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Bayesian inference for within-herd prevalence of Leptospira interrogans serovar Hardjo using bulk milk antibody testing.

Lewis FI, Gunn GJ, McKendrick IJ, Murray FM - Biostatistics (2009)

Bottom Line: Using a percentage positivity cutoff in bulk milk of at most 41% ensures that there is at least a 97.5% probability of less than 5% of the herd being exposed to L. hardjo.Our analyses provide strong statistical evidence in support of the validity of interpreting bulk milk samples as a proxy for individual animal serum testing.The combination of validity and cost-effectiveness of bulk milk testing has the potential to reduce the risk of human exposure to leptospirosis in addition to offering significant economic benefits to the dairy industry.

View Article: PubMed Central - PubMed

Affiliation: Epidemiology Research Unit, Scottish Agricultural College, King's Buildings, West Mains Road, Edinburgh EH9 3JG, UK. fraser.lewis@sac.ac.uk

ABSTRACT
Leptospirosis is the most widespread zoonosis throughout the world and human mortality from severe disease forms is high even when optimal treatment is provided. Leptospirosis is also one of the most common causes of reproductive losses in cattle worldwide and is associated with significant economic costs to the dairy farming industry. Herds are tested for exposure to the causal organism either through serum testing of individual animals or through testing bulk milk samples. Using serum results from a commonly used enzyme-linked immunosorbent assay (ELISA) test for Leptospira interrogans serovar Hardjo (L. hardjo) on samples from 979 animals across 12 Scottish dairy herds and the corresponding bulk milk results, we develop a model that predicts the mean proportion of exposed animals in a herd conditional on the bulk milk test result. The data are analyzed through use of a Bayesian latent variable generalized linear mixed model to provide estimates of the true (but unobserved) level of exposure to the causal organism in each herd in addition to estimates of the accuracy of the serum ELISA. We estimate 95% confidence intervals for the accuracy of the serum ELISA of (0.688, 0.987) and (0.975, 0.998) for test sensitivity and specificity, respectively. Using a percentage positivity cutoff in bulk milk of at most 41% ensures that there is at least a 97.5% probability of less than 5% of the herd being exposed to L. hardjo. Our analyses provide strong statistical evidence in support of the validity of interpreting bulk milk samples as a proxy for individual animal serum testing. The combination of validity and cost-effectiveness of bulk milk testing has the potential to reduce the risk of human exposure to leptospirosis in addition to offering significant economic benefits to the dairy industry.

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Posterior distributions for serum ELISA sensitivity (a) and specificity (b). See Table 3for summary statistics.
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fig3: Posterior distributions for serum ELISA sensitivity (a) and specificity (b). See Table 3for summary statistics.

Mentions: Figure 3 shows estimates of the posterior densities for the serum ELISA specificity (C) and sensitivity (S) from our optimally fitting model (see Table 3 for 95% confidence intervals). The test is extremely good at correctly predicting unexposed animals; however, there is considerably more uncertainty regarding the correct classification of exposed animals. The uncertainty in our estimate of sensitivity could be due, at least in part, to the relatively small proportion of animals that tested nonnegative.


Bayesian inference for within-herd prevalence of Leptospira interrogans serovar Hardjo using bulk milk antibody testing.

Lewis FI, Gunn GJ, McKendrick IJ, Murray FM - Biostatistics (2009)

Posterior distributions for serum ELISA sensitivity (a) and specificity (b). See Table 3for summary statistics.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2742498&req=5

fig3: Posterior distributions for serum ELISA sensitivity (a) and specificity (b). See Table 3for summary statistics.
Mentions: Figure 3 shows estimates of the posterior densities for the serum ELISA specificity (C) and sensitivity (S) from our optimally fitting model (see Table 3 for 95% confidence intervals). The test is extremely good at correctly predicting unexposed animals; however, there is considerably more uncertainty regarding the correct classification of exposed animals. The uncertainty in our estimate of sensitivity could be due, at least in part, to the relatively small proportion of animals that tested nonnegative.

Bottom Line: Using a percentage positivity cutoff in bulk milk of at most 41% ensures that there is at least a 97.5% probability of less than 5% of the herd being exposed to L. hardjo.Our analyses provide strong statistical evidence in support of the validity of interpreting bulk milk samples as a proxy for individual animal serum testing.The combination of validity and cost-effectiveness of bulk milk testing has the potential to reduce the risk of human exposure to leptospirosis in addition to offering significant economic benefits to the dairy industry.

View Article: PubMed Central - PubMed

Affiliation: Epidemiology Research Unit, Scottish Agricultural College, King's Buildings, West Mains Road, Edinburgh EH9 3JG, UK. fraser.lewis@sac.ac.uk

ABSTRACT
Leptospirosis is the most widespread zoonosis throughout the world and human mortality from severe disease forms is high even when optimal treatment is provided. Leptospirosis is also one of the most common causes of reproductive losses in cattle worldwide and is associated with significant economic costs to the dairy farming industry. Herds are tested for exposure to the causal organism either through serum testing of individual animals or through testing bulk milk samples. Using serum results from a commonly used enzyme-linked immunosorbent assay (ELISA) test for Leptospira interrogans serovar Hardjo (L. hardjo) on samples from 979 animals across 12 Scottish dairy herds and the corresponding bulk milk results, we develop a model that predicts the mean proportion of exposed animals in a herd conditional on the bulk milk test result. The data are analyzed through use of a Bayesian latent variable generalized linear mixed model to provide estimates of the true (but unobserved) level of exposure to the causal organism in each herd in addition to estimates of the accuracy of the serum ELISA. We estimate 95% confidence intervals for the accuracy of the serum ELISA of (0.688, 0.987) and (0.975, 0.998) for test sensitivity and specificity, respectively. Using a percentage positivity cutoff in bulk milk of at most 41% ensures that there is at least a 97.5% probability of less than 5% of the herd being exposed to L. hardjo. Our analyses provide strong statistical evidence in support of the validity of interpreting bulk milk samples as a proxy for individual animal serum testing. The combination of validity and cost-effectiveness of bulk milk testing has the potential to reduce the risk of human exposure to leptospirosis in addition to offering significant economic benefits to the dairy industry.

Show MeSH
Related in: MedlinePlus