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Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain.

ABSTRACT: The objective of this study was to contribute to a risk assessment to review the over 30 months (OTM) scheme in cattle, whereby all cattle aged over 30 months slaughtered in the UK are removed from the human food chain. We use back-calculation methods to estimate the impact of changes to the OTM rule, by using passive and active surveillance data collected between 1 July 2001 and 30 June 2002. There are two types of change considered: increasing the age limit and allowing animals born after a certain date into the food chain. Results indicate that under the OTM rule less than 1 animal in the last 12 months of the incubation period would enter the food chain in 2003. The birth date changes considered and small changes to the upper age limit would increase this number by a relatively small amount.

SUBMITTER: Arnold M

PROVIDER: S-EPMC1691480 | biostudies-literature | 2003 Oct

REPOSITORIES: biostudies-literature

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Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain.

Arnold Mark M Wilesmith John J

Proceedings. Biological sciences 20031001 1529

The objective of this study was to contribute to a risk assessment to review the over 30 months (OTM) scheme in cattle, whereby all cattle aged over 30 months slaughtered in the UK are removed from the human food chain. We use back-calculation methods to estimate the impact of changes to the OTM rule, by using passive and active surveillance data collected between 1 July 2001 and 30 June 2002. There are two types of change considered: increasing the age limit and allowing animals born after a ce ...[more]

PMID: 14561277

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Project description:The number of cattle herds placed under movement restrictions in Great Britain (GB) due to the suspected presence of bovine tuberculosis (bTB) has progressively increased over the past 25 years despite an intensive and costly test-and-slaughter control program. Around 38% of herds that clear movement restrictions experience a recurrent incident (breakdown) within 24 months, suggesting that infection may be persisting within herds. Reactivity to tuberculin, the basis of diagnostic testing, is dependent on the time from infection. Thus, testing efficiency varies between outbreaks, depending on weight of transmission and cannot be directly estimated. In this paper, we use Approximate Bayesian Computation (ABC) to parameterize two within-herd transmission models within a rigorous inferential framework. Previous within-herd models of bTB have relied on ad-hoc methods of parameterization and used a single model structure (SORI) where animals are assumed to become detectable by testing before they become infectious. We study such a conventional within-herd model of bTB and an alternative model, motivated by recent animal challenge studies, where there is no period of epidemiological latency before animals become infectious (SOR). Under both models we estimate that cattle-to-cattle transmission rates are non-linearly density dependent. The basic reproductive ratio for our conventional within-herd model, estimated for scenarios with no statutory controls, increases from 1.5 (0.26-4.9; 95% CI) in a herd of 30 cattle up to 4.9 (0.99-14.0) in a herd of 400. Under this model we estimate that 50% (33-67) of recurrent breakdowns in Britain can be attributed to infection missed by tuberculin testing. However this figure falls to 24% (11-42) of recurrent breakdowns under our alternative model. Under both models the estimated extrinsic force of infection increases with the burden of missed infection. Hence, improved herd-level testing is unlikely to reduce recurrence unless this extrinsic infectious pressure is simultaneously addressed.

Dataset Information

Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain.

Publications

Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain.

Similar Datasets

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