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Milk quality issues: what
does it take to get somatic cell count down to 100,000 and keep
it ther? |
RISK FACTORS FOR ENVIRONMENTAL
MASTITIS |
It is now accepted that the most common mastitis
problems in herds with low bulk tank somatic cell counts (i.e.
have controlled contagious pathogens) is clinical mastitis due
to environmental pathogens. There are a variety of factors that
determine the new infection rate or rate of clinical mastitis
by these pathogens.
Housing
It is a general observation that housed cattle are at greater
risk of environmental mastitis than cattle on pasture (Hogan and
Smith, 1987). An interesting study by Meaney (1981) compared major
pathogen prevalence at calving in heifers that were housed or
reared on pasture. The quarter infection prevalence at calving
was 2.4-fold higher in housed heifers (18.8%) compared with that
in pastured heifers (8%). Seventy percent of infections or clinical
cases in the pastured heifers and 98% in housed heifers were by
environmental pathogens or no pathogen was isolated.
Situations that increase cow density may contribute to increased
environmental pathogen load, even in pastures. The congregation
of cattle around shaded areas in pastures during hot summer months
resulted in environmental pathogen numbers exceeding 10 million
per g dry matter in the soil (Harmon et al., 1992). These levels
were as high as that observed in soiled sawdust in free stalls.
Bedding
Bedding materials are a significant source of environmental pathogens.
Organic materials such as straw, shavings and sawdust have been
shown to support higher numbers of environmental bacteria than
inorganic materials such as sand or limestone (Hogan and Smith,
1987; Hogan et al., 1989b). In addition, increased ambient temperature
and moisture tend to enhance growth of pathogens.
Coliform counts were highest in summer and fall. Klebsiella spp.
counts were higher in sawdust than in chopped straw, and streptococcal
counts were higher in straw than in sawdust. Hogan et al. (1989b)
reported a significant linear relationship between total rates
of clinical mastitis during lactation and Gram-negative bacterial
and Klebsiella spp. counts in bedding. Thus, there is an apparent
relationship between level of exposure and new infection rate
or clinical incidence of mastitis.
Season
Hogan et al. (1989a) studied the rates of clinical mastitis in
nine well-managed, low somatic cell count herds. Environmental
pathogens and bacteriologically negative samples accounted for
82.3% of clinical cases. Only 4.9% of bacteriologically positive
samples were from contagious pathogens. They found that clinical
cases were highest in summer and fall. This compares with a previous
three-year study showing high frequencies of IMI and clinical
cases in summer months caused by environmental pathogens (Smith
et al., 1985a). |
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