Discipline: mastitis; Keywords: CNS/NAS strains, SCC, Staphylococcus aureus, Streptococcus agalactiae, retrospective study, TMR vs pasture-based production systems.

Coagulase negative Staphylococcus or non-aureus Staphylococcus (CNS/NAS) pathogens are relatively new isolates affecting udder health. The most common strains isolated from a study on 20 SA dairy herds were Staph. chromogenes, Staph. epidermidis and Staph. haemolyticus. The numbers of the CNS/NAS strains increased as lactation progressed.The results were also evaluated to identify a probable correlation between the CNS/NAS strains and somatic cell count (SCC) levels. Staph. epidermidis had the highest percentage of isolates with SCC in excess of 1 million cells per ml milk followed by S. haemolyticus. The most commonly isolated CNS/NAS overall was S. chromogenes.

Heifers may calve down with within udder infections and udder damage. Pre-weaned calves housed together (co-housed) that are fed fresh milk may suckle each other’s teats and may infect udders of fellow heifer calves with pathogens present in the milk. In a study to investigate prevalence, CNS/NAS, Staph. aureus, Streptococcus agalactiae, environmental streptococci and coliforms in heifers was 26%, 0.9%, 0.08%, 1.4% and 0.4% respectively, whereas samples with no bacterial growth were in the majority at 66%. The overall prevalence ratio between heifers and cows for Staph. aureus within udder infections was 0.76 to 1.

In a retrospective study to investigate trends in the bacteria isolated from milk samples obtained over the period 1996 to 2007 in South Africa, milk samples were obtained from 7 of the 9 provinces. The results showed that mastitis have not decreased over the test period. Both mastitis and teat canal infection increased from 2002 to 2006, from 8.1% and 24.1% to 15.4 and 30.0% respectively. The percentage of mastitis-causing pathogens isolated from cows over these years also varied. Previously unknown or almost eradicated pathogens such as α,β haemolytic Staph. aureus, which is thought to be of human origin, Streptococcus agalactiae and Enterococcus canis were responsible for numerous mastitis outbreaks. CNS/NAS were the most frequently isolated bacteria in milk samples from both lactating and dry cows, followed by Staph. aureus and S. agalactiae.

In a study to establish differences in the prevalence of the mastitis pathogens between years and farming systems (TMR versus pasture-based), it was found that CNS/NAS were the most prevalent pathogens in both TMR and PAS dairy farms during 2008 and 2013. Staph. aureus decreased in prevalence between 2008 and 2013, whereas S. agalactiae showed a steep increase in prevalence during the same period, potentially because of localised outbreaks. Streptococcus uberis increased in prevalence, both overall and within affected herds, with the highest prevalence in pasture-based dairy farms during 2013. As a consequence, the results suggest treatment regimes also need to be adapted to maintain efficacy.


Meissner, H.H., 2021. Summary of Research Progress with Mastitis 2015 to 2021. Report to the Milk SA Board, November 2021, Milk SA, Pretoria.