Denmark’s dairy herd health data reveals persistent Streptococcus agalactiae challenge despite advanced surveillance

Despite comprehensive data systems and mandatory reporting, Denmark faces ongoing issues with Streptococcus agalactiae in its dairy herds, highlighting the limitations of bulk tank milk testing and the challenge of infection relapse.

Denmark’s dairy sector has built one of the most detailed herd-health data systems in industrial farming, yet Streptococcus agalactiae remains a stubborn problem. According to the Danish analysis, near-universal use of the Dairy Management System and mandatory veterinary reporting give the country unusually strong visibility on mastitis and antimicrobial use. Even so, the share of positive herds has risen since the early 2000s, after an eradication scheme from the 1950s was replaced by passive surveillance.

The new assessment found that bulk tank milk testing is useful but not perfect. In a 2025 study of 2,029 herds, the bulk tank approach missed some infected units later identified through cow-level samples, implying sensitivity of 85.8%. The authors argue that low within-herd prevalence and intermittent shedding can leave infections hidden.

They found little evidence that dry-cow therapy or automatic milking systems are driving the rise. Infection rates were broadly similar in robotic and conventional herds, while dry-cow treatment volumes have shifted in composition rather than expanded enough to explain the trend.

The strongest signal was relapse: herds recently cleared of S. agalactiae were far more likely to become positive again within six months. That points to persistent infection, reintroduction through animal movement, or other management factors. Earlier Danish research has similarly linked eradication to culling, mastitis protocols and transmission at milking.

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