Detection of highly pathogenic avian influenza A(H5N1) virus, clade 2.3.4.4b, genotype B3.13 in placentas from aborted bovine fetuses.

Infection by highly pathogenic avian influenza (HPAI) A(H5N1) virus (family Orthomyxoviridae, taxon species Alphainfluenzavirus influenzae), clade 2.3.4.4b, genotype B3.13, was first diagnosed in dairy cattle in the United States in March 2024. Here, we present a cluster of bovine abortions in whose placentas HPAI A(H5N1) was detected. Only placentas were submitted in all 5 cases received. Grossly, 3 placentas had multifocal edema; no significant gross abnormalities were observed in the other 2 placentas. Microscopically, one placenta had random areas of mineralization and another placenta had mild neutrophilic placentitis. Reverse-transcription quantitative real-time PCR for influenza A virus and avian influenza A virus subtype H5N1 were positive in all 5 placentas. RNAscope in-situ hybridization for matrix protein gene was positive in one placenta. Routine ancillary tests for bovine abortion diagnostic work-up did not detect any other abortifacient pathogens. We speculate that abortion was caused by the HPAI A(H5N1) virus.