Susceptibility to Infection Varies During the Dry Period

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.

The elevated rate of new infections during the early dry period may be due to one or several of the following: 1) flushing of colonized bacteria in the teat canal during milking is terminated; 2) udder sanitation and teat dipping are discontinued; 3) the teat canal becomes dilated and shortened due to milk cessation which allows organisms to enter the udder; 4) phagocytes are involved in removing milk components instead of bacteria; and 5) reduced activity of lymphocytes.

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; and 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; and 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.