Published on 21 July 2025

Studies on the stability of H5N1 influenza viruses in raw milk products

An infection of dairy cows with the H5N1 avian influenza virus was first observed in the USA in 2024. The milk of the infected animals contained large quantities of infectious viruses. In a joint study conducted by Agroscope and the Institute of Virology and Immunology (IVI), the safety of yoghurt and cheese made from raw milk has now been examined. The result: Pasteurisation and the commonly used thermisation procedures reliably inactivate the viruses. However, without thermal pretreatment, some H5N1 strains can ‘survive’ the cheese-making process.

• Why is this study important?

  H5N1 influenza viruses of clade 2.3.4.4b have spread worldwide in recent years causing the death of millions of poultry and wild birds. These H5N1 viruses have also succeeded in repeatedly overcoming the species barrier infecting various mammalian species as well as humans. The atypical replication of these viruses in the mammary glands of lactating dairy cows is particularly concerning as it goes hand in hand with a high viral load in the milk. Studies by American scientists have also shown that laboratory mice have been infected by orally ingesting milk containing the virus. For this reason, we decided to study the stability of H5N1 viruses in cheese and yoghurt made from raw milk.

• Description of the study

  The stability of two H5N1 virus strains at different temperatures and pH values in fresh and fermented cow’s milk was investigated. One H5N1 strain had been isolated from a duck in Hokkaido, Japan, in 2004; the other strain was isolated from the milk of an infected cow in Texas, USA, in 2024.

• What are the main findings?

  - H5N1 influenza viruses remain infectious in bovine milk for four weeks at a temperature of 4 °C.
  - Pasteurisation and thermisation can efficiently inactivate H5N1 viruses in milk
  - Treatment at 50 °C for 30 minutes reduces infectious H5N1 titers only slightly.
  - H5N1 influenza viruses are sensitive to pH-dependent inactivation.
  - In cheese, bovine H5N1 virus is relatively resistant to pH-mediated inactivation.

• What are the next steps?

  The results of the study were considered by the European Food Safety Authority (EFSA) as part of a risk assessment of H5N1 viruses in foodstuffs.

Title and link to publication (International Journal of Food Microbiology)
Impact of pH and temperature in dairy processing on the infectivity of H5N1 avian influenza viruses

Authors
Nicole Lenz-Ajuh, Leonie Rau, Lisa Butticaz, Étori Aguiar Moreira, Bettina Zimmer, Vincent Beuret, Florian Loosli, Jan-Erik Ingenhoff, Barbara Wieland, Gert Zimmer

More Infos
Influenza viruses – a global threat to humans and animals