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Mammary Physiology During the Dry Period
Physiology of the mammary gland during the dry period differs markedly from
that during lactation. For the sake of convenience, the dry period can be divided
into three phases :
Active Involution
Steady State Involution
Redevelopment and Colostrogenesis
Active involution
Active involution begins with the cessation of periodic milk removal, either by
drying off a cow or by weaning the young. Halting milk removal results in milk
stasis. In the cow active involution is probably complete by 21 to 30 days after
dry off. It is a transition phase of the mammary gland from the lactating to the
nonlactating state. Changes in the mammary secretion volume and composition
occur during active involution, including:
Milk continues to accumulate for a couple of days after drying off. Volume of
secretion in the gland increases for 2 to 4 days after drying-off, then declines
rapidly over the next week. Fluid volume continues to decrease through ~30 days
(in a 45-60 day dry period).
Concentrations of milk-specific components (caseins, a-lactalbumin, ßlactoglobulin, and milk fat) decline slowly during the first 2-3 wk of the dry
period, but never completely disappear. Lactose concentrations decline rapidly
(Aslam et al. 1994; Hurley and Rejman 1986; Hurley 1987; Hurley et al. 1987).
Proteins of serum origin (immunoglobulins, serum albumin) increase in
concentration during the first week of involution. All classes of immunoglobulins
(Ig) increase, including IgG1, IgG2, IgA, IgM. There is a transitory increase in the
selective transport of IgG1 from about days 2 to 4 of involution, but not as much
as during colostrum formation. Increased serum albumin (SA) concentrations
reflect increased permeability (tight junctions between epithelial cells are
loosened for a period of several days), but this does not approach serum levels or
levels found in the milk during acute inflammation, therefore, the permeability
barriers are not completely destroyed.
NAGase (N-acetyl-ß-D-glucosaminidase);activity increases substantially during
mammary involution. NAGase is a lysosomal enzyme that is secreted in large
quantities in the mammary gland during involution and inflammation. The specific
function of NAGase in the gland is not known, but its activity in mammary
secretions sometimes is used as an indicator of tissue changes that accompany
involution and inflammation.
Lactoferrin (LF) concentrations increase markedly during active involution
(Rejman et al. 1989). LF is an iron-binding protein, thought to compete with
bacteria for iron (see Sanchez et al. 1992). This is the basis for lactoferrin's
bacteriostatic action, although recent evidence suggests that other properties of
LF may account for its bacteriostatic action. Invading bacteria are forced to
compete with LF for iron. For coliform bacteria it is the citrate:LF ratio that is
important. Citrate chelates Fe and bacteria can use the citrate-iron complex.
During early involution citrate concentration declines, while LF concentration
increases. LF may play a role in mammary gland by affecting phagocyte function.
LF also may limit the oxidative degeneration of cellular components that can
occur during periods of tissue disruption such as during inflammation and
involution.
Cells in secretions of the involuting mammary gland:
Very few cells (less than 2%) in mammary secretions during involution are
epithelial cells. Most cells in the secretion are leukocytes. Total leukocyte
concentrations in mammary secretions increase rapidly in early involution.
Types of leukocytes found in mammary secretions during the dry period:
PMN (polymorphonuclear neutrophils). These are phagocytic leukocytes. They
predominate for the first 3-7 days (if the quarter is also infected, then they will
predominate throughout the early stages of infection).
Macrophages are the predominant cell type after ~7 days. These are also
phagocytes. Many are filled with ingested fat droplets and other debris. They play
a major role in removing large quantities of fat and cellular debris, including dead
PMN. Macrophages are also the predominant cell type in the colostrum at
parturition.
Lymphocytes are always present. They increase in proportion roughly in parallel
with the macrophages, but may only become the predominant cell type during
the mid-dry period. Specific function of lymphocytes in the involuting mammary
gland is unknown.
New Intramammary Infections During Active Involution
A number of factors contribute to the elevated susceptibility to infection during
the early dry period:
Streak Canal and Udder Fluid:
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milk is no longer periodically removed from the gland
milk is an excellent growth medium for bacteria
accumulation of a large volume of milk in the gland (for the first few days)
leakage from the teats
teat-end disinfection is stopped
Phagocytic Leukocytes (Somatic Cells) and Other Defenses:
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leukocytes begin entering the gland by day 1 after dry-off
leukocytes are occupied with ingesting milk fat, casein and debris
milk fat and casein may decrease the phagocytic function of the
leukocytes
citrate:lactoferrin ratio is very high (Lf concentration is increasing, but still
very low)
immunoglobulins (antibodies) are increasing, but still low
Intramammary dry-cow antibiotic therapy, given immediately after the last
milking, is very effective in controlling mastitis in early involution (except for
coliform caused by IMI). Dry-cow antibiotic therapy is the most effective
preventative treatment to help the udder get through the early stages of
involution.
Steady State Involution
(mid-dry period)
The length of the steady state period depends on the total length of the dry
period. If active involution takes about 4 weeks to complete in the dairy cow and
the redevelopment stage takes about 3 or 4 weeks, that accounts for the
recommended optimal 45-60 day dry period. So, cows with a 45-60 day dry
period probably have a very short steady state phase or no steady state phase of
involution at all. When the dry period is less than 40 days, the tissue is
undergoing active involution and beginning the redevelopment phase
concurrently. This may contribute to the decline in optimal milk yield in the next
lactation. However, other factors (metabolic and management factors) also
contribute to the requirement for the 45-60 day dry period.
New IMI are generally low during the steady state phase period. This is the period
of greatest resistance to intramammary infection. If an infection occurs, it usually
is spontaneously eliminated.
Streak Canal and Udder Fluid:
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teats have become sealed, no leakage
small fluid volume in the gland
composition of the fluid is less conducive to bacterial growth
Phagocytic Leukocytes (Somatic Cells) and Other Defenses:
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high concentrations of leukocytes
little milk fat, casein or debris left, leukocytes are more effective
citrate:lactoferrin ratio is lowered (Lf concentration may be very high)
immunoglobulin concentrations are elevated
Redevelopment and Colostrogenesis
Prepartum Period : Colostrogenesis and Lactogenesis
This phase of the dry period marks the transition from the nonlactating state to
the lactating state. We do not know exactly when this period begins, but it is
probably beginning at about 3 to 4 weeks prepartum. Recent work indicates that
in cows given a 60 day dry period, an increase in mammary DNA synthesis begins
occurring about 35 days prepartum (Capuco et al. 1997). This might indicate that
the early stages of redevelopment are beginning even 5 weeks prepartum.
The selective transport of IgG1 is a major activity of the epithelial cells in teh 2
weeks prior to parturition (see theimmunoglobulin transport section in the Mother
& NeonateLesson). Concentrations of the major milk components increase
beginning ~2 wk prepartum, and then increase markedly ~3-5 days prepartum.
See Lactogenesis in the Mother & Neonate Lesson for more on the two stages of
lactogenesis.
The potential for IMI is increased again because the gland is undergoing the
opposite changes from those during early involution.
Streak Canal and Udder Fluid:
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fluid accumulates in the udder as calving approaches
leakage from the teats begins as fluid accumulates
teat-end disinfection is not begun until after calving
periodic removal of milk is not begun until after calving
Phagocytic Leukocytes (Somatic Cells) and Other Defenses:
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relatively few leukocytes are in the secretion
leukocytes that are present are confronted with increasing milk fat and
casein concentrations
citrate : lactoferrin ratio is high again, lactoferrin concentrations have
already declined
immunoglobulins (antibodies) concentrations are increasing even more as
colostrum is being formed
Citrate increases and LF concentrations are relatively low, leukocyte numbers are
fairly low, phagocytic capacity of phagocytes is reduced again because of
increased numbers of fat droplets so, the incidence of new IMI is again high.
(Dry-cow therapy does not last this long.
Summary of Changes in Composition of Mammary Secretions During the
Dry Period
Milk component
Active
Involution
Steady State
Involution
Redevelopment and
Colostrogenesis
Lactose
decreasing
low
increasing (late)
Milk Proteins
decreasing
low
increasing
Milk Fat
decreasing
low
increasing
Udder fluid volume
decreasing
low
increasing
Milk components
decreasing
low
increasing
Leukocytes
increasing
high
low
Lactoferrin
increasing
high
low
Immunoglobulins
increasing
high
increasing
Concentrations of: