Dry dairy cow therapy choices
Research is still underway to determine herd characteristics that will enable herds to reduce dry cow treatment while keeping mastitis at bay
Researchers discovered more than 40 years ago that treating dairy cows at the end of lactation with an antibiotic udder infusion reduced mastitis in dairy herds. At the time the most important causes of mastitis and those targeted for dry cow treatment were the chronic and contagious mastitis infections caused by Staph aureus and Strep agalactiae. Today in Ontario, Strep agalactiae is a rare mastitis pathogen. Changes in mastitis risks, then, have changed over the years.
Given societal interest in reducing unnecessary antibiotic use in food animals and producer costs to treat cows that may not need it, some herd owners wonder if or when they should stop routinely treating all cows at dry off time with antibiotic udder infusions. In the past though, research in Canada and elsewhere has shown that herds using selective dry cow treatment ended up with higher somatic cell counts (SCCs) and clinical mastitis. In the Netherlands, very recent research has shown that eliminating complete dry cow treatment resulted in some herd owners treating more cows for mastitis in early lactation.
Blanket dry cow treatment
Routine use of antibiotic treatment at the end of lactation for all cows is called blanket dry cow treatment (BDCT). This method treats mastitis cases that start during the lactation just completed, and prevents new infections from starting in the early dry period as the teat ends are closing. Treating a cow that has just ceased milking doesn't require milk to be discarded, and it sends a signal to the cow that lactation is over, especially if the treatment is the trigger for the cow to be moved out of the lactating herd. Today, more than 95 per cent of herd owners use BDCT in their mastitis control programs.
Have important changes in mastitis occurred?
Depending on the predominant mastitis pathogens causing problems in a herd, mastitis prevention practices, such as BDCT, will assume greater or lesser importance. If mastitis infections known to have a high cure rate with antibiotic treatment are believed to be reduced or eliminated in a herd it could be time to re-evaluate the use of BDCT. Surveys show herd infection rates with Streptococcus agalactiae have decreased significantly during the last 20 years. Before 1990, more than 40 per cent of producers would have had cows infected with Strep ag. Today, this organism is rarely found on either culture or polymerase chain reaction (PCR) testing. This pathogen is highly susceptible to antibiotics so it's not surprising a major effect of BDCT has been to virtually eradicate Strep. ag.
Increasing testing of SCC has made it possible to identify more cases of subclinical mastitis, which helps identify problem herd situations sooner and show improvement when programs are successful. The decline in bulk milk SCC during the past 20 years primarily reflects the decline in prevalence of Strep. ag. On the other hand, Staph. aureus still infects at least one cow in more than 80 per cent of Canadian herds. Antibiotic-based preventive practices, such as BDCT, haven't been successful against this pathogen or other members of the Staphylococci family to the same degree.
Herd owners can better manage mastitis today if they take advantage of all the information and testing programs available. More tests are available to producers and veterinarians to identify the bacteria causing clinical and subclinical mastitis in a particular herd. Aseptic samples can be collected and cultured in a diagnostic lab, a vet practice or even a farm lab. Samples collected for milk recording can be tested at the CanWest DHI laboratory for contagious mastitis bacteria using the PCR test. Access to more testing means producers can know more precisely what bacteria are causing cases of mastitis in their herds, and use this information to fine-tune their prevention programs.
Cow SCCs provide a monthly snapshot of mastitis prevalence in the herd. Analyzing the results using herd management software means producers can keep tabs on changes in mastitis patterns. Detecting changes in a SCC pattern can trigger a timely response for further testing or determine if changes are needed for prevention programs. However, SCCs are not intended to detect cows that need antibiotic treatment either during or at the end of lactation.
Some antibiotics have changed but the families of antibiotics they belong to have not. Fortunately, research shows the resistance patterns of commonly targeted mastitis pathogens have not changed either.
Newer technologies, such as teat seal products, can replace some, but not all, of the effect of antibiotic use at dry off time. Teat seal products can prevent new infections from starting during the upcoming dry period but have no effect on infections already present when they are infused.
Detecting new infections
Before considering reducing BDCT, a herd owner needs to be sure contagious mastitis is at a low level. Only herds with consistently low bulk milk SCCs for several years should consider reducing BDCT. Additionally, the ongoing use of an acceptable testing program to identify mastitis cases and the bacteria causing mastitis is necessary to ensure contagious mastitis is at a low level. Given an opportunity, such as a lapse in what has been an effective mastitis control procedure, contagious mastitis can spread readily and rapidly within a herd. Newly infected cows can be difficult to identify quickly enough since the infections are mainly subclinical. A regular objective review of mastitis preventive procedures at a suitable interval by the herd veterinarian will ensure adequate mastitis control preventive practices are in place, being carried out in an effective manner, and adjusted to respond to mastitis risks. Modifications to a herd's mastitis control program to enhance mastitis prevention, rather than treatment, will be necessary if BDCT is to be reduced.
Owners that keep a closed herd have a low risk of introducing contagious mastitis pathogens into their lactating herd by bringing in new cows. When cattle are brought in, testing for mastitis pathogens should be done before the new cattle are milked or come into contact with the herd. Milking new cows last or separately may be okay when introductions are infrequent. However, given the time delay for culture results and the need to conduct PCRs in a laboratory rather than cow side, it would be difficult to reduce the use of BDCT when cows coming from other herds are routinely added to the home herd.
Detecting new mastitis infections early is critical to reducing BDCT. Since many new infections are subclinical, relying on visual detection of mastitis at milking time is not enough. On the other hand, having mastitis test results at every milking or every day can make it hard to sort out which responses indicate potential problems and which are insignificant.
Environmental mastitis is caused by manure bacteria that enter the teat ends. BDCT can also reduce some kinds of environmental mastitis. Even a herd with low cow SCCs and little mastitis during lactation can still have new environmental infections start in very late lactation or the early dry period. Spacious dry cow housing and bedding hygiene protocols that keep bedding clean and dry are needed to ensure the teat ends of newly dry quarters are not exposed to high levels of bedding bacteria if BDCT is to be reduced.
Talk to your veterinarian and make a plan together
If you decide to change from BDCT to treating only some cows, you will need to devise a plan for selecting the right cows to treat. You'll also need excellent herd records and a testing strategy that adequately identifies cows that need treatment at dry off.
Monthly cow SCCs are the most common method for detecting mastitis and developing mastitis treatment protocols on most Ontario dairy farms. However, they may not work well enough to pick individual cows for dry cow treatment. Dutch researchers studied about 1,700 cows in 97 herds to find out if using the last monthly DHI test before dry off would work to pick cows for dry treatment. For first lactation, cows with a last test of less than 150,000 were selected for the trial, while for older cows a last test of less than 250,000 was used. Two quarters from each cow were treated with antibiotics and the other two quarters were left untreated.
When followed into the next lactation, by 100 days in milk, the untreated quarters had 1.7 times more clinical mastitis cases. The untreated quarters also had higher SCCs on the day of calving and at 14 days of lactation. Overall antibiotic use was reduced as only 85 per cent of the amount of antibiotics were used for untreated quarters compared with treated quarters. However, some infected quarters persisted and the preventive value of BDCT in the early part of the dry period was lost, resulting in higher SCCs and more cows needing to be treated in the next lactation. The cost of lost milk and component production from persistent and newly infected quarters, and the cost of discarded milk due to lactation treatment, as well as the antibiotic residue risk, was not measured.
More research is needed to validate other kinds of tests and testing strategies to find the best way to consistently pick the right cows and-or the right quarters to treat. Determining the characteristics of herds that can successfully reduce their dry cow treatment is still in progress. To start with it's clear that herds need to have consistently low bulk milk SCCs, probably below 200,000 cells/ml. Then herd owners need to have sufficient information about the current mastitis pathogens and risk factors for mastitis to determine their herd's suitability for reducing dry cow treatment.
Reducing antibiotic use in livestock production is an important goal to work toward, but doing it by reducing BDCT may be difficult for most herds in Ontario at this time. Discuss this practice with your herd veterinarian to determine what preparation and changes to mastitis prevention programs you need to employ to reduce your reliance on BDCT.
This article was originally published in the Milk Producer Magazine.
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