It probably goes without saying, but fostering antimicrobial stewardship is the right thing to do. During the 2024 National Mastitis Council (NMC) Annual Meeting, Simon Dufour, Université de Montréal, St-Hyacinthe, Québec, Canada, reinforced that dairy producers (and veterinarians) can and should strive to use antibiotics more judiciously to mitigate antimicrobial resistance (AMR).
To explain AMR, Dufour described how AMR develops. Most living organisms are colonized with various bacterial species and other microorganisms, with most of them living in harmony with their host or environment.
“In general, these bacterial populations are extremely diverse,” he said. “However, when a host receives an antimicrobial treatment, the bacteria that are susceptible to this antimicrobial will probably be eliminated. If present, the very few bacteria that were resistant to this antimicrobial will then possibly replace this initially diverse population and become the main type of bacteria in that new ‘post-treatment’ population. As a result, the antibiotic will become ineffective for future treatments and the infection may persist – increasing the risk of spreading to other animals and humans.”
Dufour added that, in essence, AMR is like genetic selection – but applied to “bugs.” “When using antibiotics on a farm, we are imposing a selection pressure that will favor resistant bugs,” he explained. “The main problem with bugs is that they need very little time to produce a new generation.” For example, it only takes 20 minutes to create a new generation of Escherichia coli. “In addition, many bacterial species can exchange genetic materials with their neighbors, not just with their descendants, including genes coding for resistance to antimicrobials.”
Simply stated, the more AMRs we use, the more we are selecting resistant bugs. Eventually, AMRs will be ineffective. To curtail AMR, Dufour recommends limiting antimicrobial use. By doing so, selection pressure will be removed and the “bug population” will slowly regain its original diversity.
Optimize antimicrobial use
Dufour described antimicrobial stewardship as optimal use of antimicrobials. How is this achieved? Dufour shared three strategies to foster antimicrobial stewardship. First, design and implement procedures (e.g., vaccination and hygiene) to prevent diseases. Second, determine if an antimicrobial is needed for an animal with a particular condition. And third, if an antimicrobial is needed, choose the appropriate antibiotic and correct dose, route of administration and treatment duration.
To consider management practices that foster antimicrobial stewardship, Dufour encouraged NMC members to start by reviewing a dairy farm’s antimicrobial use. Assess the most common reasons (diseases) why antibiotics are being used. What can be done to prevent these diseases?
Assuming that mastitis is the most common reason for antibiotic use, Dufour suggested evaluating the prevalence of intramammary infections (IMI) at first calving by using milk bacteriology or somatic cell count (SCC). Repeated monthly (or daily for automatic milking systems), SCC measures can also be used to investigate lactational and dry period IMI dynamic. Additionally, compute the clinical mastitis incidence rate. If needed, also compute the clinical mastitis incidence rate for early vs. late lactation and/or age-specific groups of cows.
Based on these assessments, a dairy producer may want to implement preventative measures, such as vaccinating for coliform mastitis, improving milking hygiene and/or improving youngstock, lactating cow and/or dry cow hygiene. “Preventing disease is probably the most efficient method for reducing antimicrobial use,” Dufour stated.
Not all infections need treatment
Next, Dufour recommended developing (reviewing) clinical mastitis treatment and dry-off standard operating procedures. Just because a cow has clinical mastitis, it doesn’t mean you should give her an antibiotic. Dufour shared the results from Kurban et al. (2023), which evaluated milk bacteriological culture results from 43,924 clinical mastitis cases (from Brazil, Canada and United States). Researchers reported that 39.6 percent of culture results were classified as “no growth.”
“The high proportion of clinical mastitis cases with no identifiable microorganism suggest that many of these cases do not require an antibiotic treatment,” said Dufour. “An exception to that would be for severe clinical mastitis cases, where fever, anorexia, depression, etc. are observed.” In these situations, it’s likely that a significant proportion of cows have developed bacteremia (Wenz et al., 2001). “These cows could benefit from a systemic antibiotic.”
Is there a downside to delaying clinical mastitis treatment? “For mild and moderate clinical mastitis cases, neither bacteriological cure, subsequent SCC, milk yield, clinical mastitis recurrence risk, nor culling are impacted when clinical mastitis treatment is delayed by 24 hours to obtain bacteriological results from a rapid diagnostic test (de Jong et al., 2023a),” stated Dufour.
If antibiotic treatment is appropriate, choose an antibiotic that is not primarily used to treat severe human infections. These include quinolones, third- and fourth-generation cephalosporins, and polymyxins. From that list, however (in many countries), only third- and fourth-generation cephalosporins are available as commercial mastitis treatments. Do not use the latter products as the primary clinical mastitis treatment. Instead, keep those for cases where we have data (e.g., an antibiogram), indicating that the pathogen is resistant to the other antibiotics that could be used.
When administering antibiotics, follow the label! Give the right dose via the right administration route for the labelled duration and frequency. Dufour noted that when systemic (intravenous) antibiotics are given, we should determine the animal’s weight and adjust the dose to its weight.
Selective dry cow therapy supports judicious antimicrobial use
For the past decade, significant research has evaluated selective vs. blanket dry cow therapy. In a “nutshell,” practicing selective dry cow therapy (SDCT) without administering an internal teat sealant (ITS) in uninfected quarters usually leads to reduced udder health (i.e., increased dry period new IMI incidence and post-calving IMI prevalence) (Kabera et al., 2021a). Therefore, inserting an ITS in uninfected quarters is an essential component of SDCT. Also, make sure that those administering internal teat sealants follow an aseptic technique. If not done aseptically, bacteria will most likely enter the mammary gland – potentially resulting in an udder infection.
For more details regarding SDCT practices, read Dufour’s complete NMC Annual Meeting proceedings paper (found in the NMC Member Center), refer to past SDCT proceedings papers or review past Udder Topics articles (also found the NMC Member Center) that address SDCT.
Avoid unnecessary expenses
While some are hesitant to “back off” on antimicrobial use, Dufour encourages dairy producers, veterinarians and consultants to adopt a “can do” attitude as they approach antimicrobial stewardship. Overuse of antimicrobials results in unnecessary expenses (e.g., antibiotics, dumped milk), potential for an antibiotic residue in bulk tank milk, and risk of antibiotic-resistant bacteria in milk that may hinder cattle, farm workers and/or the general public.
Dufour cited a couple quotes from Canadian dairy producers who changed farming practices with antimicrobial stewardship in mind. They concurred that proactive measures (e.g., better hygiene) and reducing antibiotic use (by determining which infections to treat, with the appropriate treatment) is doable and yields several benefits, including improved animal health and dairy farm profitability.
“Antibiotic stewardship in dairy cattle requires a multifaceted approach that prioritizes animal health, food safety and industry sustainability,” Dufour concluded. “By adopting responsible practices, including veterinary oversight, recordkeeping, education and exploring alternative management practices, such as selective clinical mastitis treatment and SDCT, dairy farmers contribute to the well-being of their herds, the environment and public health. As consumers increasingly demand transparency and sustainability in food production, antibiotic stewardship stands as a fundamental principle for the dairy industry to thrive in a responsible and ethical manner.”
