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.