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Estimating the hidden burden of bovine tuberculosis in Great Britain.

Conlan AJ, McKinley TJ, Karolemeas K, Pollock EB, Goodchild AV, Mitchell AP, Birch CP, Clifton-Hadley RS, Wood JL - PLoS Comput. Biol. (2012)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Disease Dynamics Unit (DDU), Department of Veterinary Medicine, University of Cambridge, Cambridge, UK. ajkc2@cam.ac.uk

ABSTRACT
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.

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Herd level measures of efficiency of transmission and clearance of infection for SORI and SOR models.(A) Predictive distribution for the within-herd reproduction ratio (R0), plotted as a shaded density strip where the intensity of shading is proportional to the probability that R0 takes a given value [35]. We calculate  where  measures the strength of density dependence,  is the estimated transmission parameter and  the per capita rate of turnover of the herd sampled from an empirical distribution and . Both models estimate that transmission is non-linearly density dependent with point estimates for the invasion threshold (R0 = 1) of 71 (8–674,95% CI) for the SORI model and 12 (1–833, 95% CI) for the SOR model. (B) The effective sensitivity of the SICCT test within our study population of herds measured under the standard (solid line) and severe (dashed line) interpretations and relative to the gold standard of confirmation with visible lesions (dotted line). (C,D) The infectious burden remaining after movement restrictions are lifted can be characterized in terms of the number of herds with at least one infectious animal remaining when movement restrictions are lifted (C) and the expected distribution of animals on those herds (D). Predictive distributions for (B,C,D) are calculated by simulating from the empirical distribution of herds taken from our study population, representing the national distribution of herds.
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pcbi-1002730-g003: Herd level measures of efficiency of transmission and clearance of infection for SORI and SOR models.(A) Predictive distribution for the within-herd reproduction ratio (R0), plotted as a shaded density strip where the intensity of shading is proportional to the probability that R0 takes a given value [35]. We calculate where measures the strength of density dependence, is the estimated transmission parameter and the per capita rate of turnover of the herd sampled from an empirical distribution and . Both models estimate that transmission is non-linearly density dependent with point estimates for the invasion threshold (R0 = 1) of 71 (8–674,95% CI) for the SORI model and 12 (1–833, 95% CI) for the SOR model. (B) The effective sensitivity of the SICCT test within our study population of herds measured under the standard (solid line) and severe (dashed line) interpretations and relative to the gold standard of confirmation with visible lesions (dotted line). (C,D) The infectious burden remaining after movement restrictions are lifted can be characterized in terms of the number of herds with at least one infectious animal remaining when movement restrictions are lifted (C) and the expected distribution of animals on those herds (D). Predictive distributions for (B,C,D) are calculated by simulating from the empirical distribution of herds taken from our study population, representing the national distribution of herds.

Mentions: Both models estimate that the rate of cattle-to-cattle transmission within a herd increases, non-linearly, with herd size. The potential for transmission within a herd can be characterized by the basic reproductive ratio R0, defined as the expected number of secondary cases within a herd of size N on the introduction of a single infectious individual. Within the range of our study population our (median) point estimate of R0 from the SORI model increases from 1.5 (0.26–4.9; 95% CI) in a herd of size 30 up to 4.9 (0.99–14.0; 95% CI) in a herd of 400 cattle (Figure 3A). Estimates from the SOR model are smaller, but with overlapping credible intervals, increasing from 0.52 (0.1–1.6, 95% CI) in a herd of size 30 up to 3.6 (0.73–8.85, 95% CI) in a herd of size 400 (Figure 3A). As a consequence, both models predict that the efficiency of control will also scale with herd size.


Estimating the hidden burden of bovine tuberculosis in Great Britain.

Conlan AJ, McKinley TJ, Karolemeas K, Pollock EB, Goodchild AV, Mitchell AP, Birch CP, Clifton-Hadley RS, Wood JL - PLoS Comput. Biol. (2012)

Herd level measures of efficiency of transmission and clearance of infection for SORI and SOR models.(A) Predictive distribution for the within-herd reproduction ratio (R0), plotted as a shaded density strip where the intensity of shading is proportional to the probability that R0 takes a given value [35]. We calculate  where  measures the strength of density dependence,  is the estimated transmission parameter and  the per capita rate of turnover of the herd sampled from an empirical distribution and . Both models estimate that transmission is non-linearly density dependent with point estimates for the invasion threshold (R0 = 1) of 71 (8–674,95% CI) for the SORI model and 12 (1–833, 95% CI) for the SOR model. (B) The effective sensitivity of the SICCT test within our study population of herds measured under the standard (solid line) and severe (dashed line) interpretations and relative to the gold standard of confirmation with visible lesions (dotted line). (C,D) The infectious burden remaining after movement restrictions are lifted can be characterized in terms of the number of herds with at least one infectious animal remaining when movement restrictions are lifted (C) and the expected distribution of animals on those herds (D). Predictive distributions for (B,C,D) are calculated by simulating from the empirical distribution of herds taken from our study population, representing the national distribution of herds.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3475695&req=5

pcbi-1002730-g003: Herd level measures of efficiency of transmission and clearance of infection for SORI and SOR models.(A) Predictive distribution for the within-herd reproduction ratio (R0), plotted as a shaded density strip where the intensity of shading is proportional to the probability that R0 takes a given value [35]. We calculate where measures the strength of density dependence, is the estimated transmission parameter and the per capita rate of turnover of the herd sampled from an empirical distribution and . Both models estimate that transmission is non-linearly density dependent with point estimates for the invasion threshold (R0 = 1) of 71 (8–674,95% CI) for the SORI model and 12 (1–833, 95% CI) for the SOR model. (B) The effective sensitivity of the SICCT test within our study population of herds measured under the standard (solid line) and severe (dashed line) interpretations and relative to the gold standard of confirmation with visible lesions (dotted line). (C,D) The infectious burden remaining after movement restrictions are lifted can be characterized in terms of the number of herds with at least one infectious animal remaining when movement restrictions are lifted (C) and the expected distribution of animals on those herds (D). Predictive distributions for (B,C,D) are calculated by simulating from the empirical distribution of herds taken from our study population, representing the national distribution of herds.
Mentions: Both models estimate that the rate of cattle-to-cattle transmission within a herd increases, non-linearly, with herd size. The potential for transmission within a herd can be characterized by the basic reproductive ratio R0, defined as the expected number of secondary cases within a herd of size N on the introduction of a single infectious individual. Within the range of our study population our (median) point estimate of R0 from the SORI model increases from 1.5 (0.26–4.9; 95% CI) in a herd of size 30 up to 4.9 (0.99–14.0; 95% CI) in a herd of 400 cattle (Figure 3A). Estimates from the SOR model are smaller, but with overlapping credible intervals, increasing from 0.52 (0.1–1.6, 95% CI) in a herd of size 30 up to 3.6 (0.73–8.85, 95% CI) in a herd of size 400 (Figure 3A). As a consequence, both models predict that the efficiency of control will also scale with herd size.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Disease Dynamics Unit (DDU), Department of Veterinary Medicine, University of Cambridge, Cambridge, UK. ajkc2@cam.ac.uk

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus