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Dry Period and Mastitis

This resource describes the dry period of the cow and its relationship to mastitis.

Dry Period and Mastitis

The early dry period ( first week or two) is the time of the highest incidence of new intramammary infection (new IMI). The mid-dry period is the time of lowest incidence of new IMI. The peripartum period is also a time of high incidence of IMI.

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, ß-lactoglobulin, 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:

  • 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:

  • 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:

  • 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:

  • 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:

  • 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:

  • 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.

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