Tb bacillus

Can TB be eliminated from possums in large forest areas?

Local elimination - Proof of concept of eradication

G Nugent & B Warburton

Landcare Research, P.O. Box 40, Lincoln, N.Z

Outline

• TB history

• Proposed future for TB-possum management

• Local Elimination

• Proof of concept of eradication trial

TB lesion in cow head

TB lesion in possum armpit

Trends in TB in livestock

• 30 April 2010: 95 herds

infected, probably <50

caused by possums

• >94% reduction in

livestock TB in 15 years

=> Now no doubt that

possum control is

being successful in

most areas

NPMS review and new proposal

• Current National Pest Management Strategy for bovine TB aims to reduce TB in livestock to <0.2% by 2013– It does NOT aim to eradicate TB from possums

• TB has been eliminated from some small areas

• BUT no proof yet TB can be eradicated from possums in large tracts of native forest in which TB is long established

Can TB be eradicated from a wildlife host?

• One country (Oz) has eliminated TB from wildlife host

• But from Buffalo• Big and easy to

see, and muster or shoot

• Much harder from possums?

NPMS TB: New proposal (2009)

• Extend strategy to 2025

New objectives:• To reduce area with

infected wildlife • To obtain proof of

concept that TB can be eradicated from the wild across large forested habitats

• NB: Also, clarify legal responsibilities for reporting animal ID information from slaughter

TB vector free

Eradication requirements:

• Intensive possum control to

break cycle in possums

– Model: >99% prob of Tb

extinction in 5 y at <2% RTCI

• Maintain low possum numbers

till SPILLBACK risk is zero

– Infected female deer can remain

alive and infected for 10-15 years

YearsPro

bab

ility

of

Tb

ext

inct

ion

0 1 2 3 4 5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

1.0

5% RTCI2% RTCI1% RTCI

SPILLBACK RISK:

Possum eating pig mesenteric tissue

Predictions of a spatial

possum-Tb model

Three phases in TB eradication

1. Initial knockdown • Reducing possums from carrying capacity to below

~0.4 possums/ha.

2. Maintenance• Possum densities kept at this low level 10-15 y to

allow bTB to die out

3. Proof of Freedom (POF) • surveillance data used to assess objectively the

likelihood that bTB has been eradicated successfully

ExampleBlythe Valley; 13,000 ha hilly farmland, N Canterbury

Rapid initial knockdown (by ground-based methods)

Possum control started 2000

0%

10%

20%

30%

1991

1994

1997

2000

2003

2006

2009

RTCIPossum control

No possum control

Maintenance control

• 4 yrs trapping• Index-removal-index estimator

Only 30 possums now present 0.002/ha Too low to sustain TB?

Kill Survivors ~RTCI ~% kill

2005 39 64 0.04% 38%

2006 34 36 0.03% 49%

2007 25 30 0.02% 46%

2008 16 24 0.02% 40%

‘Proving’ freedom: Step 1 Have we done enough?

• Model predicts 99% prob Tb eradicated from possums by 2005 IF the high intensity of control had been applied evenly

Year

Predicted effect on possum Tb

0.0

0.2

0.4

0.6

0.8

1.0

2000

2002

2004

2006

2008

Pro

bab

ilit

y o

f T

b

exti

nct

ion

Proving TB FreedomStep 2: Validate model predictions with

surveillance data

xP

HxPHPxHP ii

i

||

Multiple data sources:• Livestock Tb testing• Wildlife necropsy• Possum trapping

Bayesian Framework

Period Updated/posterior probability

2006/07 99.0%

2007 /08 99.7%

2008 /09 99.8%

Prob TB absence in possums Blythe Valley

2006 predicted probability = 10%*

If possums almost eliminated a single survey can

provide >99% confidence Tb is absent

* Conservatively set high

0

10

20

30

40

50

N s

kin

-tes

t +

ve

Tb levels in deer and cattle

Local elimination of possums Control at zero density

Three components

1. Maximise initial knockdown: >99% reduction• Already achievable - 0% RTCI already common• Focus now on cost reduction

2. Mop up: Detect and eliminate any survivors• Requires cheap detection method

3. Preventing all or most reinvasion:• Difficult but not issue if surrounding area

controlled

Initial knockdown:

Hauhungaroa 2005 Example

• 88,000 ha treated with aerial 1080

– 2 pre-feeds, cost $60/ha

– Monitored by 512 traplines

RTCI = 0.045% (50 x lower than 2% target)

• Predicted prob Tb extinction > 99% in <5 yr

• But repeat control still needed - because of deer

spillback risk

Mop-up of survivors key step

• Development of Chew Card as low-cost but sensitive detection devices

• To map where survivors are still present

So they can be targeted by follow-up or mop-up control

Possum Detection Probability DNA extracted from Chewcards and trapped possums

>80% of possums detected by Chewcards

(when spaced 50m along lines 250m apart)

BUT only 40% of the possums present trapped

=> not ideal, but adequate if control annual

Local eliminationCurrent status

• Conventional aerial ops can deliver knockdown to near zero density

• Feasible to prevent population recovery through annual detect-and-mop-up control• If not reduce further

• Difficult to prevent re-invasion • But with regional-scale control to low density not an issue

Proof of concept of eradicationproposed trials

• Two areas ID’d as possibilities:

– Hokonui, Southland

– Rangitoto/Hauhungaroa (RHR)

• RHR: Established focus of TB

– >50% adult female deer infected, most pigs

historically.

– Some controlled since 1994, others not till

2005.

– TB-infected deer killed in 2008

– Farms periphery still infected 2009

TB lesion, red deer lungs

‘Standard’ eradication recipe

for ‘deep’ forest

• Conventional broadcast aerial

1080 with 1-2 prefeeds

– Reliable intensive control

• Repeat 3 time at ~5 y intervals

– To keep possums v low for >15 yr

• RTCI monitoring

– To confirm UNIFORM low possum

density

• Pig and deer Tb survey

AHB Design best practice (Draft)

Otago Daily Times 21/01/2008

Alternative 2:

Low-cost aerial

• 2nd & 3rd aerials at half cost

• If first control highly effective

• Use low-sow techniques

• Potentially = broadcast?

• But even if not adequate to prevent spill back?

Option 3:

Targeted aerial

• Reduce cost of 2nd & 3rd aerial by targeted low sow

• Use detection surveys (Chew Cards) to identify areas with most possums

• Same cost as option 2, but less 1080.

• Plus extra surveillance data from simultaneous possum/pig/deer survey

2006 Chew Card survey:

Large areas with no possum

detections

Option 4: Detection and Ground-based

Mop-up’

• Use detection surveys (Chew Cards) to identify areas with possums

• Annual or biennial.

• Plus extra surveillance data from simultaneous possum/pig/deer survey

• Only need aerial1080 for first knock down

Eradication proof of concept trial

• 2011-2025:

• Compare speed & cost effectiveness of 4 diff approaches

1. Standard – conventional broadcast aerial, RTCI monitoring, pig and deer

Tb survey

2. Low sow (half price) standard aerial RTCI monitoring, pig and deer Tb

survey

3. Targeted low sow: Low intensity detection CTCI surveys to

systematically map possum distribution, and target option 2 at <50% of

area

4. Ground-based detection & mop up incorporating possum, deer and pig

surveillance annually.

Eradication proof of concept Illustrative scenario ($/ha)

2010/11 2011/12 2012/13 2013/14 2014/15 2015/16

1. Std intensive aerial

$40 aerial

$5 RTCI

$3 Pig/deer survey

$3 Pig/deer survey

$5 TrendRTCIStop?

2. Low-cost aerial

$20 aerial

$5 RTCI

$3 Pig/deer survey

$3 Pig/deer survey

$5 TrendRTCIStop?

3. Targeted low-cost aerial

$5 detect (mapping)

$5 detect(mapping)

$10 aerial (half area)

$5 RTCI

$12 detect,

mop-up, deer/ pig

control

$12 detect,

mop-up, deer/ pig

control

Stop?

4. Ground detect & mop-up

$12 detect,

mop-up, deer/ pig

control

$12 detect,

mop-up, deer/ pig

control

$12 detect,

mop-up, deer/ pig

control

$12 detect,

mop-up, deer/ pig

control

$12 detect,

mop-up, deer/ pig

control

Stop?

Summary I

We can:

• Reduce possums to near zero

• Detect survivors with high sensitivity

• Mop up survivors at > reproductive rate

=> Local elimination feasible

Summary II

At the same time we can also :

• Show many sites have no possums (= no TB) Proving TB can be eradicated may be easy? And it may be achievable more cheaply and

quickly than current practice