MSU Extension

If you have cultured the milk of cows with mastitis you have probably found what are called “Coagulase-Negative Staphs.” or CNS. In fact, these bugs are becoming the most prevalent pathogen identified for mastitis but may be one of the least understood. However, some new research is opening the door to a better understanding of CNS.

Let’s talk about how it gets its name. In the process of identifying the bacteria that cause mastitis or any disease, various procedures are used to separate out different types of bacteria based in their response to lab procedures.

Whether culturing on-farm or in a lab, an investigator looks at the media on which growth occurs. On a bi-plate containing blood agar and MacConkey agar, a milk sample containing only CNS bacteria will yield growth on blood agar only. That narrows the possibilities of bacteria mainly to streptococcus and staphylococcus. Hydrogen peroxide is used on a glass slide to see if a colony bubbles. A bubbling reaction is called catalase-positive and indicates a staphylococcus species, whereas non-bubbling, or catalase-negative, indicates streptococcus bacteria.

To further differentiate a catalase-positive colony, colonies are placed in a test tube and rabbit serum is added. If the serum thickens to a gel (that is, it coagulates), the bacteria is called coagulase-positive and is most likely Staphylococcus aureus. Otherwise, the bacteria are labeled as a Coagulase-negative Staph. or CNS.

But CNS is not an individual type of bacteria – it is really a group of species of bacteria, and these do not all behave the same in the udder. A study by Fry published in the August 2014 edition of the Journal of Dairy Science showed 20 different CNS species bacteria could be identified in milk samples from 555 cows that had subclinical mastitis on 89 farms in Canada. Cows were sampled during late lactation, dry period, at calving, in the early post-partum period and during lactation. They used gene sequencing to determine the specific bacteria.

Some species occurred more commonly than others in the milk samples. Of the 20 different species differentiated, three species accounted for 77 percent of the CNS identified. Only nine species where identified in persistently-infected quarters, some for a period of greater than 200 days. The other 11 species appeared to be transient infections, or were cured. Some species of CNS were associated with significantly higher somatic cell count (SCC) than uninfected control mammary quarters. However, some milk samples from which CNS were isolated did not have a higher SCC than their uninfected quarters.

Michigan State University Extension researchers do not classify CNS as either strictly contagious or environmental pathogens. Instead, they designate them as skin flora opportunists since CNS are part of a normal skin flora. CNS have the ability to colonize the teat canal and some species may also be found free-living in the cows’ environment. Some species have been shown to act more like contagious pathogens than others.

The point is that we cannot assume that the impact of a CNS infection on one cow or one farm is the same as it was on another, or that it will respond the same. In fact, Fry and her co-authors say that in the research literature, “conflicting results as to the importance of CNS as mastitis pathogens is likely due to the failure to acknowledge variations within and between these species.”

Many studies report CNS as the leading cause of subclinical mastitis, especially in herds in which the major pathogens, such as Staph. aureus and Strep. ag are controlled. Therefore, you will most likely have to deal with it. Prevention is the best way to handle it.

Infection with CNS is most commonly found immediately after calving, indicating that the infection most likely occurred during the dry period prior to calving. It is also more common in first lactation animals than in later lactations so protecting springing heifers is important. Prevention therefore should be focused on providing a clean, dry environment to cows and heifers during the weeks prior to and during the time of calving, fly control and preventing suckling between heifers.

In addition, because the bacteria often are established on teat skin, milking preparation that effectively disinfects the teat end, equipment function that does not damage the teat end and post-milking teat dip are important to prevent CNS infections. Routine dry cow treatment is effective in reducing intramammary infection rates.

It is important to realize that because CNS inhabits the teat skin and even in the teat canal, it can be a common cause of contamination of milk samples. So before taking a sample for culture, prepare the teats – and especially the teat ends – thoroughly by scrubbing with alcohol and working from far teats to near teats when prepping and working from near teats to far teats when sampling.

Talk with your veterinarian about CNS and together work to decrease the prevalence in your herd.