Mastitis in Beef Cows
Several surveys have demonstrated that about 15% to 40% of beef cows have IMI in at least one mammary gland.521,522 Similar to heifers and small ruminants, NAS are the predominant bacteria isolated, but S.
aureus has been recovered from 7% to 10% of milk samples. Milk from infected glands has significantly higher SCCs as compared to milk collected from healthy glands.521,523,524 Clinical mastitis occurs rarely and is diagnosed and treated when economically feasible, but most mild cases of clinical mastitis are likely undetected.Several modes of transmission of mastitis pathogens have been proposed for beef cattle. Flies are believed to serve as vectors for staphylococci, and fly-induced teat scabs of beef heifers have been shown to harbor staphylococci.525 Calves may also serve as vectors by transmitting oropharyngeal organisms to the teats or carrying bacteria-laden milk from teat to teat. Unsanitary housing conditions probably increase the risk of infection with environmental pathogens.521,525 As with dairy cows, poor udder conformation and teat injuries may predispose to mastitis.
Several studies have demonstrated numeric or significant decreases (up to 40 kg) in weaning weights of calves from beef cows with subclinical mastitis,521,522,524 probably as a result of reduced milk production.523 The adverse effect on weaning weight appears to increase with the number of infected glands.521 Use of dry cow antimicrobial therapy has been associated with decreased IMI in the subsequent calving and increased body weight gain of calves.526 However, the economic justification for the routine use of antibiotic treatment at dry-off is lacking. Selective intramammary antibiotic treatment at weaning based on CMT reaction has been suggested, but CMT score is unlikely to be an accurate predictor of infection. Sanitary housing conditions, appropriate stocking density, and good fly control should help to minimize mastitis risk in beef cattle.