Proceedings of the 10th International Colloquium on Paratuberculosis

#218 Johne’s disease in <strong>the</strong> New Zealand national dairy herd - an analysis <strong>of</strong> a decade <strong>of</strong>
dairy cow culling records
Hinrich Voges
LIC, New Zealand
The LIC National Dairy Cow Database allows New Zealand (NZ) dairy farmers to record culling (including
deaths) due to Johne’s disease (JD). Analysis <strong>of</strong> <strong>the</strong>se voluntary JD culling records over ten dairy seasons
from 1998/99 to 2007/08 reveal that approximately 8% <strong>of</strong> New Zealand dairy herds cull 0.5% <strong>of</strong> <strong>the</strong>ir cows with
signs <strong>of</strong> Johne’s disease per annum. JD risks in <strong>the</strong> NZ dairy cow population have remained essentially static
over that period with 0.05% reported annual incidence nationally.
In spite <strong>of</strong> typically high levels <strong>of</strong> dairy herd and cattle movements in NZ, <strong>the</strong> data reveals distinct regional
and breed differences in JD culling rates. The Northland region records <strong>the</strong> lowest level <strong>of</strong> Johne’s disease with
2% <strong>of</strong> dairy herds culling less than 0.2% per annum, while <strong>the</strong> West Coast is <strong>the</strong> only region where within-herd
culling rates exceed 1% amongst herds recording JD culls. Jersey breed is associated with significantly higher
risk than Friesian (RR = 4.26; p < 0.001), although it remains unclear whe<strong>the</strong>r this reflects differences in breed
susceptibility.
Forty percent <strong>of</strong> all JD culls and deaths occur over three months in spring – coinciding with calving and
onset <strong>of</strong> lactation. As expected JD risks are strongly correlated with increasing age and peak amongst eightyear-old
cows, which are almost three times as likely to succumb to Johne’s disease as three-year-old cows.
#223 A stochastic and age-structured compartmental model for MAP infection on a US dairy
herd with test-based culling intervention
Zhao Lu, Ynte H Schukken, Rebecca L Smith, Yrjo T Grohn
College <strong>of</strong> Veterinary Medicine, Cornell University, USA
Modeling <strong>of</strong> <strong>the</strong> transmission <strong>of</strong> MAP on dairy herds helps us understanding <strong>the</strong> dynamics and evaluating <strong>the</strong>
effectiveness <strong>of</strong> control options. When herd size is small, stochastic effects such as fadeout become important
and stochastic modeling is necessary. To study <strong>the</strong> phenomena <strong>of</strong> highly variable prevalence and fadeout <strong>of</strong>
MAP infection, a stochastic compartmental model, with age structure including calves, heifers, and cows, was
developed. We <strong>the</strong>n investigated <strong>the</strong> impact <strong>of</strong> various testing and culling strategies on <strong>the</strong> prevalence over
time and <strong>the</strong> cumulative distribution function <strong>of</strong> fadeout. To incorporate <strong>the</strong> model parameter uncertainty and
find <strong>the</strong> most important parameters for <strong>the</strong> probability <strong>of</strong> fadeout, we performed global parameter uncertainty
and sensitivity analyses. Our results show that: (1) <strong>the</strong> threshold property <strong>of</strong> <strong>the</strong> reproduction ratio, R, is generally
not an accurate criterion to determine <strong>the</strong> invasion and persistence <strong>of</strong> MAP infection; (2) evaluation <strong>of</strong> <strong>the</strong>
effectiveness <strong>of</strong> testing and culling strategies based on prevalence over time and <strong>the</strong> cumulative distribution
function <strong>of</strong> fadeout is more appropriate; (3) for a farm <strong>of</strong> good management (low transmission rate), culling <strong>of</strong>
only high shedders based on ELISA tests is effective to eradicate MAP infection within 20 ~ 30 years; however,
for a farm <strong>of</strong> poor management (high transmission rate), culling <strong>of</strong> only high shedders is not enough to eradicate
MAP infection within 50 years, although <strong>the</strong> prevalence still decreases; (4) higher test-based culling rates
for low and high shedders increase <strong>the</strong> probability <strong>of</strong> fadeout which may be important for <strong>the</strong> eradication <strong>of</strong>
MAP infection.
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