Proceedings of the 10th International Colloquium on Paratuberculosis

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Milk quality assurance for paratuberculosis: effects <strong>of</strong> infectious young stock.
Maarten F Weber 1 , Huybert Groenendaal 2
1 GD Animal Health Service, Deventer, The Ne<strong>the</strong>rlands
2 Vose Consulting US LLC, Boulder, CO, USA
INTRODUCTION
A bulk milk quality assurance program (BMQAP) for paratuberculosis in Dutch dairy herds
was initiated in 2006, aiming at a reduction <strong>of</strong> <strong>the</strong> concentration <strong>of</strong> Mycobacterium avium
subsp. paratuberculosis (Map) in milk delivered to <strong>the</strong> milk factories (Weber and van Schaik,
2007). The development <strong>of</strong> <strong>the</strong> program was supported by various modeling studies (van
Roermund et al., 2005; Weber et al., 2005; Franken, 2005; Velthuis et al., 2006), in which it
was assumed that cattle do not become infectious before adulthood. However, shedding <strong>of</strong>
Map in young stock has been described in several studies (Rankin, 1959; Rankin, 1961;
McDonald et al., 1999; Waters et al., 2003; Weber et al., 2005; van Roermund et al., 2007),
and transmission <strong>of</strong> Map amongst young stock was found in an experimental study (van
Roermund et al., 2007). Therefore, we evaluated <strong>the</strong> effect <strong>of</strong> transmission amongst young
stock on key output parameters <strong>of</strong> <strong>the</strong> BMQAP in simulated dairy herds.
MATERIALS AND METHODS
Closed dairy herds participating in a BMQAP were simulated with a stochastic simulation
model, JohneSSim. Transmission <strong>of</strong> Map amongst young stock was simulated, assuming
various probability distributions for <strong>the</strong> age at onset <strong>of</strong> infectiousness and various contact
rates between individuals.
RESULTS
Transmission amongst young stock resulted in an increased average within-herd prevalence,
and a decrease <strong>of</strong> <strong>the</strong> beneficial effect <strong>of</strong> preventive management measures directed at a
separation <strong>of</strong> adults and young stock on herd-level and within-herd prevalence’s. When
including transmission amongst young stock, infected herds were detected earlier within <strong>the</strong>
simulated BMQAP. Consequently, <strong>the</strong> proportion <strong>of</strong> certified herds (‘green’ herds, i.e. status
A in <strong>the</strong> Dutch BMQAP) decreased. Moreover, <strong>the</strong> beneficial effect <strong>of</strong> preventive measures
on <strong>the</strong> proportion <strong>of</strong> certified herds was reduced. However, <strong>the</strong> effect <strong>of</strong> transmission <strong>of</strong> Map
amongst young stock on <strong>the</strong> concentration <strong>of</strong> Map in milk from certified herds was small and
became negligible beyond year 10 after <strong>the</strong> start <strong>of</strong> <strong>the</strong> program.
CONCLUSION
It is concluded that transmission <strong>of</strong> Map amongst young stock does not affect <strong>the</strong> quality
assurance <strong>of</strong> milk from certified herds. However, <strong>the</strong> proportion <strong>of</strong> herds that are certified in a
BMQAP is decreased by transmission <strong>of</strong> Map amongst young stock.
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