Selective Dry Cow Therapy: It’s Complicated

RJ Erskine
Michigan State University

Because of public health concerns for developing antimicrobial resistance among bacteria, the dairy industry is striving to practice prudent antimicrobial drug use. As of January 1, 2017 new guidelines for the use of antimicrobial drugs in feed have been established by the FDA (Veterinary Feed Directive) that will increase the role of veterinarians in prescribing the administration of these drugs. While antimicrobial drug use in feed is rare on most dairy farms (for adult cattle), a recent survey found 92 % of Michigan dairy producers rank reducing antimicrobial use as important or very important. Selective dry cow therapy (SDCT) has been proposed as a potential opportunity to decrease antimicrobial use on dairy farms. Overall, antimicrobial resistance among mastitis pathogens has changed very little over the past 30 years, and there is little evidence of a direct cause and effect link between antimicrobial use in dairy cattle and increasing frequency of resistant pathogens in humans. Nonetheless, methods to reduce antimicrobial use on a farm should be explored and this article will discuss the possible benefits, but also pitfalls of SDCT.

Background

For more than 40 years, studies have overwhelmingly found that blanket dry cow therapy (BDCT-treating all cows at the end of lactation with intramammary antimicrobials) reduces the prevalence of intramammary infections (IMI-mastitis) at calving and early lactation, decreases the incidence of clinical mastitis in early lactation, and improves milk quality. Dry cow therapy is seen as an efficient and simple practice to both cure existing infections and prevent new infections during the dry period. As recent as last year, a report from over 600 herds in three states found that herds that used BDCT were more likely to have lower bulk tank somatic cell counts (SCC) than herds that did not use BDCT (Schewe et al., 2015). When BDCT is combined with internal teat sealants, the overall impact on decreasing IMI and clinical mastitis in fresh cows is even more profound. BDCT offers a relatively low risk of antimicrobial residues in milk and meat relative to lactating cow therapy, in part because of the “planned dosing” of BDCT as opposed to “reactionary dosing” of lactating therapy, most often in response to a case of clinical mastitis. Additionally, cure rates of IMI are often considerably higher for BCDT compared to lactation therapy. Especially in Europe, SDCT has been proposed as a method to decrease antimicrobial drug use. If efficacious, SDCT could decrease drug use and labor. But what do you need to know to make this decision?

Previous SDCT Studies

The biggest challenge with SDCT, compared to BDCT, is to decide which cows get treated and which cows do not; or how to identify infected cows, who are treated, as opposed to non-infected cows, who are not treated. Any protocol to select infected from non-infected cows has to be simple, cost-effective, and accurate. A summary of previous studies from Ohio, Netherlands, Canada, and Germany is in Table 1.

Table 1

As can be seen from the table, when cows were selected not to be dry cow treated based on SCC or clinical mastitis history (Rajala-Schulz, 2010), if SCC at dry off were below 250,000 cells/mL for older cows or less than 100,000 cells/mL for first lactation cows Scherpenzeel, 2014), or when three previous DHI test dates were below 100,000 cells/mL (Kiesner, 2016), the consequences of this decision could result in increased IMI during early lactation and clinical mastitis. This is a critical problem on two fronts:

1) Lost milk production and premature culling- In a study from Idaho and Washington, DHI Records from over 160,000 cows in 22 herds found that cows that begin their lactation with a linear SCC score of ≥ 4 (200,000 cells/mL) will produce 1,580 lbs (700 kg) LESS milk, and be three times more likely to have a case of clinical mastitis by 60 DIM than cows with lower SCC (Kirkpatrick and Olson, 2015). Additionally, cows with clinical mastitis were twice as likely to have died or be culled by 60 and 120 DIM. A recent study on the impact of clinical mastitis in the first 30 DIM found that the average cost per case is $444, and about 70% of the losses result from indirect costs such as premature culling and lost milk production throughout lactation (Rollin et al., 2015).

2) Increased use of antimicrobials in early lactation cows, which are at greater risk to be culled for market. In the Dutch study cited above (Scherpenzeel et al, 2014) low SCC cows that were not treated with dry cow therapy were 1.8 times more likely to be treated with systemic antimicrobial therapy in early lactation as a result of increased risk of clinical mastitis.

Accurate identification of infected cows for SDCT was reported if cows with SCC below 200,000 cells/mL were further screened by culturing composite milk samples before dry off. Cows that were negative on culture (presumably not infected) were not treated as opposed to cows that had positive cultures for bacteria that were dry cow treated. It is important to note that regardless of the decision to use antimicrobials, all cows were infused with internal teat sealants (Cameron et al., 2014).

Pitfalls

The key points for successful SDCT as a management tool are 1) an accurate selection of infected from non-infected cows, and 2) a reliable record system to determine if use of SDCT, rather than BDCT, affected milk quality. Omitting the necessary steps to meet either one of these points can lead to disaster. DO NOT RELY ON HERD AVERAGE OR BULK TANK SCC TO DETERMINE SUCCESS OF SDCT! Herd average SCC are not the best indicator of the prevalence of mastitis in a dairy herd and can mask problems until mastitis becomes a serious problem in a herd. For more on this topic refer to a previous QMA article:

http://qualitymilkalliance.com/2015/01/17/is-cell-count-a-good-measure/

Based on current research, if SDCT is to be considered as a management tool:

1) Individual SCC are a must, whether performed by DHI testing, California Mastitis Test (CMT), or conductivity in milk (with conductivity equipment that is calibrated regularly and validated to accurately detect subclinical mastitis). If this is not available in a herd it is essentially impossible to attempt SDCT.

2) Additionally, ALL low SCC cows (< 200,000 cells/mL) need to be cultured before dry off and positive cows are treated

3) Internal teat sealants should still be used, aseptic preparation of teats for infusion is a must!!!

4) Track both clinical mastitis rate in the first 60 DIM and the proportion of cows that are infected at Test date 1 (linear SCC ≥ 4). Also, the trend in dry cow infection rate (based on SCC, see article in weblink below). If these metrics are not available to a herd manager, it is nearly impossible to assess the full impact of SDCT in a herd.

http://qualitymilkalliance.com/2013/08/12/monitoring-new-infections-in-dry-cows-2/

5) What is the labor culture of the herd? In the Canadian study, the average herd size was 75 cows. In larger herds that rely on employees to collect milk samples, and infuse intramammary antimicrobials or internal teat sealants, training and protocol compliance can be variable? If employees are involved with the selection, identification and infusion of cows in a SDCT program, how well have they been trained?

6) Finally, ensure that everything else is done to help lower the impact of dry cow mastitis in a herd, for example, lowering milk production at dry off, maintaining a clean environment, and using proper teat infusion technique (helpful articles and videos links listed below).

http://qualitymilkalliance.com/2014/01/12/whats-lurking-on-the-teat-ends-of-your-cows/

http://qualitymilkalliance.com/2014/04/27/video-how-to-infuse-antibiotics-to-treat-mastitis/

http://qualitymilkalliance.com/2014/09/10/video-reduccion-de-la-mastitis/

Selective dry cow therapy can reduce antimicrobial use in a dairy herd, but will not decrease labor and management of individual dry cows. If done without care, it can have many detrimental effects.

Leave a Reply

Your email address will not be published. Required fields are marked *