Streptococcus agalactiae in dairy cows – that`s easy to handle, right?

“That`s easy to handle, right?” is the usual response to the fact that Denmark probably holds the record in Streptococcus agalactiae identified positive dairy herds in the industrialized world in herds shipping milk. But before we dive into the more specific aspect of S. agalactiae, understanding the prevalence requires some general background on the structure of the data available in Denmark.

Data availability for management purpose

Because nearly 100% of Danish dairy farms use the Dairy Management System (DMS®) provided by SEGES Innovation, we rely on this system as the central data platform. The software functions as the farmer’s primary management tool, where all cow-related events are recorded, many of which are even mandatory to record. The DMS® is integrated with multiple external systems, including but not limited to; IA services, dairy processors, abattoirs, veterinary service, DHI, and feeding systems. The DMS® provides a wide range of action lists, reports, analytical tools, Power BI, AI, and decision-support functions for herd management, which the farmer uses daily as their one-stop shop for farm information.

It’s mandatory for dairy farmers to record data continuously, and veterinarians automatically transfer data from their own recording systems each night. Herds can only access antimicrobials through a herd health contract with a designated dairy veterinarian. This veterinarian is solely responsible for prescribing medicine, transmitting prescriptions to the pharmacy, and ensuring that consumption-data is reported to the national database VetStat as well as to the central national cattle database. These registrations are mandatory by Danish legislation. The data are recorded at the herd number and are subsequently available within the DMS® system, allowing farmers, veterinarians and the authorities to monitor antimicrobial usage.

The herd health contract requires dairy farmers to record each treatment as part of the herd diagnoses to have access to treatment. For example, during mastitis treatment, the farmer records the cow identification number, selects the relevant herd diagnosis, and ticks a box in the system. The DMS® software then automatically logs the treatment details according to the protocol, such as the use of intramammary tubes and Non Steroid Anti Inflammatory Drug (NSAID). These recordings form the basis for all subsequent reports, analyses, and decision-support tools and AI which incentivize accurate and timely data entry.

Some farmers attempt to misreport data; however, this represents a minor proportion. Dairy farmers have learned the benefits of systematic recording clearly outweigh the effort required. The system provides extensive opportunities for monitoring and supports evidence-based decision-making rather than reliance on subjective judgment.

Key indicators of Danish milk production based on data from the DHI database (January 2026) are as follows:

• We enroll 94% of all dairy cows in the DHI system.
• The average herd size is 280 cows, including dry cows.
• The average milk production is 11.935 kg of energy corrected milk.
• Approximately 35% of herds use robotic milking systems.
• The average bulk tank somatic cell count (BMTSCC) is 171,000 cells/mL, with 96.6% of recorded milk delivered for processing.
• The new infection rate is 10% for lactating cows.
• The incidence of treated clinical mastitis is 0.17 cases per cow per year.

Where does this lead us?

We have a robust, integrated recording system that provides a more comprehensive and reliable picture of herd- and country-level dynamics than what is typically available elsewhere. The combination of mandatory reporting, centralized data handling, and continuous data flow enables consistent monitoring and evidence-based interpretation.

In contrast, conference presentations from other countries often rely on selectively reported indicators or incomplete datasets – for example, citing national BMTSCC values or claiming a low occurrence of S. agalactiae without acknowledging the absence of systematic surveillance. Without structured, mandatory, and independently managed data collection, such conclusions are inherently limited.

From a practical perspective, when working directly with dairy farmers, it becomes evident that conditions such as S. agalactiae are likely to be underreported in systems lacking mandatory surveillance and third-party data management.

Historical dynamics of S. agalactiae

An eradication program targeting S. agalactiae was established in Denmark in the 1950s by the Danish Dairy Board in response to its zoonotic potential. We discontinued this program in early 2000s and replaced it with a mandatory passive surveillance system. At the same time, we lifted the ban on the sale of adult animals from infected herds due to increased demand for dairy cattle and the assessment that the remaining infected herds had a negligible impact on pathogen transmission. However, since abandoning the eradication program the prevalence of S. agalactiae in Denmark has been increasing as illustrated in Figure 1.

Monitoring for S. agalactiae

All dairy processors submit payment milk samples to one single accredited laboratory in Denmark. Within this system, SEGES Innovation are responsible for operating the surveillance program and mandating biannual bulk tank milk (BTM) testing. The analysis is based on the DNA Diagnostic Mastitis 4® PCR assay performed by Eurofins Milk Testing Denmark.

All BTM samples with a Ct value < 40 are classified as positive, and SEGES record the herd status in a national online open-access registry. If a herd that is registered as free of S. agalactiae have a positive BTM sample, we analyze two additional BTM samples collected 8–10 days apart to verify the result. If one or both samples test positive, we change the herd status to infected.

Due to concerns regarding the sensitivity of testing on BTM only with the inherent risk of dilution, we expanded the criteria in 2021. We now also include PCR testing of samples from subclinically infected cows prior to dry-off, particularly in cows where antimicrobial treatment is planned. Accordingly, if a cow-level PCR test yields a Ct value < 30, or if a veterinarian reports a S. agalactiae positive clinical or subclinical quarter milk sample to the national database in a herd previously classified as free of S. agalactiae, we change the herd status to infected. In Denmark it’s also mandatory to analyze and report quarter milk sample result from mastitis cases treated with antimicrobials to a central database.

Sensitivity in the surveillance program of S. agalactiae

In 2025 we analyzed a total of 2,029 dairy herds in a study, of which 739 herds had individual cow samples collected within one month before or after the BTM sampling date. This overlap allowed for direct comparison between herd-level BTM results and individual cow-level test outcomes. S. agalactiae was detected in 97 of the 739 herds based on BTM analysis, while 642 herds tested negative. Among these BTM-negative herds, S. agalactiae was subsequently identified in 16 herds through individual cow samples analyzed either by PCR or bacteriological culture, indicating the presence of undetected infections at the herd level.

When these 16 herds were considered false negatives, the estimated sensitivity of the BTM-based surveillance method for detecting S. agalactiae in Danish dairy herds was 85.8%. This result suggests that while BTM surveillance provides a robust and efficient screening approach, a considerable proportion of infected herds may remain undetected due to low within-herd prevalence or intermittent bacterial shedding. Such limitations are consistent with previous findings that herd-level sensitivity depends on the proportion of infected cows and bacterial load in the milk sample (Skarbye et al., 2020).

Table 1. Detection of S. agalactiae in bulk tank milk (BTM) samples and individual cow samples during spring 2025.

Obtaining free status again can be done in two ways

BTM Sampling:
The milk processor collects four BTM samples over a 30 to 45-day interval. All samples must test negative for S. agalactiae using PCR analysis, with cycle threshold (Ct) values of 40, indicating no detectable bacterial DNA.

Individual animal testing:

• Quarter or composite milk samples must be obtained from all lactating cows on the same day.
• In addition, all dry cows and cows recently treated with antimicrobials were sampled 5–7 days post-calving or following the designated withdrawal period.
• All samples must test negative on PCR Ct 40/culture

What is the risk of re-entering the positive group of herds after this process of obtaining free status?

Herds that have recently belonged to the S. agalactiae positive group have a substantially higher risk of returning to the S. agalactiae positive group within 6 months compared to herds that have been free for a longer period. In 2017, the change in status between bi-annual testing showed that S. agalactiae positive herds had a relapse rate of 17.6% (27 out of 153 herds), whereas S. agalactiae negative herds had a corresponding proportion of farms that turned positive of approximately 2% (about 40 out of 2,000 herds per half year).

This implies that recently cleared herds have a roughly 8–9 times higher risk of becoming positive within six months. Overall, the results indicate a clearly elevated short-term risk of relapses following exit from the S. agalactiae group. Alternatively, the results reveal that the criteria for changing from positive to negative status are too inaccurate and thereby leading to erroneous status of completed eradication.

Can the prevalence of S. agalactiae be related to the Danish selective dry cow therapy approach?

The development of dry-cow therapy (DCT) from 2015 to 2024 can be described as progression through three main phases, as illustrated in Figure 2.

From 2015 to 2018, total DCT use increased steadily from approximately 0.23 to 0.29 doses per cow per year. This growth was driven by both beta-lactamase-sensitive penicillins and other penicillins, reflecting a general expansion in dry-cow treatment activity.

In 2019–2020, a clear structural shift occurred. The use of beta-lactamase-sensitive penicillins dropped sharply, almost disappearing, while other penicillin’s – particularly cloxacillin – increased markedly and compensated for this decline. Despite this substitution, overall DCT use continued to rise, reaching a peak of around 0.30–0.31 doses per cow per year during this period.

From 2021 onwards, DCT use declined slightly and stabilized at a somewhat lower level, fluctuating between approximately 0.26 and 0.29 doses per cow per year. During this phase, other penicillins remained the dominant antimicrobial class. Beta-lactamase-sensitive penicillins reappeared but at lower and more variable levels than before 2019, while other antimicrobial classes, such as cephalosporins, contributed consistently at a smaller scale.

Overall, DCT use increased until 2020, followed by a modest decline and stabilization. The most notable long-term change is not in the total volume of use, but in the sustained shift in antimicrobial composition, with other penicillins becoming the primary treatment choice after 2019. At the same time the prevalence of S. agalactiae positive herds has increased, therefore the increase has not been driven by more limed use of DCT at dry-off.

Can the dynamics of S. agalactiae be related to the milking system?

In 2025, we made an analysis based on DMS recordings to identify potential association between milking system and S. agalactiae. To be included in the dataset, herds had to meet the following criteria throughout the entire period:

1. No change in S. agalactiae infection status.
2. No change in milking system.

In total, 1,692 milk-delivering herds met the inclusion criteria. Of these, 610 used one of the four automatic milking brands on the Danish marked. The proportion of herds with an official positive S. agalactiae status varied slightly between milking systems (Table 2 and 3). The proportion of infected herds was 12.6% in the automatic milking system group and 11.6% in the group with conventional milked systems, indicating no substantial difference in infection prevalence between the milking systems.

Table 2. Streptococcus agalactiae status between automatic milking system (AMS) and conventional milking.

However, when comparing within automatic milking system -brands, infection rates ranged from 11.5% to 21.4%. But the number of herds using Brand 3 and Brand 4 was small (14 and 13 herds, respectively). Therefore, observed differences should be interpreted with caution.

For herds with a positive S. agalactiae status, the average herd size, dry-off treatment rate, somatic cell count (SCC), and bacterial count were assessed. Conventional herds had on average larger herd sizes (425 cows) compared with automatic milking system herds (231 cows).

Table 3. Descriptive statistics between automatic milking system and conventional milking.

What are the main obstacles in control and eradication of S. agalactiae?

• Because of environmental constraints we cannot have a substantial culling rate of lactating cows and stay profitable, therefore most dairy farms have limited access to replacement heifers.
• We are challenged by limited access to diagnostic support with quick turnaround and high sensitivity of analysis.

Discussion

The Danish data highlights that S. agalactiae remains a significant and increasing challenge in Danish dairy production based on a highly advanced and integrated surveillance system. The Danish context is unique due to its near-complete data coverage, mandatory reporting, and centralized databases, which provide a level of transparency and analytical depth that is rarely achieved internationally. This makes Denmark particularly well suited for evaluating true herd-level prevalence and epidemiological patterns.

The observed increase in the proportion of S. agalactiae-positive herds over the past decade cannot be explained by limitations in data quality or surveillance sensitivity alone. Although the BTM surveillance system demonstrated a relatively high sensitivity of 85.8%, the identification of false-negative herds confirms that low within-herd prevalence and intermittent bacterial shedding remain important diagnostic challenges. This suggests that the true prevalence may be underestimated, even in a robust system.

Importantly, the findings indicate that neither changes in dry cow therapy (DCT) practices nor the adoption of automatic milking systems automatic milking system appear to be primary drivers of the increasing prevalence. The introduction of single-cow samples to the eradication program in 2021 hampers discrimination between the observed rise in S. agalactiae-prevalence since 2021 and the change in surveillance-methodology.  However, up until 2019 there has been a steady rise in both DCT and S. agalactiae herd-prevalence which robustly indicates that the DCT-regime is not the primary explanation of the S. agalactiae situation in Denmark.

Likewise, the lack of meaningful differences in infection prevalence between conventional and automatic milking systems suggests that milking technology alone is not a decisive risk factor.

Instead, the results point toward more complex epidemiological dynamics. The markedly higher relapse rate in recently cleared herds (approximately 8–9 times higher than long-term negative herds) indicates that elimination of infection at herd level is difficult to obtain/sustain. This may reflect incomplete clearance of infection, persistence in subclinical carriers, or reintroduction through animal movement, management practices or even cases of anthroponoses with farm-employees acting as a reservoir. The unpredictable shedding patterns and the weak association between infection and somatic cell count further complicate detection and control, as infected cows may remain unnoticed within the herd.

Structural and practical constraints also play a critical role. Limited opportunities for extensive culling, combined with restricted access to rapid and high-quality diagnostic testing, reduce the effectiveness of traditional control strategies. In this context, the shift from an eradication program to passive surveillance appears to have contributed to the gradual re-emergence of the pathogen.

Finally, the Danish experience raises important questions about international comparisons. In countries without mandatory surveillance and centralized data systems, the prevalence of S. agalactiae may be substantially underreported. This underscores the importance of standardized, high-quality data collection when assessing and comparing udder health across regions.

Conclusion

In conclusion, S. agalactiae remains a persistent and growing challenge in Danish dairy herds despite comprehensive surveillance and management systems. The increase in prevalence cannot be attributed to changes in DCT or milking technology but is more likely driven by complex infection dynamics, limitations in detection, and structural constraints in herd management.

The high relapse rate in recently cleared herds highlights the difficulty of achieving sustained freedom from infection and suggests that current control strategies may be insufficient. Future efforts should focus on improving diagnostic sensitivity, strengthening biosecurity—particularly around animal movement—and developing more effective herd-level eradication protocols.

The Danish data infrastructure provides a valuable model for evidence-based herd health management, but it also reveals that even the most advanced systems cannot fully compensate for biological and practical challenges associated with S. agalactiae. Addressing these challenges will require a combination of improved surveillance, targeted interventions, and potentially renewed consideration of coordinated control or eradication strategies at the national level.

References

• DANMAP. DANMAP 2024 – Use of antimicrobial agents and occurrence of antimicrobial resistance in bacteria from food animals, food and humans in Denmark. Copenhagen, Denmark: National Food Institute, Technical University of Denmark and Statens Serum Institut; 2025. Available at: https://www.danmap.org
• DANMAP. DANMAP 2024 – Use of antimicrobial agents and occurrence of antimicrobial resistance in bacteria from food animals, food and humans in Denmark. Copenhagen, Denmark: National Food Institute, Technical University of Denmark and Statens Serum Institut; 2025. Available at: https://www.danmap.org

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