09 July 2026
In February 2025, researchers at the Cornell Animal Health Diagnostic Center (AHDC) Virology Laboratory detected and isolated live highly pathogenic avian influenza (HPAI) A(H5N1) virus (clade 2.3.4.4b, genotype B3.13) from a commercial lot of fetal bovine serum (FBS) during routine safety screening. This unprecedented discovery highlights a novel biosafety risk in bovine-derived biological products. This study reveals that the virus can penetrate deeper into the bovine systemic circulation, occasionally breaching the placental barrier to reach the fetus.
The virus was uncovered during routine adventitious agent testing, a regulatory requirement for animal-origin biologics, on a pooled FBS lot collected from five states: California, Kansas, Nebraska, Pennsylvania, and South Carolina. The Virology Laboratory inoculated Vero cells and primary fetal bovine kidney (FBK) cells with media supplemented with 15% of the test FBS. By day 7, a distinct cytopathic effect (CPE), was observed in the Vero cells. Follow-up testing via real-time reverse transcription PCR (rRT-PCR) and embryonated chicken egg (ECE) inoculation demonstrated a very low viral load within the lot. Despite this low concentration, the researchers successfully isolated viable, active virus. Spurred by these findings, the team screened 768 individual fetal bovine serum samples collected from slaughterhouses in California and Texas. One sample tested positive via rRT-PCR, providing solid evidence that sporadic maternal viremia allows the virus to access various tissues, including the fetus. Fetal bovine serum (FBS) is harvested from the blood of bovine fetuses following the slaughter of pregnant cows in the meat industry. FBS is the most widely utilized growth supplement for cell culture media due to its rich concentration of growth factors, and proteins necessary for cell proliferation.
Isolation of Infectious Highly Pathogenic Avian Influenza A(H5N1) Virus from Fetal Bovine Serum, United States, 2025. Emerging Infectious Diseases, Volume 32 Number 8 - August 2026
