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Managing your Bovine Mastitis Index: The role of Non-Aureus Staphylococci

When most veterinarians receive the results of their milk samples, they often see that NAS’s result (Non-aureus Staphylococci) has come back positive. They tend to dismiss this positive result as something normal, without being aware that this kind of infections can cause sub-clinical cases of mastitis in dairy cows. The more we know about NAS’s, the more confident we’ll be when it comes to interpreting results, being able to decide if an infection with this family of bacteria is a risk we want to take…


Aetiology and epidemiology

NAS’s are Gram-positive cocci that inhabit both the outside and inside of infected udders.

Often they are called “opportunistic flora of the skin”, because they can be isolated from the skin of the teat, the teat canal, vagina, and the coat and nostrils.

This group of bacteria includes over 50 species and subspecies (Pyöräla S. et al. 2009).

Mastitis in dairy cows: Non-aureus Staphylococci infections (NAS's) play a very important role in Non-Aureus Staphylococci usually cause mild infections and sub-clinical cases of mastitis in dairy cows.

Species such as Staphylococcus epidermitis, Staphylococcus saprophyticus, Staphylococcus simulans and Staphylococcus warneri belong to the normal bacterial flora of the teat skin, while other species such as Staphylococcus xylosus and Staphylococcus sciuri seem to come from the environment.

Staphylococcus chromogenes may colonize the skin of the teat and other parts of an animal’s body such as hair, the vagina and teat canal.

It seems that there are differences in the pathogenicity of different species of NAS’s that are investigated by techniques of molecular diagnosis (Zadoks and Schukken, 2006).

We found species with different antimicrobial susceptibility and diverse virulence factors of NAS’s isolated from mastitis in dairy cows (Taponen S. et al. 2009).


Incidence & prevalence of NAS infections: When are they higher?

The incidence of new infections is highest during the cow’s dry period and prior to calving. Therefore, the percentage of quarters infected is high at the time of calving.

The highest prevalence of NAS’s is in primiparous animals rather than in mature cows.

Unfortunately, many producers mistakenly believe that their heifers are healthy, and the presence of mastitis in dairy cows is not observed until calving. Heifers represent future lactation and care for the udder is basic for ensuring the profitability of dairy farms.

Although NAS infections are usually mild or sub-clinical, it has also been shown that they can cause more severe and persistent processes, causing an increase in the somatic cell counts and a decrease in milk quality and production due to damage to breast tissue (Taponen S. et al. 2009; Gillespie B.E. et al. 2009).


Mastitis in dairy cows caused by NAS’s: Characteristics

  • Are usually mild infections and cause sub-clinical cases of mastitis in dairy cows.
  • Increase in SCC.
  • Can induce persistent clinical processes that do not respond to antibiotic treatment.
  • Milk appearance is normal, but it can induce intramammary infections with alterations in milk (floccules).
  • High prevalence in primiparous animals (especially in the time around calving).
  • Higher incidence of new infections in the cows’ dry period.
  • The general state of the animal is not usually affected, nor are there severe systemic signs.



Content originally created for “STARTVAC Library No. 3”

Author: Clara Navarro Sansano.

Milk quality consultant | KERSIA/HYPRED | Spain.



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