The good news: the dreaded scourge of Staph. aureus mastitis, and its annoying contagious counterpart, Strep. ag., are on a measurable decline.

The bad news: environmental nuisances like E. coli, Strep. uberis, Strep. dysgalactiae, Klebsiella and coagulase-negative Staph have all too eagerly taken their place.

Here’s why environmental mastitis now takes center stage on most farms.

Why the shift?

For decades, NMC (formerly known as the National Mastitis Council) promoted its “five-point mastitis control plan” to educate producers on how to combat contagious mastitis. Those five action steps include:

  • Practice sanitary milking procedures, including teat-dipping.
  • Dry treat every quarter of every cow.
  • Treat clinical mastitis in lactating cows with approved antibiotic therapy.
  • Properly maintain milking equipment.
  • Cull problem cows. 

Dairy producers nationwide adopted the plan. The progress made has been incredible.

“We’ve done a tremendous job at bringing contagious mastitis organisms under control,” says Austin Belschner, manager of dairy veterinary operations for Pfizer Animal Health, based in Saugatuck, Mich. So good a job that contagious mastitis pathogens rarely show up in diagnostic reports. In fact, adds Belschner, on most diagnostic reports Strep. ag. only shows up as a tiny fraction among causative mastitis organisms -— if at all.

This progress comes with an unfortunate downside. The decrease in contagious, also called gram-positive bacteria, left a void in the mammary gland. That void was quickly filled by other bugs. Namely, some hardy gram-negative bacteria that survive well in the everyday environment on dairies, such as in manure, urine, water, bedding and feed. The classic gram-negative mastitis pathogen is E. coli.

New challenges

Gram-negative pathogens can be difficult to treat because they are more resistant to antibiotics than gram-positive bacteria, says Ron Erskine, veterinary researcher at the Michigan State University College of Veterinary Medicine. The gram-negative pathogens cause more mastitis infections today -— not because there are more of them, but because they seized the opportunity to pick up where the contagious, gram-positive bugs left off.

For example, a recent study by Erskine and his colleagues showed that dairy herds with exceptionally low somatic cell counts in the range of 50,000 to 75,000 cells/mL were the herds most likely to have environmental mastitis cases caused by organisms like E. coli. Diagnostic data generated by Erskine and other labs across the country currently show a general trend of E. coli and “no growths” making up more than half of all clinical mastitis infections seen on farms. 

Veterinarian Pat Gorden, of Dairy Veterinary Services in Chandler, Ariz., says those numbers ring true in his practice. While the herds he serves still fight occasional battles with Staph. aureus, Gorden says the overwhelming majority of mastitis cases cultured in his practice turn out to be either E. coli or “no growth.” In addition, most of the E. coli cases are not the hot, acute, shocky, kill-the-cow-type cases that are often associated with that organism. Rather, they tend to look just like any other case of mild to moderate clinical mastitis.

Housing plays a role

In addition to improved management of contagious organisms, Belschner attributes the rise in environmentals to the way most cows are housed and managed today. While a higher concentration of cows in smaller spaces allows for more attentive management and more controlled cow comfort, it also increases their exposure to the bacteria that are ever-present in that environment.

Gorden notes that environmental mastitis infections tend to be seasonal in nature, “although you will see environmental mastitis whenever the environment is bad.” Whether in Wisconsin (where he practiced for several years) or Arizona, more cases of environmental mastitis tend to occur in the summer, when heat stress, increased water used for cooling, and increased bacterial loads raise the risk factor. Even on very well-managed dairies, when you put the right weather conditions in place, or if protocols temporarily become lax, the door is open for environmental pathogens to strike.

The rise in prevalence of environmental mastitis is a frustration to dairy producers and their veterinarians alike.

“Most of these infections look the same when they become clinical cases, which has forced us to do more diagnostic work to pinpoint the bug,” explains Gorden. “But that allows us to now target therapy more accurately, plus we get a larger picture of what’s going on in each herd over time. That’s powerful information.”

Environmental mastitis has indeed taken center stage on most dairies.

Maureen Hanson is a freelance writer in La Porte City, Iowa.