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Impact of Intramammary Infections caused by Coagulase-Negative Staphylococci

Because mastitis caused by coagulase-negative staphylococci (CNS) is usually characterized by mild elevations of somatic cell counts (SCC) and subclinical changes, this group has usually been accepted as a minor pathogen in dairy herd health. However, with global control of contagious mastitis pathogens such as Streptococcus agalactiae and Staphylococcus aureus, CNS have become the most common mastitis pathogens cultured from udder secretions.

Recent studies have found the prevalence of subclinical mastitis due to CNS to be between 16% and 50%, depending on country. CNS are the predominant pathogen isolated from pre-calving heifers. Somatic cell counts from quarters infected with CNS usually remains below 500,000 cells/ml. Since increases in SCC over 100,000 cells/ml are associated with reduced milk production, herds with a high prevalence may see an impact from CNS mastitis. Additionally, where a farm goal is to have an extremely low bulk milk SCC (<100,000 cells/ml), many cows infected with CNS may lead to a high herd SCC and loss of income from premiums.

To elucidate the impact CNS may have on production and bulk milk SCC, fifteen years of individual cow culture results (352, 614) and data (milk production and SCC) from New York state were evaluated. The study found that overall, approximately 9% of all cows were infected with CNS while at the herd level an average of 15% of cows were infected with CNS. Lowest SCC cow-level SCC was associated with no infection; moderate SCC level with CNS and Corynebacterium spp. and high level SCC with S. agalactiae and S. aureus.

The percentage contribution to SCC in herd with BMSCC <200,000 was ~18%; in herds with BMSCC between 200,000 and 400,000 it was ~12% and in herd with BMSCC greater than 400,000 cell/ml, the percent contribution by CNS was 8%. In low BMSCC herds, CNS as a group had a larger contribution than any of the other major mastitis pathogens and are important contributors to the total number of somatic cells in bulk milk.

As with all other types of mastitis, prevention is the key but management strategies aimed specifically at CNS need additional investigation. Since CNS can be resident microflora on teats and udders, they have long been considered opportunistic pathogens; however, recent studies have shown that certain species are more adapted to the udder environment (S. chromogenes and S. hyicus). Strategies aimed at decreasing bacteria on teats (e.g. dipping) have been shown reduce CNS populations, as have strategies aimed at environmental hygiene. The use of internal and external teat sealants also have been shown to have a preventive effect against new CNS and other intramammary infections in both heifers and cows.

Spontaneous elimination rate of CNS in lactating dairy cattle is high and so treatment may not be indicated for all cases though it should be recommended for quarters with moderate to severe clinical mastitis and/or persistent infection. Selection of antimicrobials should be based on susceptibility testing.

Understanding of CNS mastitis is still quite limited and more research into understanding intramammary infection dynamics, management and cost-effective treatment strategies is encouraged.