Factors Other Than Infection Status Have Little
Impact on SCC
From the NMC Newsletter "Udder Topics", August 1994
The major factor affecting somatic cell count (SCC) in milk is an intramammary
infection. Other factors are often implicated in observed increases in
SCC, however few have a significant impact. The following article reviews
some of the other factors which may influence SCC and clarifies some misconceptions
regarding changes in SCC.
Age and Stage of Lactation: Generally SCC increases with advancing
age and stage of lactation, however little change is observed in uninfected
quarters. The major influence of parity and stage of lactation on SCC is
related to intramammary infection status.
Stress: Various types of stress have been implicated as causing
increases in SCC. However, attempts to induce SCC changes experimentally
for uninfected cows have shown only modest or no effects on SCC. Although
SCC of milk from heat-stressed cows has been shown to increase, some of
this increase may be due to decreased production that is observed with
heat stress. While stray voltage may cause behavioral changes in cows,
no evidence exists that stray voltage directly influences SCC in healthy
udders. It has also been reported that estrus does not effect SCC.
Season: Somatic cell counts are generally lowest during the winter
and highest during the summer. The high temperature and humidity per se
does not cause increased SCC; rather, the increase is due to greater exposure
of teat ends to pathogens, resulting in more new infections and clinical
cases during the summer months.
Other factors: A normal variation in SCC occurs with the fraction
of milk collected throughout a milking, and diurnal variation occurs during
the time between milkings. In general, SCC is highest in the strippings
and lowest immediately before milking. The naturally elevated SCC may persist
for up to four hours after milking and then gradually decline. The difference
in high and low SCC varies from four- to 70-fold for individual quarters.
Because marked increases in SCC are a result of cells being attracted to
the mammary tissue in response to the mediators produced during a local
infection, events that do not affect udder health are unlikely to have
a direct or dramatic effect on SCC. Little evidence exists that any factor
other than normal diurnal variation has a major influence on SCC in the
absence of intramammary infection.
Source: Journal of Dairy Science, Vol. 77, No. 7, p. 2103 (1994)
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