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Volume 32, Number 8—August 2026
Dispatch
Isolation of Infectious Highly Pathogenic Avian Influenza A(H5N1) Virus from Fetal Bovine Serum, United States, 2025
Table
Summary results of further testing of fetal bovine serum samples for highly pathogenic avian influenza A(H5N1) virus, United States, 2025*
| Sample no. | Treatment | Cell culture virus isolation† | Embryonated egg virus isolation‡ | rRT-PCR |
|---|---|---|---|---|
| 046093-25-1 | Not HI | ND | ND | ND |
| 046093-25-2 | Not HI | Detected, Ct 9.50 | ND | ND |
| 046093-25-3 | Not HI | ND | ND | ND |
| 046093-25-4 | Not HI | ND | ND | Detected, Ct 37.78 |
| 046093-25-5 | Not HI | ND | ND | ND |
| 046093-25-6 | Not HI | ND | Detected, Ct 4.73 (UC), Ct 31 (10:1 dilution) | ND |
| 046093-25-7 | Not HI | ND | Detected, Ct 31 (10:1 dilution) | ND |
| 046093-25-8 | Not HI | ND | ND | ND |
| 046093-25-9 | HI | ND | ND | Detected, Ct 39.10 |
| 046093-25-10 | HI | ND | ND | ND |
| 046093-25-11 | HI | ND | ND | ND |
| 046093-25-12 | HI | ND | ND | ND |
| 046093-25-13 | HI | ND | ND | ND |
*Thirteen FBS samples were received (8 bottles of non–HI FBS and 5 bottles of HI FBS). Samples were tested by using virus isolation in cell culture (Vero and Cal-1 cell lines), embryonated eggs, and avian influenza virus rRt-PCR. In embryonated eggs, FBS was inoculated undiluted, diluted 10:1, diluted 10:2, and UC. Ct, cycle threshold; ND, not detected; HI, heat inactivated; rRT-PCR, real-time reverse transcription PCR; UC, ultracentrifuged. †H5N1 considered detected if cytopathic effect observed and cell culture supernatant positive by rRT-PCR. ‡H5N1 considered detected if allantoic fluid positive by rRT-PCR and hemagglutination assay.
1These first authors contributed equally to this article.