and lymph nodes) • Epidemiology: young deer only become infected once independent and low disease mortality Possums Deer • Wild deer are “spillover” hosts for Tb

4. Possums aerially poisoned in 1994 and 2000 Possums poisoned Possums not poisoned Control possums → Tb in deer declines? • Apparently yes... Deer Tb prevalence by year, Hauhungaroa Range: Source: Nugent (2005)

5. But... • Plenty evidence possums scavenging and investigating deer carcasses • If carcasses infected – potential for possums to become infected = “Tb spillback” Possums Deer Photos: G Nugent

6. Deer as a temporal vector of Tb Longevity of infected individuals: Spillback risk period c.f. time scale of Tb eradication program

7. Is it worth controlling deer? • Reduction in spillback risk period? • Reduction in no. spillback events? • Cost-effective? Photo: I Yockney

8. 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

9. Case study: Hauhungaroa Range Combined VCZs = 915 km2

10. Control scenarios • Possums: 3 aerial poisoning operations, 5 years apart • Deer: • None • Non-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 control All deer control scenarios included “background” hunting by recreational hunters which had a bias towards culling males Photo: LCR archives

11. Simulation results • No possum or deer control: Deer population structure

12. Simulation results • “Standard” possum control strategy: Possums

13. Simulation results • “Standard” possum control strategy (and 30% by-kill of deer with initial possum control): Deer

14. Simulation results • Selective deer control, targeting females, over 5 years: Deer

15. Reduction in deer to possum spillback through deer control?

16. 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

17. 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

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