When consistently high bulk tank SCC persists due to *Streptococcus agalactiaeinfections, what precise culling and segregation strategy, beyond targeted antibiotic therapy, provides the most rapid and sustained herd-level reduction?

When consistently high bulk tank Somatic Cell Count (SCC) persists due to *Streptococcus agalactiaeinfections, the most rapid and sustained herd-level reduction is achieved through a precise culling and segregation strategy focused on eliminating the primary reservoir: the infected mammary gland. *Streptococcus agalactiaeis an obligate intramammary pathogen, meaning it lives almost exclusively within the udder of infected cows and is highly contagious, spreading primarily during milking.
The initial critical step is the definitive identification of all infected animals. This is best accomplished through individual cow milk culture, which involves collecting milk samples from each quarter of suspected cows (those with high individual SCC records or positive California Mastitis Test results) and culturing them in a laboratory to specifically identify the presence of *Streptococcus agalactiae*. This ensures accurate identification of the source animals, as other mastitis pathogens might also cause high SCC.
Once identified as positive for *Streptococcus agalactiae*, infected cows must be immediately segregated to prevent further transmission within the herd. The most effective segregation method is to consistently milk these culture-positive cows last in the milking parlor. Milking them last ensures that any bacteria shed during milking are not transferred to uninfected cows via shared milking equipment, such as teat cups or clusters, thereby minimizing the risk of new infections. This also allows for thorough cleaning and disinfection of milking units after the infected group has been processed.
Concurrently with segregation, a stringent culling strategy is implemented to permanently remove the source of infection from the herd. Culling, the removal of an animal from the herd, is paramount for *Streptococcus agalactiaecontrol due to its contagious and obligate nature. Specific criteria for culling include: cows that repeatedly culture positive for *Streptococcus agalactiae*, especially those with chronic infections indicated by persistently high individual SCC or those that have shown limited or no response to previous antibiotic therapy. Additionally, cows with extensive mammary gland damage, such as significant fibrosis or multiple infected quarters, are strong candidates for culling as they are unlikely to clear the infection and will remain persistent shedders. Prompt culling of these identified infected animals is crucial for rapid herd-level reduction because it directly removes the infectious load from the population, breaking the chain of transmission and preventing new infections. By systematically eliminating the infected reservoir animals, the pathogen cannot re-establish itself within the herd, leading to a rapid and sustained reduction in bulk tank SCC and, ultimately, the potential eradication of *Streptococcus agalactiaefrom the herd.