From the NMC Newsletter "Udder Topics", August, 2000
Coagulase-negative staphylococcus (CNS) species are the organisms most frequently isolated from bovine milk samples. Mastitis researchers do not classify CNS as contagious or environmental pathogens. They designate them as "skin flora opportunists" since CNS are a part of the normal teat skin flora. CNS can colonize the teat canal. Some species are also found free-living in the environment.
In the mastitis literature, researchers report CNS intramammary infections to occur in 10 to 20% of quarters. The infection rate is generally higher in primiparous cows. Most CNS infections are transient. Cows and heifers will have a higher prevalence of CNS infection after calving, with a rapid decline over the first week or two of lactation. Researchers believe cow to cow spread is rare.
CNS are of low pathogenicity. Infections are usually subclinical and result in quarter somatic cell counts (SCC) only about two- to three-fold above that of uninfected glands. The impact on composite sample SCCs (such as those on the DHI report) will therefore be minor. Despite their low pathogenicity, CNS infections can occasionally contribute to clinical cases of mastitis in dairy herds, but CNS are rarely a major cause.
A positive culture may show recovery of an organism but this does not mean inflammation of the mammary gland has occurred. Because CNS are commonly found on teat skin and in the streak canal, they are a common cause of contamination of milk samples. Repeated isolation from a particular quarter sampled multiple times builds the case for a persistent and important infection.
Isolation in association with elevated SCC also supports the diagnosis of mastitis versus non-significant infection. The most likely mastitis-causing bacteria should be ruled out before CNS are considered significant in suspected mastitis cases.
To improve the interpretation of culture results, prevent the contamination of milk with CNS from skin sites when collecting milk samples. Prepare teat ends carefully using cotton swabs moistened with alcohol. Scrub the teats on the far side of the udder first, then those on the near side. Begin aseptic sample collection from the closest teat and move to teats on the far side of the udder (the reverse order from cleaning). Do not allow the tube to touch teat end. Do not allow milk entering tube to touch fingers or hands. Frequent isolation of CNS from milk samples suggests either poor sampling technique and/or poor teat end hygiene.
Excerpted from the Ontario Ministry of Agriculture, Food and Rural Affairs "Animal Health News, Vol. 7, No. 4, December 1999