The role of wild deer as a temporal vector of bovine tuberculosis (Tb) 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) 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 PossumsDeer 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 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” PossumsDeer Photos: G Nugent

Deer as a temporal vector of Tb Longevity of infected individuals: c.f. time scale of Tb eradication program 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 PossumsDeer

Case study: Hauhungaroa Range Combined VCZs = 915 km 2

Possums: 3 aerial poisoning operations, 5 years apart Deer: 1.None 2.Non-selective – one-off foliage baiting 3.Selective – 5 yrs ground hunting of females, after 1 st possum control 4.Selective – 5 yrs ground hunting of females, after 3 rd 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 No possum or deer control: Deer population structure

Simulation results “Standard” possum control strategy: Possums

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

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

Reduction in deer to possum spillback through deer control? Control scenarioSpillback risk period Prob. of spillback Cost of deer control ($/km 2 ) No deer control Non-selective Female-targeted Female-targeted

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)