Replacement heifers are critical to herd productivity because they represent the future milking and breeding stock in all dairy operations. Unfortunately, most producers regard young heifers as uninfected, and the presence of mastitis is not observed until freshening or until the first clinical flare-up in early lactation. Thus, animals may carry intramammary infections for a year or more before they are diagnosed with mastitis. The greatest development of milk-producing tissue in the udder occurs during the first pregnancy, so it is important to protect the mammary gland from mastitis pathogens to ensure maximum milk production during the first lactation.
How do heifers contract mastitis? No one knows for sure. However, sources may include:
Bacteria that are the normal flora on udder skin, which are in an opportunistic position to colonize the teat end and enter the teat orifice
Bacteria harbored in the oral cavities of calves, which suckle other calves
Bacteria present in the heifers' environment, such as those found in soil, manure, and bedding material
Bacteria present on biting flies that congregate on teat ends
Normal flora would be almost impossible to control, as these microorganisms are naturally found on the udders and teat skin. Daily teat-dipping might reduce bacterial populations, but this practice would be highly impractical. The transfer of mastitis-causing bacteria through cross-suckling of calves fed mastitis milk can be prevented by housing calves in individual hutches, and this management practice has become fairly routine. As with attempts to control normal udder flora, the control of environmental mastitis-causing bacteria has its limitations; however, the percentage of intramammary infections caused by environmental streptococci and coliforms in heifers is low, except just prior to freshening. Flies have certainly been implicated in the spread of mastitis-causing bacteria among heifers and should be a major focus of control.
This information was taken from the article "Mastitis Detection, Prevention, and Control in Dairy Replacement Heifers" (S.E. Nickerson and W.E. Owens).
Source: NMC (formerly the National Mastitis Council)