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Canadian Bovine Mastitis Research Network

All About Mastitis I Management / Prevention

Bacterial populations I Variation in the teat streak I Resistance mechanisms I Advantages of therapy

Factors Affecting Susceptibility of Dry Cows to Mastitis
Mammary glands are highly susceptible to new infections during the early dry period and near calving. Increased incidence of infection during the dry period results in an elevated number of infected quarters at calving and is responsible for the high level of intramammary infections during lactation in many herds. Without dry cow therapy, approximately 8 to 12% of quarters in herds with average infection levels will become infected during the dry period. Such infections cause inflammation and affect mammary cell differentiation prior to calving, resulting in decreased milk production during lactation.
Several risk factors contribute to the variation in susceptibility to new intramammary infection during the dry period. These factors include:

Bacterial populations on the teat end

The cessation of milking hygiene practices, such as teat dipping, allows bacterial populations on teat skin to increase. Staphylococcus aureus and environmental Streptococci bacterial numbers on teat skin are high immediately after drying-off. Coliform organisms are more prevalent on teat skin late in the dry period and at calving time.

Variations in the teat streak canal

Studies suggest that the teat canal is more easily penetrated by bacteria during the early dry period. Similarly, swelling of the mammary gland, the increasing volume of secretion, and the leaking of colostrum contribute to the high risk of new infection during the prepartum period.
Resistance mechanisms within the mammary gland
Throughout the dry period, there are marked changes in the composition of mammary gland secretions. There is an increase in the concentration of protective factors such as leucocytes, immunoglobulins, and lactoferrin. These changes influence the variation in susceptibility to both environmental and contagious pathogens. When the gland is completely involuted, resistance to new intramammary infections is high.

Conversely, the rate of new infections during the mid dry period is very low. Mammary gland resistance during this time may be attributed to:

  1. Formation of a keratin plug in the teat canal which prevents mastitis pathogens from entering the udder;
  2. Antibacterial factors such as lactoferrin and immunoglobulin are present in the udder and provide protection against invading pathogens.

Susceptibility to infection again increases near calving. This may be due to:

  1. Increased fluid volume and dilation of the teat canal;
  2. Decreased lactoferrin concentration;
  3. Reduced leukocyte numbers and phagocytic ability;
  4. Utilization of milk components for bacterial growth.

Although not effective against all species of bacteria, dry cow treatment with antibiotics is the most effective method of reducing the rate of new intramammary infections during the early dry period. However, antibiotic therapy at drying off is not effective in preventing infections at calving. Therefore, clean, dry environmental conditions are necessary to reduce infections at this time.

Advantages of dry cow therapy

 The most effective time to treat subclinical udder infections is at drying off. Dry cow therapy has the following advantages over lactation therapy:

  • The cure is higher than that achieved by treatment during lactation, particularly for Staphylococcus aureus;
  • A much higher dose of antibiotic can be used safely;
  • Retention time of the antibiotic in the udder is longer;
  • Tissue damaged by mastitis may be regenerated before freshening;
  • The risk of contaminating milk with drug residues is reduced when the milk withholding time after calving is properly observed.
Adapted of: NMC Factsheet “Dry Cow Therapy” (2006), NMC Newsletter "Udder Topics", June-July 2002 et National Mastitis Council Regional Meeting Proceedings (1999) pg. 35. Source : www.nmconline.org


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