In March 2024, highly pathogenic avian influenza (HPAI) virus of A(H5N1) clade 2.3.4.4b was detected in mammalian livestock, mostly dairy cattle, in the United States and has now been reported in dairy herds in nine U.S. states.1 High titers of infectious virus and genome copies of HPAI A(H5N1) virus have been found in milk from these infected dairy cows.2 Molecular testing has also revealed the presence of HPAI A(H5N1) genetic material in approximately 20% of the samples obtained from retail pasteurized milk products, but researchers have been unable to culture virus from these samples.3 In this study, we measured the stability of HPAI A(H5N1) virus in raw milk at 63°C and 72°C, the temperatures most commonly used in commercial pasteurization.4
We diluted HPAI A(H5N1) virus A/mountain lion/MT/1/2024 (clade 2.3.4.4b; Global Initiative on Sharing All Influenza Data [GISAID] accession number, EPI_ISL_19083124) in raw (unpasteurized) cow’s milk to 106 50% tissue-culture infectious doses (TCID50) per milliliter of medium. We heat-treated the milk in a temperature-controlled heat block at 63°C and 72°C. We quantified infectious virus by means of end-point titration in Madin–Darby canine kidney cells, using 1:10 serial dilutions. HPAI A(H5N1) genome copies were quantified by quantitative reverse-transcriptase–polymerase-chain-reaction (qRT-PCR) assay.