26 June 2026
The journal Eurosurveillance has published a detailed report on the investigation of H5N1 influenza antibodies detected in a Dutch dairy farm. The incident, which unfolded in January 2026, initially centered on rumors of antibodies in just a single cow, however, the official report reveals a much broader scope.
The timeline began in December 2025, when a four-month-old domestic cat residing on a dairy farm in the Netherlands fell ill and tested positive for the highly pathogenic avian influenza (HPAI) H5N1 virus, specifically clade 2.3.4.4b and genotype DI.2.1. This concerning finding immediately triggered a comprehensive epidemiological investigation of all livestock on the premises.
While direct virological testing via RT-qPCR on milk and tissue samples yielded no traces of active viral RNA, the serological profile painted a starkly different picture:
- Dairy Cows (Sera): Specific H5N1 antibodies were identified in the sera of 34 out of 72 dairy cows (47.2%).
- Dairy Cows (Milk): Antibodies were detected in the milk of 14 out of 70 cows (20.0%).
- Heifers and Calves (1–2 years old): The highest exposure rate was recorded in this cohort, with antibodies detected in 24 out of 38 animals (63.2%).
To ensure utmost accuracy, researchers utilized a combination of advanced laboratory assays. Initial screening was conducted using an NP-ELISA (which detects antibodies against the viral nucleoprotein), followed by a Virus Neutralization Test (VNT). The VNT is widely considered the gold standard confirmatory assay because it physically measures the ability of serum antibodies to neutralize live virus in a cell culture. The VNT assays successfully confirmed nearly all of the initial positive results.
The investigation further revealed that the infection swept through the herd almost entirely sub-clinically. Aside from a single dairy cow that developed clinical mastitis in all four quarters in December 2025, the farmer reported no distinctive signs of illness or production drops across the herd.
The high seroprevalence rate suggests that the virus spread widely and silently, pointing to either a massive initial environmental exposure or highly efficient cow-to-cow transmission. Researchers hypothesize that the cattle were exposed via environment or feed contaminated by migratory wild birds, which were present in large numbers in the surrounding pastures during those months.
Ultimately, the study demonstrates that serum antibody testing is a more sensitive and reliable tool for long-term serological surveillance in cattle compared to milk testing, as antibody concentrations remain significantly higher and more sustained in blood.
Importantly, all members of the farmer's family were tested and found to be negative for the virus.
Ballmann et al. (2026) Detection of antibodies against avian influenza in European dairy cattle, the Netherlands, January 2026. Eurosurveillance. 2026;31(25):pii=2600464
