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

4.1.1. Annual level
There is a strong relationship between the annual geometric mean of the bulk SCC and the
estimated prevalence of infected ewes, based on a decision rule considering an udder as infected
if at least two individual SCC (iSCC) are higher than 1.2 million cells/ml.
As shown in Figure 8, an annual bulk SCC of 650,000 cells/ml corresponds to a prevalence
of 15 %. A change of 100,000 cells/ml of the annual bulk SCC reflects a 2 to 3% change in
prevalence of infected ewes.
Figure 8. Relationship between the annual geometric mean of bulk SCC and the estimated
prevalence of IMI (from Berthelot et al. 2006)
Estimated prelavence of
infected ewes
60
55
50
45
40
35
30
25
20
15
10
5
0
4.1.2. Punctual level
y = 0.0201x
r2 = 0.7951
0.0 500.0 1000.0 1500.0 2000.0
Annual geometric mean of bulk SCC (x 1000 per ml)
As shown in the Figure 9, the punctual SCC of the bulk milk reflects the repartition of SCC
of the ewes:
- for a tank SCC equal to 900,000 cells/ml, ewes with more than 1,000,000 cells/ml represent
more than 75 % of the bulk cells but only 17 % of the production of milk,
- for a tank SCC equal to 400,000 cells/ml, the SCC contribution is comparable but the milk
contribution of the high SCC ewes is limited to 7% of the dairy production.
Thus it is obvious that control of SCC has to focus on these “infected” ewes which are not so
numerous (have a limited contribution to the milk production of the flock), but have a great
contribution to bulk tank SCC and are an important source of IMI and reservoir of bacterial
infections for healthy ewes.
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