Proceedings of the 10th International Colloquium on Paratuberculosis

persistence in an infected herd. The latter findings are consistent with <strong>the</strong> well established
need for control strategies that will, among o<strong>the</strong>r things, prioritize <strong>the</strong> interruption <strong>of</strong> MAP
transmission during early calfhood.
Importantly, we captured <strong>the</strong> inverse relationship between <strong>the</strong> incidence rate <strong>of</strong>
seroconversion and infection with time: <strong>the</strong> incidence rate <strong>of</strong> milk seroconversion increases,
while that <strong>of</strong> infection decreases with age (Figure 1). Young calves are more susceptible to
MAP infection and susceptibility to infection decreases with aging (Taylor, 1953), while highparity
dairy cattle are more likely to become seropositive than low-parity dairy cattle (Nielsen
et al., 2002).
Our model heavily depends on correct prior specification for ui. We, <strong>the</strong>refore, had to
base our priors on adequate, relevant and scientifically justified information. Within an
infected herd most animals acquire MAP early in <strong>the</strong>ir life. In natural infections, <strong>the</strong>
immunological and infectious properties <strong>of</strong> MAP infection are not fully characterized.
Susceptibility to infection decreases over time, while environmental (Tiwari et al., 2009) and
genetic (Koets et al., 2000) factors, not fully conceptualized yet, play a critical role on
whe<strong>the</strong>r initial entrance and persistence <strong>of</strong> MAP will lead to clinical manifestations, be
restrained during <strong>the</strong> productive life <strong>of</strong> infected animals or even be cleared out. Most clinical
cases occur between 3 to 5 years (Chiodini et al., 1984). In experimental infections, <strong>the</strong>
latent period is generally shorter (Lepper et al., 1989) but can only be indicative for natural
infections because <strong>of</strong> <strong>the</strong> variation in infective doses occurring during natural infections
(Nielsen and Ersbøll, 2006). Specified priors captured both our prior belief and <strong>the</strong> variation
<strong>of</strong> latent period occurring at natural infections.
The proposed model adjusts for <strong>the</strong> latent period <strong>of</strong> infection and can be quite useful
for <strong>the</strong> analysis <strong>of</strong> chronic infections – like MAP – with a long latent period, which, if ignored,
will have a severe impact on estimations.
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