The role of antibiotic dry cow therapy (aDCT)
Antibiotic dry cow therapy (aDCT) has a dual purpose:
- Curing the existing subclinical infection at dry off
- Preventing new infections before calving
The use of dry cow therapy has been a cornerstone of the 5-point plan for mastitis management for more than 30 years (Neave et al., 1969. J Dairy Sci 52:696-707). This plan has resulted in a decrease of contagious mastitis pathogens in many well-managed herds.
With the appropriate DCT, existing infections can be cured in up to 85% of cases and the chance of contracting new infections can be halved.
Selective dry cow therapy
As judicious use of antibiotics is becoming more important in order to preserve the action of antimicrobials in both human and veterinary medicine, selective dry cow therapy is also becoming more common.
Antibiotic use creates a selective pressure on bacterial populations and contributes to the development of antimicrobial resistance. Organizations, such as the WHO, recommend reducing the use of antibiotics, especially of those classes of molecules that are most important for human health. The classes of antibiotics which should be avoided as first option treatment are 3rd and 4th generation cephalosporins, fluoroquinolones and macrolides (ref WHO 2011).
However, selective dry cow therapy must not be performed blindly. A study in 1990 showed that a decrease in DCT led to dramatic consequences, such as a 10 times higher incidence rate of clinical mastitis in quarters dried out without antibiotics (Schukken et al., 1993). In this trial, however, a comparison on the effect of DCT was performed without differentiating between infected and uninfected cows before deciding to treat with DCT or not. Such an informed decision, based on culture or cell count, and the use of internal teat sealants can now offer a certain level of protection to those cows that did not get antibiotics at dry off.
Application of dry cow therapy
The act of administering antibiotics to a potentially sterile quarter is the first potential area of hazard, and the need for cleanliness in the procedure cannot be over-emphasised.
Using a partial insertion technique of the tube cannula:
- minimises physiologic damage to the streak canal
- reduces the risk of reaming out keratin, which subsequently will be needed to allow plug formation.
Bradley and Green 2004. The importance of the nonlactating period in the epidemiology of intramammary infection and strategies for prevention. Vet Clin Food Anim 20:547-568