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

2) The persistence of subclinical IMI during lactation may be high. In a survey carried out in
France in 6 flocks (768 udder-halves), the average duration of a pathogen was 3 to 4 months
(Bergonier et al. 2003).
It is clearly established that IMI is the major cause of variation for SCC in dairy sheep:
- with all SCC (or geometric mean) below 250,000 or 500,000 cells/ml an udder may be
considered as healthy,
- with two SCC over 1,000,000 cells/ml, an udder may be considered as infected,
- in the other cases, an udder may be classified as doubtful
Sometime, it would be interesting to take into account the non-pathological variation factors
for the interpretation or the comparison of the SCC. For example, we can observe large
fluctuations of SCC (at the individual level but also at the bulk milk level) during the suckling x
milking period, or some days just after the weaning of the lambs, or at the beginning of a grazing
period after a barn period…
2 – Somatic Cell Counting
2.1. Direct method
The principal methods for somatic cell counting are:
- counter coulter method: the principle is based on the count of the particles in the milk.
The count may be influenced by the size of the particles, the presence of grass
globules…
- fluoro-opto-elecronic method: the principle is based on the count of the DNA nucleus in
the milk after coloration. It is the method currently used now. Due to the high level of
SCC found in dairy sheep, the counter should be calibrated using samples from 0 to
2,000,000 cells/ml.
- the reference method (International IDF standard 148A/1995) is based on a DNA
coloration and microscopic counting.
The influence of sample conservation on the counting results has been well established by
Gonzalo and Martinez (Gonzalo et al. 2003, Martinez et al. 2003):
- for unpreserved milk (temperature of conservation 4°C): the first 2 days of conservation
have little quantitative influence on SCC.
- for preserved milk (temperature of conservation 4°C): the age of the milk has little
quantitative influence on SCC within the first week of conservation.
- frozen milk: on average SCC is lower after freezing compared to refrigeration. But
using a good analytical temperature (40°C), the Fossomatic device can be used for
frozen samples over 60 days of conservation.
2.2. Indirect method
The California Mastitis Test (CMT) is based on the following principle: a reagent destroys
the milk cells and reacts with DNA. The gelling of the milk is correlated with the quantity of
DNA. CMT is an on-farm method, easy to be performed for trained people and not very
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