research on pre-movement testing in ireland tracy clegg centre for veterinary epidemiology and risk...

Research on Pre-Movement

Testingin Ireland

Tracy Clegg

Centre for Veterinary Epidemiology and Risk Analysis,

University College Dublin

Animal movement and infection spread

History of pre-movement testing in Ireland

Animal movement and bovine tuberculosis

Research on pre-movement testing in Ireland

GB findings

Pre-movement testing in Ireland

• Prior to 1996:

Animals were tested prior to movement (except to slaughter) if a test had not been passed in the previous 60 days

Pre-movement testing in Ireland

• Since 1996:

Animals can move from an unrestricted herd provided they passed a tuberculin test in the previous 12 months

Research on Pre-movement testing in Ireland

How many tuberculosis breakdowns are due to purchasing an infected animal?

When an infected animals is purchased how many other animals in the herd become infected?

Prior to 1996 % of reactors detected by pre-movement and subsequent testing

Studies prior to 1996

Questionnaire Survey completed by a Veterinary Inspector for all new herd breakdowns in Cavan in 1989

6.9% of breakdowns were probably directly due to purchased cattle

a further 8% were possibly due to this cause

Griffin 1993:

Case-control study in East Offaly of risk factors at the herd and animal level

Herd level: Purchased cattle was not a significant factor

Animal level: Purchased cattle were less likely to be positive at the next test compared to animals present at the previous herd test.

Griffin 1996:

Between 1982 and 1994 pre-movement testing and subsequent testing accounted for 7.8% of all reactors

When consequential contiguous tests are included this figure increases to 11%

Hahesy 1996:

Studies after 1996

Data sourcesAnimal Health system:

– All Tb tests since 1989

Cattle Movement and Monitoring system:– Records all: births, deaths on farm,

slaughters, exports, movements since 2000

Factory lesion database:– Results from laboratory testing of lesions from

non-reactor animals found at factories since 2000

Retrospective study

• Study population:

– Herds restricted following a tuberculosis outbreak during:

– 1st April 2003 and 31st March 2004

Retrospective study

Recently Introduced Reactors:

Introduced into the herd since the last clear test.

Potential for previous exposure?

Retrospective study6,252

Breakdown herds

1,163 (18.6%)

Breakdown herds with…

Recently Introduced Reactors

441 (7.0%)

Breakdown herds with…

Exposed Recently Introduced Reactors

18,824

Reactors

2,033 (10.8%)

Recently Introduced Reactors

622 (3.3%)

Exposed Recently Introduced Reactors

Retrospective study

• Link between introduction and infection

Introduced animals in: 18.6% of breakdown herds

Exposed introduced animals: 7.0% of breakdown herds

Coincidental link…

Retrospective study

• Assumptions:

– Exposure has resulted in infection

(overestimate)

– No latent infection

(underestimate)

Prospective study

Study population:

• Herds de-restricted following a tuberculous breakdown between:

1st October 2001 - 30th September 2002

3,947 de-restricted herds

2,646 de-restricted herds

Sold 55,410 animals

13,170

Purchasing herds

Prospective studyMovements between de-restriction and the next herd test

13,170 Purchasing herds

(55,410 animals purchased)

1,576 Purchasing herds restricted at the next test

(5,454 reactors)

88 (5.6%) Restrictions involved a purchased animal

(120 purchased animals were reactors)

Prospective study

Prospective studyAssumptions:

• Animals purchased from other herds did not cause the disease

(underestimate)

• The purchased animal was the cause of the breakdown

(overestimate)

Tuberculosis restrictions attributed to a purchased animal

Retrospective study:

7.0% of breakdowns involved an animal purchased from a previously infected herd

Prospective study:5.6% of restrictions in the destination herds involved a purchased animal

Tuberculosis restrictions attributed to a purchased animal

7.0% of restrictions due to purchased animals

7.2% of restrictions due to purchased animals

Prospective study

Pre-movement test• 3.4 movement events per herd• 9089 movement tests• 0.97 restrictions identified per 100 herd tests

National testing regime:• Annual test: 2.6 restrictions per 100 herd tests• Post de-restriction test: 7.2 restrictions per 100

herd tests

Tuberculosis outbreaks due to purchasing an infected animal

Around 6% to 7% of tuberculosis outbreaks in Ireland are due to purchasing an infected animal

Onward spread

In 12 herds “probably” infected by a purchased animal only 3 further homebred animals were positive.

“little evidence of lateral transmission of tuberculosis in the purchasing herds”

Griffin 1993:

Strain typing of animals to establish whether the animal was infected before or after arriving in the purchasing herd

Onward spread ranged from 0 to low

Flanagan 1998 :

Flanagan and Kelly 1996:

Study of 77 breakdowns in County Offaly in 1989-1990. All breakdowns had purchased animals.

Compared breakdowns with purchased animals previously exposed to infection to breakdowns with purchased animals not previously exposed to infection

Flanagan and Kelly 1996:

Findings:

Significantly fewer reactors in herds with purchased animals previously exposed to infection (1.8% v 4.2% of animals were reactors).

“the onward transmission of tuberculosis from purchased tuberculous cattle was at a low level.”

Clear herds tested following the detection of a single animal found to have a lesion at slaughter in 2003.

Olea-Popelka 2007 (submitted):

Purchased animal:

lower risk of a positive herd test following detection of an animal with a lesion at slaughter

Olea-Popelka 2007 (submitted):

Case-control studies to look at risk factors for transient and persistent breakdowns

Transient breakdowns: <= 6 months

Persistent breakdowns: > 6 months

Reilly and Courtenay 2007:

..”the probability of transient breakdown is most strongly influenced by the purchase of cattle over other management variables and covariates.”

“..the probability of persistent breakdown appears to be mostly affected by management factors relating to type of herd enterprise and silage storage in addition to the relative density of badgers.”

Reilly and Courtenay 2007:

Animals that do not move are at a higher risk of becoming a reactor at the next test compared to animals that do move

(0.5% v 0.2%).

Suggestive of an ongoing locality effect in the source herd.

Clegg 2007 (submitted):

Onward spread from purchased cattle

Onward transmission is at a low level

Resultant breakdowns tend to be transient

GB Findings

Gilbert 2005:

Core areas: 5km squares

TB in 2 of the last 3 years

Remote areas: 5km squares

all other areas

Gilbert 2005:

“BTB occurrence in core areas was associated with:

• Previous year’s BTB status

• Disease in the previous year in surrounding areas.

Gilbert 2005:“In remote areas, BTB occurrence was associated with:

variables describing past BTB status; the status of the disease in the previous year in surrounding areas; the proportion of inward movements from infected areas; the density of cattle; the proportion of grassland; and the mean of the normalized deviation vegetation index.”

Differences between GB and Ireland

• Difference in testing regimes

• Epidemiological differences between Britain and Ireland

• Cattle to cattle spread an important factor for disease spread in Britain

Summary

Cattle movement plays only a limited role in the spread of tuberculosis in Ireland.

Around 6% to 7% of tuberculosis outbreaks in Ireland are due to purchasing an infected animal

Onward spread following purchasing of an infected animal is limited and resultant breakdowns tend to be transient

Summary

Pre-movement testing would involve intensive testing to prevent a small number of restrictions (0.97 restrictions prevented per 100 herds tested)