Interference of Streptococcus agalactiae Blitz Therapy in Staphylococcus aureus Microbiological Diagnosis in Subclinical Bovine Mastitis.

Simple Summary: In this study, we looked to evaluate the variation in sensitivity, specificity, predictive values, and accuracy (Kappa index) of the microbiological diagnosis for Staphylococcus aureus in a dairy cattle herd subjected to blitz therapy to eradicate Streptococcus agalactiae. The studied dairy herd had an average of 160 lactating Holstein cows and microbiological diagnosis was carried out on all lactating cows for 5 consecutive months. After treating the animals that presented a pure culture of S. agalactiae in microbiological diagnosis in the first and second milk sample collections, isolation of S. aureus was observed in these animals in the following milk sample collections, increasing the sensitivity of the microbiological diagnosis. The results indicate that due to the release of S. agalactiae being greater than that of S. aureus, due to competition in a nutritive culture medium, it was not possible to isolate S. aureus in the first collections. However, after the elimination of S. agalactiae through intramammary antimicrobial treatment, S. aureus began to be identified in the microbiological examination as there was no longer competition between the pathogens in the nutritive culture medium. Bovine mastitis is an important and costly disease to dairy cattle. Diagnostic methods usually performed in Brazil are somatic cell counts (SCC) and milk microbiology. Low bacteria shedding in milk implies no colony growth in microbiological tests and false negative results. Streptococcus agalactiae and Staphylococcus aureus are principal pathogens of mixed mastitis. However, S. agalactiae has a higher bacterial release from the mammary gland than S. aureus, affecting microbiological sensitivity to diagnose S. aureus. This study aimed to estimate the SCC and total bacterial count (TBC) from cows according to pathogen isolated in milk and to evaluate variation in S. aureus diagnosis by a microbiological test during S. agalactiae treatment, which is called blitz therapy. Both S. agalactiae and S. aureus presented high SCC means, although S. agalactiae showed shedding of bacteria 2.3 times greater than S. aureus. Microbiological sensitivity to S. aureus increased for 5 months during S. agalactiae treatment. The prevalence of S. agalactiae fell after 5 months of therapeutic procedures. The prevalence of S. aureus increased to 39.0. The results showed that due to high sensitivity, the polymerase chain reaction (PCR) could be used at the beginning of blitz therapy with the goal of S. agalactiae eradication from the dairy herd. [ABSTRACT FROM AUTHOR]