Mastitis control: Think positive

The dairy industry’s most costly health issue is well known to every producer: mastitis. This topic was discussed at length at the recent 47th annual meeting of the National Mastitis Council, where one point resonated loud and clear: The vast majority of subclinical intramammary infections are caused by gram-positive bacteria. 

Armed with this knowledge, dairy producers can save significant money and milk by altering their treatment protocols to combat the problem more selectively and more effectively.

In a three-year U.S./Canadian study conducted by the University of Minnesota, University of Wisconsin and Ontario’s University of Guelph, researchers evaluated 4,044 quarters from 1,028 fresh cows in 11 distinct herds. Of the intramammary infections (IMI) detected, a striking 91% were shown to be caused by gram-positive pathogens.  Furthermore, of the relatively small number of infections caused by gram-negative bacteria, most self-cure without treatment. 

The most important conclusion producers can take from these findings is the value of aggressively treating gram-positive infections with proven antibiotics, while leaving gram-negative infections to self-cure under observation.

The facts support “positive thinking.”

  • Gram-positive pathogens accounted for 91% of subclinical IMI in a recent study. 
  • Gram-negative pathogens accounted for less than 9% of subclinical IMI. 
  • Persistent subclinical infections are usually established by gram-positive pathogens. 
  • Two to three weeks after calving, many subclinical, gram-positive IMI are still present in the udder. Such persistence may require antibiotic treatment.
  • One persistent gram-positive pathogen, Coagulase-negative Staphylococcus spp., was found to be prevalent in 51% of infected animals.
  • E. coli, the study’s most common gram-negative pathogen, turned up in just 5% of such cases. 1



For gram-negative pathogens, no treatment may be the best treatment.

  • Infections caused by gram-negative bacteria tend to be mild to moderate in nature, and frequently require no treatment at all. 
  • Within 24 hours, most gram-negative infections have been shown to spontaneously resolve themselves –- without treatment. This finding comes from a Michigan State University study of 3,200 cows.
  • Another study found 90% of subclinical, environmental streptococcal and coliform IMI were eliminated without antibiotics by two to three weeks after calving. (Staph. spp. and Staph. aureus, on the other hand, persisted at least two to three weeks in more than 60% of cases.) 


Producers save milk and money through selective, gram-positive protocols

  • Administering antibiotic treatment on a broad scale for all cows with IMI can lead to longer-than-necessary periods of unsalable milk.
  • Selectively treating only those cows with gram-positive infections can significantly reduce antibiotic costs, reduce the number of treatments and reduce milk discard. 
  • Antibiotics have been shown to be highly effective in treating gram-positive infections.

Staph. aureus – positively a major concern

  • Staph. aureus is a contagious, gram-positive bacterium that continues to be a problem for many dairies. 
  • A 2007 survey revealed Staph. aureus makes up 43% of contagious mastitis organisms. 1
  • One leading researcher described Staph. aureus as “a highly successful mastitis pathogen in that it has evolved to produce infections of long duration with limited clinical signs.”
  • This bacterium was found in 30% to 40% of bulk tank milk samples in one university study.