Dr. William B. Epperson, Extension Dairy Veterinarian, The Ohio State University
Mastitis is generally regarded as the most costly infectious disease on dairy farms. For most farms, the main bacteria causing mastitis have shifted to the environmental bacteria (coliforms and environmental streptococci), which are transferred to teat ends at times other than milking.
The dry period is an important time for the establishment of environmental intramammary infection (IMI). Of environmental streptococci IMI present in lactation, 50.5% originated in the non-lactating period (Todhunter et al., 1995). Similarly, 61.2% of clinical coliform mastitis cases observed in lactation were due to organisms that originated with infection in the dry period (Todhunter et al., 1991). Together, these facts emphasize the relative importance of the dry period on mammary gland health.
Within the traditional 8 week dry period, the first 2 weeks and the last 2 weeks have been shown to be high risk times for new IMI (Smith et al., 1985). These times are the transitions into and out of involution, and are accompanied by high volumes of milk/colostrum in the gland. Therefore, preventive strategies that impact either or both of these high risk times are likely to be beneficial.
Numerous field studies have related high environmental bacterial loads to increased mastitis. Providing a clean, comfortable dry cow environment will decrease teat end exposure to pathogens. Inorganic bedding materials support lower bacterial loads, improving hygiene and leading to improved udder health. Cracked skin on teat ends, as commonly seen with hyperkeratotic teat ends (often termed "teat rings", "prolapsed sphincters" or the like) has been shown to increase risk of IMI in the dry period, probably by promoting bacterial colonization (Dingwell et al., 1994).
Teat canal plug formation following dry off is an important event in limiting movement of bacteria into the mammary gland. Due to intramammary pressure immediately after dry off, the teat canal is forced to dilate. Epithelial cells accumulate and fill the canal, causing constriction over 1 to 2 weeks (Dingwell et al., 2003). If these cells are not removed, the canal eventually seals closed. Teat canals that remain open in the dry period are almost twice as likely to acquire a new IMI. A recent study indicated that about 50% of teat canals close in the first week. However, by the 6th week, only 77% were closed, suggesting a considerable fraction of quarters remained open in the dry period.
Factors which impact teat canal closure are not known, but presumable include milk yield, teat shape and anatomy, and trauma. Recent work has indicated that milk production of > 46 lb/day the day prior to dry off was associated with a delay in teat canal closure. A preliminary report from Ohio indicted that with each 11 lb/day increase in milk production above 27.5 lb on the day prior to dry off, IMI at calving increased 77% (Rajala-Schltz et al., 2005). Future udder health programs may include specific recommendations for managed decrease of milk production immediately prior to dry off in an effort to improve teat canal closure and limit dry period IMI.
Teat canal sealing products were devised to lessen the impact of delayed teat canal closure. Administration of the internal teat sealant (Orbeseal®, Pfizer Animal Health) has been shown to decrease new IMI at calving by approximately 30% (Godden et al., 2003). In addition, reductions of clinical mastitis and early lactation somatic cell counts have been observed in some studies. External teat sealers are specially formulated teat dips intended to produce a film barrier on the teat skin. No work with these products has been reported in scientific journals. While the internal teat sealer has been shown effective and is broadly recommended (concurrent with dry cow therapy), external teat sealers have not undergone the same study and scrutiny, and therefore, are not universally recommended at this time.
Optimal nutrition promotes host defense and contributes to the overall well-being of the cow and the mammary gland. The benefit of adequate levels of vitamin E (1000 mg/head/day in diet) and selenium (minimum of 0.3 ppm in diet) in mammary health are well known (Hogan et al., 1993). Dry and transition cows with energy imbalance will often express ketosis shortly after calving. Cows with ketosis within 1 week after calving have an increased risk of clinical mastitis in the period from calving to 63 days in milk (Leslie et al., 2001). Cows with postpartum metritis show a tendency for increased susceptibility to subsequent IMI (Epperson et al., 1993). As each of these conditions (mastitis, ketosis, and metritis) is associated with decreased white blood cell function, the occurrence of one disease identifies those cows at risk of other diseases.
Antibiotic therapy of all quarters of all cows at dry off (total dry cow therapy; DCT) has been a longstanding recommendation for US dairy producers. This is based on the following facts:
1) DCT will help eliminate a high proportion of existing IMI caused by Streptococcus agalactiae, C. bovis, and coagulase negative staphylococci (Dingwell et al., 2003),
2) DCT enhances the cure rate of Staphylococcus aureus (Dingwell et al., 2003),
3) Available evidence indicates that DCT will prevent new environmental streptococcal IMI from establishing in the early dry period (Smith et al., 1985), and
4) There is no substantial evidence to indicate that antimicrobial resistance in mastitis pathogens is emerging in a widespread manner as a consequence of DCT (National Mastitis Council, 2004).
Present day DCT products are active principally in the first weeks of the dry period. Antimicrobial activity declines to negligible levels by the final 2 weeks of the dry period. The cost:benefit of DCT continues to be re-evaluated, and new products/techniques may offer alternatives to DCT. However, until that time, total DCT is recommended.
*A complete list of research references is available on request.