Introduction
Dry cow therapy (DCT) is the treatment of cows at the end of lactation with a long acting antibiotic preparation with or without a teat sealant. This is to treat for any intra-mammary infections (IMI) contracted during lactation and provides protection against new infections during the dry period.
Dry cow therapy is a component of the five point plan.
Infection dynamics
During a cows lactation, the probability of infection increases from repeated milking and potential exposure in the parlour and environment. Although many infections will self cure either during lactation or the dry period, some will persist and cause an elevated somatic cell count (SCC) which if not treated may persist into the following lactation. Additionally, using antibiotics at drying off is more successful at eliminating IMI than treatment during lactation since higher concentrations of antibiotic can be used. This is one component of the rationale behind using an antibiotic at drying off.
DCT is also useful to protect against new IMI during the dry period which can lead to clinical or subclinical mastitis during the subsequent lactation. Most cases of clinical mastitis occur in the first several weeks after lactation and it has been shown that over 50% of these infections originate from the dry period (Bradley and Green, 2000). On cessation of milking at the end of lactation, the teat canal closes and forms a keratin seal. This process may take up to two weeks to complete and therefore presents a risk period for new IMIs to develop. The other main risk period is in the last couple of weeks before calving when the integrity of the teat canal seal is compromised as the udder prepares for the subsequent lactation. Providing protection against new IMI is the rationale behind using a long acting antibiotic preparation and the use of a teat sealant. However, the chances of contracting infection during these periods are increased by poor environmental management.
DCT is also thought to be protective against summer mastitis.
Internal teat sealants
A problem may arise when using just long acting antibiotics for DCT as they may not provide sufficient protection alone for the whole dry period. This is because during the second risk period prior to calving, antibiotic levels may have reached low levels insufficient to confer protection against new IMI. This is the rationale behind using an internal teat sealant which prevents new IMI for a prolonged period of time.
Currently available internal teat sealants use an inert bismuth based paste, which is infused into the teat canal. This may be performed alone or after infusion of an antibiotic. If used alone, it is important to ensure that there is no pre-existing subclinical infection. This may be through the use of a California Mastitis Test (CMT) although due to the poor sensitivity, individual SCC records are a better indicator. However, problems arise when using milk recordings because these are composite samples (i.e. all four quarters mixed together) so that a single infected quarter may be diluted by the other three quarters and therefore the cow may appear uninfected. It is therefore advisable to use individual SCC records from the whole lactation in a combination with a CMT if deciding whether to use an internal sealant alone or in combination with a long acting antibiotic. Alternatively individual quarter SCC can be measured although the cost and inconvenience may be prohibitive.
A common problem with using teat sealants is their misapplication by farmers who introduce the paste in a similar way to a normal dry cow antibiotic. It is important that the paste resides within the teat canal only in order to be effective. This is effectively performed by pinching the top of the teat between two fingers of one hand while infusing with the other. Failure to do so will mean using the product will be ineffective in preventing new IMI during the dry period.
External teat sealants
External teat sealants are in the form of a barrier film that is applied to the teat skin as a dip. Its duration of action is thought to be approximately 7 days. However, it is considered to be a less effective preventative of new infections during the dry period than the internal teat sealants and therefore is less widely used. The efficacy can be increased by regular application during the dry period although this is likely to be too inconvenient for the farmer to consider.
Administration
It is of extreme importance that DCT is administered aseptically. This is to prevent the infusion of pathogens into the udder that may cause mastitis either during the dry period or after calving.
It is not recommended that drying off occurs at the regular milking time and rather as a separate group after milking. This is so the operator can concentrate on this particular task and therefore optimise the aseptic technique as well as decrease the likelihood of accidental application to the wrong cow. Before infusion, the cow should be milked out and each quarter thoroughly cleaned and disinfected. The cleaning process is aided by the use of a pre-milking teat dip. For disinfection, often alcohol swabs are supplied with the purchased dry cow product. The furthest quarters away should be prepared first, followed by the near quarters. This is to prevent contamination of the prepared near teats by the operator leaning to reach the teats further away. For the same reason the near teats should be infused prior to those further away. A partial insertion technique is recommended to try to minimise damage to the teat canal. Infusion may be followed by massaging the udder to aid dispersal of the antibiotic. Post-milking teat disinfection can then be applied. Throughout the process, the operator should be wearing clean disposable gloves. The cow should then be moved to a clean penned area for a minimum of 30 minutes to prevent them lying down and allow closure of the teat canal. The cows should then be moved to a specific dry cow group. If dry cows remain with the milking herd, there is a risk of the cow being milked which may lead to antibiotic residue failures of the bulk tank.
Withdrawal periods
Due to the long acting nature of the antibiotics used, there is a prolonged milk withdrawal period associated with their use. It is important that if the cow calves early that the instructions are appropriately followed for each antibiotic product. Failure to do so may lead to an antibiotic residue failure. Milk can be tested using an inhibitory substance test.
There is no milk withdrawal period when an internal sealant is used alone. However it is important that the paste is thoroughly stripped out before the cow’s milk is allowed to enter the bulk milk tank. The presence of bismuth salts in milk may be associated with a condition known as “Black Spot”, whereby black spots appear on Cheddar cheese (Lay, et al., 2007). Although is not associated with a known public health risk, its appearance makes it unsellable by cheese producers.
Selective DCT
Some herds may wish to use antibiotics on selected cows based upon their mastitis history from the previous lactation. However, due to the uncertainty of freedom of subclinical infection based upon four quarter composite SCC results, this approach is rarely recommended. The same principle applies for herds that wish to only use teat sealants which have no antibacterial action. However, with the prudent use of antibiotics becoming more of an issue in the veterinary profession particularly in food producing animals, a selective approach may become more necessary with changes in legislation.
In organic herds, dry cow antibiotics may not be used. In these herds, excellent parlour and environmental management is required to maintain mastitis levels at an acceptable level.
Chronic high SCC cows
In cows with a history of a high SCC during the previous lactation, there is an opportunity at drying off to use a combination of antibiotic treatments to try to maximise the probability of bacterial cure. This may involve using an intramammary preparation in combination with parenteral antibiotics. The choice of antibiotics used should be based on milk cultures or, alternatively, farm pathogens known to be present.
Heifers
Occasionally, farms have problems with mastitis (clinical or subclinical) after heifers have calved. This is likely to be due to their environmental management pre calving. Some farms have had success in using internal teat sealants on heifers to prevent this occurring. Alternatively a barrier teat dip may be applied on a regular (perhaps weekly) basis prior to calving. The latter approach may also allow regular introduction to the milking parlour which will ease the heifer’s transition to the milking herd.