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Selective dry cow therapy – We can do this!

S. Godden, E. Royster, K. Patel and J. Timmerman

What is selective dry cow therapy and why should we consider adopting it?

With increased public health concerns regarding the potential impacts of antimicrobial use in food animals, and because the majority of antimicrobials used on dairy farms are for the prevention or treatment of mastitis, we must continually evaluate how we use antimicrobials and, where possible, develop and adopt new mastitis control practices that can reduce antimicrobial use while still maintaining or improving udder health. Blanket dry cow therapy (BDCT), the practice of infusing all quarters of all cows with a long acting antimicrobial at the time of dry off, has been a standard practice for decades, curing intramammary infections (IMI) present at dry off and preventing new IMI caused by susceptible organisms in the early dry period. Historically, BDCT has played an important role in reducing the prevalence of contagious mastitis pathogens and consequently in lowering bulk tank somatic cell counts (BT SCC). However, given steadily improving udder health in U.S. dairy herds, BDCT may no longer be a necessary practice for many well managed herds.

Selective dry cow therapy (SDCT) is an approach whereby antimicrobial treatment at the end of lactation is based on assessment of the infection status of the cow or quarter. If applied at the cow-level, then only those cows thought to be infected in one or more quarters would receive intramammary antibiotic therapy in all 4 quarters at dry off (Figure 1). If applied at the quarter-level, then only those individual quarters thought to be infected would receive intramammary antimicrobial therapy at dry off (Figure 2). If successfully applied, SDCT programs can significantly reduce antimicrobial use at the time of dry off while maintaining the same udder health and future production potential for the cow as for BDCT programs. SDCT programs targeting quarter level treatment decisions will have a greater impact on reducing antimicrobial use than programs targeting cow level treatment decisions. Some Nordic countries have already banned BDCT, leaving producers to use SDCT.

Why did early attempts at Selective Dry Cow Therapy programs fail?

While early studies demonstrated that SDCT was better than no treatment at all (negative control), these early studies also reported that SDCT programs were not as effective as BDCT programs, in that measures of udder health were worse in the subsequent lactation in cows treated using a SDCT program as compared to a BDCT program. The first factor limiting the success of early SDCT studies may be the use of imperfect on-farm screening tests, such as SCC, CMT and/or clinical mastitis history, to identify infected cows or quarters requiring antimicrobial treatment at dry off. Indirect tests such as SCC or CMT indicate the presence of inflammation, but do not always indicate the actual infection status of a cow or quarter. Studies have reported that the sensitivity of the CMT test to identify infected cows or infected quarters is less than 70% and 50%, respectively (Middleton et al., 2004; Sanford et al., 2006). Similarly, the sensitivity of using SCC test results (cutpoint > 200,000 cells/mL) to identify infected cows or infected quarters has been reported to be approximately 69.7% and 62.4%, respectively (Torres et al., 2008; Pantoja et al., 2009). The concern with using tests with imperfect sensitivity is that many truly infected cows or quarters will go undiagnosed and untreated at dry off. A second factor contributing to the failure of early SDCT programs may be that there was no mechanism in place to protect against new IMI during the dry period in cows or quarters not infused with an antibiotic at dry off.

Why is our industry now in a position to succeed with selective dry cow therapy?

We now have technologies available to address both former limitations (discussed above). First, more sensitive diagnostic tests such as on-farm culture (OFC) can now be used to find a higher proportion of infected cows or quarters that require antimicrobial treatment. For example, the sensitivity of the Petrifilm rapid culture system (3M Canada or 3M United States) at cow level was reported to be 85.2% (Cameron et al., 2013). In a recent multi-reader study that included 3 untrained (lay) readers and 2 experienced readers, the sensitivity of the MN Easy Culture System SDCT OFC plate (University of Minnesota, St. Paul, MN) averaged 92.4% (range 91.6% to 93.4%) at quarter level (Royster et al. 2016). Using direct tests with higher diagnostic sensitivity will result in fewer truly infected cows or quarters going undetected and untreated, thereby improving the udder health outcomes for a SDCT program. A second important component to a successful SDCT program will be the use of a teat sealant (TS), such as Orbeseal (Zoetis) in all 4 quarters of all cows. The TS will be particularly important to prevent new IMI during the dry period in non-infected quarters or cows that are not infused with an antimicrobial at dry off.

Two recent controlled SDCT studies, both of which incorporated OFC and TS into the program, have reported a significant reduction in antimicrobial use while successfully maintaining the same udder health as for the BDCT group. The first of these studies applied SDCT at the cow level and used on-farm culture (Petrifilm system, 3M) of a composite milk sample to identify infected cows in the SDCT group. Infected cows were treated with both a long acting antibiotic and internal teat sealant in all quarters, while noninfected cows were infused only with the teat sealant in all quarters (Figure 1). Cultures performed at dry off and after calving revealed that cows enrolled into the SDCT and BDCT programs were not different as relates to bacteriological cures, new IMI, prevalence of IMI at calving, or clinical mastitis risk in the first 120 DIM (Cameron et al., 2014). However, antimicrobial use was reduced by 21% in the SDCT program. A second more recent pilot study, conducted at the University of Minnesota St. Paul campus dairy, applied SDCT at the quarter level and used OFC (MN Easy Culture System SDCT system) of quarter milk samples to identify infected quarters in the SDCT group. Infected quarters were infused with both antibiotic and internal teat sealant while noninfected quarters were infused only with the teat sealant (Figure 2). Cultures of all quarters at dry off and after calving revealed that quarters enrolled into the SDCT and BDCT programs were statistically and numerically not different as relates to bacteriological cures, new IMI and prevalence of IMI at calving (Patel et al., 2016). However, antimicrobial use was reduced by 48% in the SDCT program. These studies demonstrate that SDCT can maintain udder health while reducing antimicrobial use.

Figure 1. Example of a Cow-Level selective dry cow therapy Program
Figure 1
Figure 2. Example of a Quarter-Level selective dry cow therapy Program
Figure 2

Selective dry cow therapy (SDCT) is an approach whereby antimicrobial treatment at the end of lactation is based on assessment of the infection status of the cow or quarter.

What are the logistics of adopting a selective dry cow therapy program?

As compared to blanket dry cow therapy (BDCT), a SDCT program will require more time and commitment on the part of management and staff to correctly implement and achieve full value from the program. The most important decision to be made will be what screening or diagnostic testing program will be used to identify infected cows or quarters needing antimicrobial treatment.

If using an on-farm test, such as on-farm culture (OFC), to screen cows to make strategic treatment decisions, it will be necessary to sample the cow 1-2 days in advance of the intended day of dry off in order to conduct the necessary testing. Farms using OFC would need to have a designated clean lab area, the necessary equipment and supplies, and trained personnel to correctly perform the necessary testing and interpretation of results. (Note: Farms already using OFC to guide clinical mastitis treatment decisions will already have these resources set up.) The herd veterinarian can play an important role in training and ongoing oversight of the on-farm lab. On the day of dry off, cows (or quarters) will need to be correctly identified and matched to the correct treatment, that being either antimicrobial plus teat sealant for likely infected cows or quarters, or teat sealant alone for likely uninfected cows or quarters. And finally, the staff administering these treatments must be well trained, using clean infusion techniques. This will be particularly important for those cows or quarters infused only with a teat sealant, since dirty infusion practices could introduce infection into an unprotected gland. However, this can be done successfully by trained producers or farm staff, as was demonstrated in the aforementioned 16-herd study conducted in Atlantic Canada and the pilot study conducted in Minnesota (Cameron et al., 2014; Patel et al., 2016).

For herds that are not able or willing to invest in the expense or hassle of setting up an on-farm culture lab, then an alternative model is to have the local herd veterinary practice offer these culture services and report the results and treatment assignments back to the herd. Additionally, local veterinary practices could set up a regular weekly service by which a team of trained animal health technicians are dispatched to the farm to collect milk samples for testing, and later to administer the appropriate treatments on the day of dry off. This full service model is already successfully being used in New Zealand where teams of trained animal health technicians employed by veterinary practice provide the service of drying off cows for farms, and in some cases, administering treatments (e.g. internal teat sealants) to prepartum heifers. In our opinion, this is just one more opportunity for North American veterinary practices to enhance their use of animal health technicians, both for the benefit of client farms as well as the practice.

If herds choose to use indirect tests such as DHIA test day SCC data, CMT, and/or other information (e.g. clinical mastitis history) as their screening test(s), then this only requires being on a DHIA testing program and keeping accurate records of clinical mastitis events. However, someone on the farm will need to carefully evaluate these records and make cow-level treatment decisions before the day of dry off. Also, managers need to remember that these indirect tests have poorer diagnostic sensitivity, potentially reducing the success of the SDCT program.

What are the economics of adopting a selective dry cow therapy Program?

Economic data are, so far, very limited estimating the cost-benefit of adopting a SDCT program. However, for the aforementioned quarter-level pilot study conducted at the University of Minnesota, the additional cost of extra labor to sample quarters 2 days prior to dry off plus culture materials (e.g. media plates, swabs, gloves, incubator) was more than offset by the savings in reduced antimicrobial use, resulting in a net benefit of $0.82 per cow assigned to the SDCT group. Obviously these economics won’t apply equally to all herds under all situations, and larger multi-herd studies are needed to examine the relative economic and biological success of different types of SDCT programs applied across different herd scenarios. However, even if SDCT programs only just breakeven as compared to BDCT, it is important to know that they are at least economically viable. That said, there are probably some scenarios under which a SDCT program may cost more to implement than a BDCT program. For example, one recent study used a computer model to predict the biological (udder health) and economic outcomes of using different SCC cut points as a means to make SDCT treatment decisions at the cow level. Due to the imperfect sensitivity of the SCC test, it was predicted that udder health measures (clinical mastitis) would be worse in the SDCT group in the following lactation, and in only 2 of the 7 SDCT program scenarios considered was there an economic advantage to adopting SDCT over BDCT (Scherpenzeel et al., 2016). Another consideration impacting the economics of a SDCT program will be the prevalence of IMI at dry off in the herd (discussed below).

What herds should consider adopting a selective dry cow therapy program?

As compared to low SCC herds, high SCC herds will have a smaller opportunity to reduce antimicrobial use in a SDCT program, since a greater proportion of cows/quarters will be infected and therefore require treatment at dry off. Research is needed to suggest science-based criteria for identifying herds most likely to benefit when adopting a SDCT program. However, and until more data are forthcoming, we will suggest that it might be reasonable to consider SDCT programs in well managed herds with a BT SCC < 250,000 cells/mL.

Summary

Our dairy industry is under increasing pressure to demonstrate responsible drug stewardship, reducing antimicrobial use where possible while still protecting animal health. The selective use of antimicrobials at the time of dry off represents one such opportunity. We now have the necessary tools available for successful and economically viable SDCT programs that can significantly reduce the use of intramammary antimicrobials at dry off, while maintaining the future udder health, production potential and wellbeing of the dairy cow. Farms with good udder health should consider working with their veterinarian to develop and implement a SDCT program. More information can be found by contacting the University of Minnesota Laboratory for Udder Health.

 

June 2016

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