Proceedings of the 10th International Colloquium on Paratuberculosis

#136 Error-adjusted, variance partitioning coefficients to assess <strong>the</strong> association between
clustering <strong>of</strong> MAP infection and calf management risk factors in Danish dairy herds
Polychronis Kostoulas, Søren Saxmose Nielsen, Leonidas Leontides
Laboratory <strong>of</strong> Epidemiology, Biostatistics and Animal Health Economics, University <strong>of</strong> Thessaly, Greece; University
<strong>of</strong> Copenhagen, Denmark
The variance partition coefficient (VPC) measures <strong>the</strong> clustering <strong>of</strong> infection/disease among individuals with
a specific covariate pattern. Failure to adjust for imperfect diagnostic tests downwards biases VPCs. Erroradjusted,
covariate-pattern-specific VPCs provide insight to <strong>the</strong> groups that exhibit great infection heterogeneity
and should be targeted for intervention. We developed a Bayesian discrete mixed model for <strong>the</strong> estimation
<strong>of</strong> error-adjusted, covariate-pattern-specific VPCs and applied it to <strong>the</strong> identification <strong>of</strong> calf management risk
factors associated with MAP infection. Details on practices regarding colostrum and milk feeding were obtained
from 633 herds (64,945 animals). Individual animal records on MAP antibody ELISA status and age were retrieved
from <strong>the</strong> Danish Cattle Database. We initially ran (i) a naïve model that ignored <strong>the</strong> imperfect accuracy
<strong>of</strong> <strong>the</strong> ELISA and subsequently (ii) our model that adjusted for <strong>the</strong> varying with age imperfect Se and Sp. Ignoring
age-dependent Se and Sp greatly biased VPC estimation. Animals fed colostrum from multiple cows were
more likely to be infected compared with those fed only from <strong>the</strong>ir own dam. Calves suckling with foster cows
were more likely to be infected than calves fed milk replacer. Error-adjusted, covariate-pattern-specific VPCs
revealed that herds feeding colostrum to calves only from <strong>the</strong>ir own dam and milk replacer were <strong>of</strong> <strong>the</strong> least
heterogeneous. Herds feeding colostrum from multiple cows and/or allowing suckling from foster cows were
<strong>of</strong> <strong>the</strong> most heterogeneous. Much <strong>of</strong> <strong>the</strong> overall heterogeneity <strong>of</strong> MAP infection among herds remained unexplained
after adjusting for <strong>the</strong> significant calf management risk factors. Thus, additional risk factors exist, which
operate at <strong>the</strong> herd level and contribute to <strong>the</strong> between herd heterogeneity <strong>of</strong> MAP infection. Error adjusted,
covariate-pattern-specific VPCs could be utilized in control programs to optimize sample sizes according to <strong>the</strong>
herd-specific component <strong>of</strong> risk factors.
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