high numbers of polymorphonuclear leukocytes which can rise to levels
of 10^7 ml^1 in infected milk.
In addition to acute mastitis, a substantial proportion of the national
dairy herd is subclinically infected. In these cases there may be no
obvious signs of infection yet the causative organism can be present in
the milk at about 10^5 cfu ml^1 and will contribute to an increase in the
overall count of bulked milk.
Many organisms can cause mastitis, the most important being
Staphylococcus aureus,Escherichia coli,Streptococcus agalactiae,Strep.
dysgalactiae,Strep. uberis,Pseudomonas aeruginosaandCorynebacterium
pyogenes. Several of these are potential human pathogens and a number
of other human pathogens such asSalmonella,Listeria monocytogenes,
Mycobacterium bovisandMycobacterium tuberculosisare also occasion-
ally reported.
Infected cows are treated by injection of antibiotics into the udder.
Milk from these cows must be withheld from sale for several days
following treatment because antibiotic residues can cause problems in
sensitive consumers and inhibit starter culture activity in fermented
milks. Attempts to control mastitis by good milking hygiene, use of a
disinfectant teat dip after milking and an antibiotic infusion at the end of
lactation have helped to reduce streptococcal and staphylococcal infec-
tions but have had little success in preventingE. colimastitis.
Theudder exterior and its immediate environmentcan be contaminated
with organisms from the cow’s general environment. This is less of a
problem in summer months when cows are allowed to graze in open
pasture and is worst when they are housed indoors and under wet
conditions. Heavily contaminated teats have been reported to contribute
up to 10^5 cfu ml^1 in the milk. Contamination from bedding and manure
can be a source of human pathogens such asE. coli,Campylobacter, and
SalmonellaandBacillusspecies may be introduced from soil. Clostridia
such asC. butyricumandC. tyrobutyricumcan get into milk from silage
fed to cows and their growth can cause the problem known as late
blowing in some cheeses.
A number of measures can be taken to minimize milk contamination
from the udder exterior and considerable advice on this topic is available
to dairy farmers. Some of the recommendations made by the Milk
Marketing Board, formerly the principal purchaser of milk in England
and Wales, included:
(1) providing enough clean bedding and replacing it as necessary;
(2) removing slurry (faeces and urine) from concrete areas at least
twice daily;
(3) preventing muddy areas wherever possible;
(4) shaving udders and trimming tails;124 Microbiology of Primary Food Commodities