Why Shorter, Targeted Mastitis Treatments Can Mean More Milk

When it comes to mastitis treatment in dairy cows, advances in understanding the course of the disease have changed antibiotic recommendations for the better — not only for the animal’s well-being but also the producer’s bottom line. Distinctions between infection and inflammation, different classes of pathogens, and short- and long-duration therapies are important parts of the conversation.

At the 2025 World Dairy Expo, Boehringer Ingelheim veterinarian Jen Roberts spoke with us about how knowing those distinctions can lead the way toward more responsible antibiotic use in the parlor.

SF: When a cow has mastitis, how can a producer tell if it’s an infection that truly needs antibiotics versus just inflammation?

JR: The surest way is to culture each case of mastitis — whether that involves sending culture out to the lab or doing on-farm culture to know if we have a pathogen present in that udder — and then using that as a guide to direct our treatment and try to reduce our antibiotic use on the farm. When a cow comes into the parlor and has clinical mastitis, whether that’s flakes and chunks in the milk or a swollen, hot, hard udder, there’s really no way to tell just from the outward clinical signs whether there’s bacteria there, or it’s just inflammation.

What we typically see in the case of mastitis is that within the first three days is where we’re going to have infection present. And then, once we’ve resolved the infection, either with the use of antibiotics or the cow just clearing it on her own, we usually see that it takes about another three days for the inflammation to resolve. So, whether the cow has a positive culture at the time of diagnosis or not, it’s usually going to take about six days for the effects of the inflammation to resolve and the milk to return to normal.

SF: Let’s talk about how culture results can guide treatments.

JR: If we take an aseptic sample and either culture that on-farm or send it off, what we’re really looking for is just broadly categorizing the bacteria as Gram-positive, Gram-negative, or no growth. Once we’ve made that identification, our recommendation is to treat accordingly, using the shortest-duration antibiotic treatment labeled for a Gram-positive pathogen and treating only those cows.

For the Gram-negative cases, research has shown that cows clear those very effectively on their own. Unless that cow is systemically ill, there’s no need to treat her. And even if she is systemically ill, we would treat her with supportive care, as directed by the farm’s protocol and veterinarian, and no intramammary antibiotics.

When we think about mastitis cases being in those three categories, and we’re going to focus our treatment just on those cows with Gram-positive mastitis, we’re going to reduce our antibiotic use by about two-thirds, if we use that strategy.

SF: Why should Gram-positive cases be targeted by short-duration antibiotic treatment?

JR: The Gram-positive cases are the ones that are more likely to result in chronic high-cell-count cows if we don’t treat them with antibiotics, so that’s why we’ve developed that guideline to focus on the Gram-positive cases and use a short-duration antibiotic therapy that is labeled for Gram-positive pathogens.

What I mean by short-duration therapy is therapy labeled for two or three days of treatment, or two to three treatments. There are some protocols that involve a longer duration, but there has been some research looking at the duration of treatment that has shown there’s no difference in clinical cures or bacteriological cures in those cows treated with a short-duration treatment compared to a longer duration.

Historically, treatment protocols often included things like, “Treat for five days or until milk visually returns to normal,” or “Treat for two days beyond return to normal milk.” But we’ve already talked about the infection versus inflammation piece, and if the bacteria have been cleared by day three — and let’s say we’re looking at a three-day versus a five-day treatment — we stop treatment at day three, and milk’s still going to be abnormal for about three more days beyond that. Previously, before people understood this distinction between infection and inflammation, the perception was: “We treated for three days and the cow still had abnormal milk, so the treatment didn’t work. But when we used a five-day protocol, the day after treatment was done, her milk was back to normal.” So, the five-day protocol got the credit for the milk returning to normal. But really, it was just the normal course of the disease and the rest of the time that it takes for the resolution of mastitis after the bacteria has been cleared from the udder.

When we think about our responsibility as dairy farmers or as dairy veterinarians in ensuring a safe and consistent food supply, if we can reduce our antibiotic use as an industry and tell that story to the customer that we are trying to use antibiotics in a judicious manner, that’s a great way for the dairy industry to build consumer confidence.

SF: Plus, it’s less time getting back to marketable milk.

JR: Absolutely. And that’s a great point, because the largest cost associated with a case of clinical mastitis is the milk discard. So the cost of the antibiotic, when you look at that versus the milk discard, it’s really negligible. You’re going to have maybe $20–30 involved in the antibiotic cost and the labor to administer that treatment. But the milk discard — if that cow’s milking 80 pounds a day, and you’re having to discard her milk for seven or eight days because she’s on a longer duration protocol versus having to discard her milk for only maybe four or five days — then that’s two or three days more milk in the tank, and that adds up very quickly.

SF: What role does antimicrobial stewardship play in shaping future mastitis protocols?

JR: I think to be able to implement those protocols and to make use of them long-term, farms need to have a way to consistently collect and send off those samples to culture. Otherwise, the option is blanket treatment in which any cow that shows signs of clinical mastitis gets treated with an antibiotic. Some farms do blanket treatment because they don’t have the resources, such as someone on the farm who can manage the on-farm culture, or a lab that’s near enough to them. In those cases, it’s still important to treat with short-duration therapy to reduce treatment costs and milk discard.

It’s hard to see a cow with mastitis and say, “OK, we’re going to collect a milk sample from her and we’re going to culture it, and we’re going to wait a day to treat, because there’s this sense of urgency in trying to get that cow better as quickly as possible.” But there’s also been research to show that delaying treatment, while waiting for culture results, does not ultimately impact the clinical and bacteriologic cures or outcomes. So, farmers can be confident knowing that if they identify that cow with mastitis today, collect a sample, and wait the 24 hours to have the culture results before initiating treatment, they’re not going to risk setting that cow back by delaying treatment.

It can be very cost-effective to do on-farm culture. There are some culture plates available that use selective media, and their availability has made it very easy for on-farm culture when we’re just trying to identify Gram-positive, Gram-negative, and no-growth. With those selective media plates, they can culture on the farm, look at the plate the next day, and know which category it falls into. I think it’s just a matter of adopting some of these protocols.