Mastitis can be caused by many pathogens, but if you ask any dairy veterinarian or farmer which is the most damaging, the odds are good that their response will be Staphylococcus aureus.
Staph. aureus is especially dangerous because it persistently infects cows — meaning that a diseased cow should almost always be considered a diseased cow capable of spreading it to other herdmates for the rest of her life. But complicating control efforts is the fact that it is hard to pinpoint exactly how prevalent Staph. aureus is in the U.S. dairy population. On an episode of the American Association of Bovine Practitioners podcast, “Have You Herd?”, Justine Britten of Udder Health Systems gave the example that at their diagnostic labs that handle an average of 7,800 bulk tank milk samples per year, roughly 45% of tests are positive for Staph. aureus. That’s up from the 20% prevalence they saw consistently between 2017 and 2020, and it doesn’t account for the real possibility that positive cows can be diluted out in a bulk tank sample by negative cows.
“In my personal experience, every dairy is impacted to some degree by Staph. aureus,” said the milk quality specialist. The presence of the pathogen doesn’t mean an outbreak is occurring, but if cows are infected, one could appear at any time.
Is it lurking in your herd?
Britten said that most farmers don’t realize that Staph. aureus is a subclinical disease. Infected animals may not drop off in production or have a mastitis event for multiple lactations. Somatic cell count (SCC) may not rise, either, and can even be markedly low before it begins to creep up. “It makes it very easy for this disease to fly under the radar if the dairy is not culturing and actively looking for it,” she cautioned.
Even if there are no clinical signs, when the pathogen is present, udder health is deteriorating, and clinical cases typically arise in older cows. All the while, these animals can transmit the bacteria to other cows through infected milk. Plus, it’s estimated that between 2% and 15% of first-lactation animals calve in with Staph. aureus. Being a closed herd reduces those odds, but it is still possible to occur, Britten said.
Culturing cows is the most proactive step to take to control Staph. aureus and identify infected animals to formulate a plan for eradication. “I’m a strong advocate for screening of all fresh cows and heifers,” Britten specified. “Dairies that are culturing only clinical cows are going to leave most of the Staph. aureus undetected in the herd.”
She reemphasized that if clinical cases are occurring or SCC is high, you are already experiencing a Staph. aureus outbreak. Treat SCC as a lagging indicator. The severity of a herd outbreak can also vary widely, and some farms with a Staph. aureus problem will never see a SCC spike. Recognize that the cost of fixing an outbreak quickly outpaces the cost of testing, Britten added.
Monthly bulk tank testing is another tool to make use of. Britten noted this is the bare minimum in monitoring pathogens because it only provides a 10,000-foot view of herd milk quality and is subject to dilution effects, but it is better than nothing. It can help you put eyes on a problem that is largely invisible — until it isn’t.
“If Staph. [aureus] was easier to see, farmers would be a lot less tolerant of it,” Britten said.
