great lakes dairy sheep symposium - the Department of Animal ...

1.3. Use of SCC in order to detect subclinical IMI in sheep
Various thresholds of individual SCC have been proposed to determine the healthy or
infected status of the udder, depending on the approach carried out.
1.3.1. Punctual approach and single threshold
Some authors have proposed a punctual approach, thus a single threshold. The cut-off point
is based on the punctual comparison of SCC of infected and uninfected udder halves. The choice
may take into account the best compromise between sensitivity and specificity. As shown in
Figure 4, the level of somatic cell score (SCS) of the three IMI statuses are partly overlapping. In
the ewe, the most common level proposed is below or over 500,000 cells/ml (SCS =2.7 on the
Figure 4) to distinguish between healthy and infected udders (Bergonier et al. 2003, Berthelot et
al. 2006).
Figure 4. Distribution of the punctual SCS from 346 ewes according to the IMI status : 1 -
healthy, 2 - briefly infected, 3 – long-term infected
%
25
20
15
10
5
0
1,5
1 2 3
1,7
1,9
2,1
2,3
2,5
2,7
2,9
This approach has the advantage to be simple. But it is necessary to modulate its use
depending on the objective: detection of infected sheep to be confirmed by a second analysis,
detection of infected sheep to be treated at drying-off, or to be culled. Moreover, according to the
chosen cut-off point, the power of detection of the truly infected (or uninfected) sheep could
change, which is a drawback of this punctual approach.
1.3.2. Dynamical approach and multiple thresholds
An important reason for the utilisation of such dynamic approaches is that staphylococcal
IMI are characterised by dynamic fluctuations and consequently SCC cyclic variations (Fig. 5.).
41
3,1
3,3
3,5
3,7
4
SCS