Antibiotic therapy continues to play an important role in the control of mastitis in dairy cows. Lactational therapy is effective against Streptococcus agalactiae, but less successful against infections caused by Staphylococcus aureus and other causes of mastitis. As a result, alternative treatment strategies have been developed, including a combination of both intramammary infusion and the parenteral administration (injection) of antibiotics to successfully cure quarters infected with S. aureus.
Likewise, extended therapy, which involves prolonged drug administration, has improved cure rates against this organism. Nonantibiotic approaches to treatment have included oxytocin injections, but relapse rates after this form of therapy can be unacceptably high. Dry or nonlactating cow therapy is almost always more successful than lactational therapy because cure rates are higher and new cases of mastitis are prevented. To reduce antibiotic usage, selective dry cow therapy is becoming popular, and teat seals are appealing because they prevent new infections without having to rely on antibiotics.
When all quarters of all cows are not treated at drying off, 8-12% of quarters will likely develop a new infection during the dry period, which will be diagnosed at calving. The prevention of only 1% of quarters from becoming infected will pay for the entire dry cow treatment program. Indeed, in herds with a low level of mastitis, preventing new infections during the dry period is more important than curing existing infections. This is because an infected quarter treated at drying off and cured at calving will produce 90% of its potential milk production during the next lactation. However, a quarter that becomes infected during the dry period or remains infected from the previous lactation will produce 30-40% less milk.
Treatment of all quarters of all cows at drying off (blanket dry cow therapy) is one of the most important components of a comprehensive plan of mastitis control. This is because dry cow therapy both cures existing infections, caused mainly by contagious pathogens, and prevents the development of new infections, caused mainly by environmental pathogens. Other advantages of dry cow therapy include the following:
- The cure rate is higher than that experienced during lactation using slow-release products that maintain effective levels of antibiotics for long periods in the nonlactating udder.
- Incidence of new infections during the dry period is reduced.
- Damaged milk-producing tissue is allowed to redevelop before freshening.
- Clinical mastitis at freshening is reduced.
- Salable milk is not contaminated with drug residues.
- All potentially infected quarters are treated with antibiotics.
- Laboratory or screening tests are not required.
A recent summary of 12 trials evaluating internal teat sealants and dry cow therapy showed that the use of both products in combination or a teat sealant alone was equally effective in reducing the number of new infections as well as clinical mastitis cases at calving. However, the addition of the dry cow therapy lowered SCC at calving, suggesting that the antibiotic component was effective in curing existing infections at the time of drying off, lowering the SCC.
Headline image courtesy of University of Georgia