Inactivation of Avian Influenza Virus in Raw Milk Kefir

Abstract

Both highly pathogenic avian influenza virus (HPAIV) and its viral RNA have been detected in raw milk during the ongoing outbreak in dairy cows. While pasteurization effectively inactivates the virus in milk, comparatively little has been published on AIV inactivation in fermented dairy products made from raw milk. We evaluated the viability of two isolates of low pathogenic avian influenza virus (LPAIV) during fermentation of kefir to assess the potential for viral persistence. Raw (unpasteurized) milk (ca. 3.0% fat) was inoculated with ca. 7.5 log 10 CFU per mL of active commercial kefir starter culture. Next, raw milk (125 mL) was inoculated with 1.0 mL of either strain A/rgGyrfalconHAxPR8/2014 H5N1 or A/turkey/Wisconsin/1968 H5N9 to achieve an average initial level of ca. 5.0 log 10 50% egg infectious doses (EID 50 ) per mL. The inoculated raw milk (ca. pH 6.55 ± 0.05) was fermented at 25 °C for ca. 36 h to an average endpoint pH of pH 4.44 ± 0.05. Results confirmed that fermentation of raw milk to produce kefir delivered an appreciable reduction of both LPAIV strains, that being a ca. 4.1 or 3.1 log 10 EID 50 per mL decrease of strains H5N1 or H5N9, respectively, from their initial levels in raw milk. In the absence of a starter culture, due to fermentation by the indigenous flora the pH of the raw milk after incubation at 25 °C for ca. 36 h also significantly decreased on average to ca. pH 5.3:, and levels of strains H5N1 and H5N9 were reduced by 4.1 and 3.5 log 10 EID 50 per mL, respectively. Our findings established that fermentation of raw milk (with or without added kefir starter culture) to ≤ pH 5.3 over ca. 36 h at 25 °C appreciably decreases titers (≥ 3.1 log 10 EID 50 per mL) of AIV strains H5N1 and H5N9 and, therefore, lowers risk associated with its consumption.

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