Dairy Sheep Symposium - the Department of Animal Sciences ...

Somatic Cell Count
Mastitis is one of the most economically important health problems in sheep and thus methods
allowing for an early diagnosis of the process or sub-clinical mammary infections show high
relevance. Among these methods we can cite the milk somatic cell count (SCC) proposed by Gonzalo
et al. (1993) that due to its effectiveness and ease of detection is currently used together with milk
composition analyses.
Somatic cell count is hardly influenced by hygienic and management practices. However, the
very few studies performed in sheep suggest an inherited component in the SCC parameter,
which would permit a prediction of response to selection. SCC has a dual significance. In healthy
animals SCC may be considered a bromatological milk variable, but SCC from infected udders
may be used as an indicator of mastitis.
A few studies have focused on this duality from a genetic point of view through the estimation
of genetic parameters separately for healthy and infected animals, as well as for the whole
population. An investigation was performed on several breeds in order to define a threshold
discriminating healthy from infected ewes, which was established in 250,000-300,000 cells/ml
(González et al., 1995). Heritability estimations were obtained for SCC under different considerations
in a study with 10 flocks from the selection nucleus of Churra sheep (El-Saied et al. 1998;
1999) and the values obtained were 0.09 for test-day measures and 0.12 for lactation measures
(Table 8). These values are in the range found for SCC in cattle (0.09-0.13) (Banos and Shook,
1990; Schutz et al., 1990; Da et al.1992, Zhang et al., 1994). Heritability was also obtained for
Churra ewes with a SCC value under the threshold indicating a healthy status (250,000 cells/ml)
which gave an estimate of 0.03 that did not significantly differ from zero. All these results
suggest that the genetic component of SCC in healthy animals is negligible whereas this is not
the case when the estimation is performed on the whole population. Then SCC as an indicator of
mastitis may be considered an adequate criterion for selection although with relatively low
expected responses.
Table 8. Heritabilities, repeatabilities and their approximate standard errors for SCC
Variable h 2
Log SCC (whole population)
SE r SE
Lactation measures 0.12 0.03 0.35 0.03
Test day measures 0.09 0.02 0.38 0.01
Log SCC (