J. S. Hogan and K. L. Smith, Department of Animal Sciences, The Ohio State University
Environmental streptococci have emerged as pathogens that uniquely affect mammary health as a primary cause of both subclinical and clinical mastitis. The bovine mammary gland appears to be most susceptible to environmental streptococcal intramammary infections during the dry period and early lactation. The importance of the dry period in control of environmental streptococcal mastitis can not be over emphasized. Rate of new IMI during the dry period is 5.5 fold greater than the rate during lactation in a total confinement herds practicing total dry cow therapy. The rate of new infection is not constant across the dry period, but is elevated during the 2 weeks following drying off and the 2 weeks prior to calving. Dry cow therapy reduces the rate of new environmental streptococcal infections during the early dry period. The high rates of new infection following drying off may relate to the lack of flushing action due to milking, changes occurring in the composition of the mammary secretion that appear to enhance streptococcal growth , and/or the lack of a keratin plug in the streak canal. The increase in susceptibility to infection in the two weeks prior to parturition may reflect the absence of milking when the gland is accumulating fluid, loss of keratin plugs from streak canals, or immunosuppression associated with the pariparturient period. Conventional dry cow therapy has no effect on rates of infection prior to calving and prepartum teat dipping was reported to be of little or no value. The environmental streptococci are a frequent cause of mastitis in heifers at calving and heifers generally suffer as many infections at calving as do older cows.
During lactation, the incidence of clinical mastitis is greatest the first week after calving and decreased throughout the first 305 d in milk. Interestingly, rate of environmental streptococcal clinical cases increases in cows with extended lactations (>305 d) to that comparable of cows in peak lactation. Therefore, the use of management practices that encourage the use of extended calving intervals, thus a larger percentage of cows with > 305 d in milk, may impact the prevalence of environmental streptococcal mastitis in a herd.