Korea VirusResearch Institute

A research team led by Director Young Ki Choi at the Korea Virus Research Institute, Institute for Basic Science (IBS), has identified the cause of the fatal pathogenicity of the H5N1 avian influenza virus circulating in North America in mammals. Pathogenicity refers to the property or ability of a pathogen such as a virus or bacterium to infect a host and cause disease. The H5N1 virus was shown not to remain limited to respiratory infection, but to infect immune cells, spread systemically, and even invade the brain. The researchers confirmed in particular that specific mutations are key factors mediating systemic dissemination and neuroinvasion.

The H5N1 avian influenza viruses reported in North America in 2022 represent a highly pathogenic avian influenza virus with a new genetic constellation characterized by systemic spread and high fatality. Especially since March 2024, outbreaks of H5N1 have been confirmed in dairy cattle in more than ten U.S. states, and viral genetic material has been detected in the milk of infected cows, raising the possibility of milk-borne transmission. During the same period, cases of infection were reported not only in other mammals such as cats but also in humans, heightening concern in the international community. However, the reason why H5N1 adapts well to mammals and causes fatal disease has not been known.

The researchers compared the North American H5N1 virus (GA/W22-145E/22) with the Eurasian lineage virus of the same subtype obtained in Korea (KR/W811/21) and analyzed differences in pathogenicity. In ferret infection model experiments—ferret: a mammal of the Mustelidae family whose respiratory structure is similar to that of humans and which is used as a representative animal model in influenza infection research—the North American strain showed 100% lethality within seven days and caused systemic infection including the brain and lymph nodes. Infection also spread to the mammary glands that secrete milk, confirming the possibility of milk-mediated transmission. By contrast, the Eurasian strain was limited to respiratory infection and exhibited low pathogenicity.

Through single-cell analysis tracking which genes are expressed at the cellular level, the researchers confirmed that the North American virus was detected in various immune cells such as T cells, B cells, and macrophages. Subsequently, infection experiments using viruses engineered to replace two amino acid mutations found in the North American virus (PB2-478I, NP-450N) with the Eurasian amino acids showed that pathogenicity was greatly weakened and infection was restricted to the respiratory tract. In addition, experiments measuring viral replication efficiency confirmed that the two mutations increased the efficiency of viral genome replication, thereby strengthening replication within mammalian cells. This shows that the two mutations are key factors driving mammalian adaptation and enhanced pathogenicity of the H5N1 virus.

Furthermore, to support reports of viral detection in cow’s milk in North America, the researchers infected bovine mammary organoids—organoid: a three-dimensional organ-like structure differentiated from adult or embryonic stem cells under laboratory conditions, mimicking the cellular composition and functions of organs—with the North American H5N1 virus and analyzed its replication pattern. As a result, the North American strain replicated actively in mammary organoid tissue, while replication of the Eurasian strain or viruses in which the mutations were changed to the Eurasian type was limited.

The researchers also used a lactating ferret infection model to verify the possibility of milk-borne transmission. Viral detection in the brains of infected mothers confirmed systemic dissemination, and viruses replicating in the mammary glands were actually observed to be transmitted to offspring through milk. In contrast, viruses without the mutations did not show such dissemination. These results scientifically explain the outbreaks in dairy cattle and concerns over milk-mediated transmission reported in North America and are expected to provide important clues to understanding the risk of cross-infection between humans and livestock in the future.

Dr. Young Il Kim, who led the study, said: “We confirmed that the H5N1 virus can infect immune cells, spread systemically, and invade the nervous system. This demonstrates a new pathological mechanism in which the virus turns the body’s defense system itself into a route of dissemination.” He added, “These results will serve as an important foundation for the development of therapeutic strategies targeting the systemic dissemination pathway of the virus and for preparing strategies against zoonotic viruses.”

Director Young Ki Choi stated: “This study is a result that elucidated, at the molecular level, how specific mutations enhance the adaptability and pathogenicity of the virus. Since highly pathogenic avian influenza is a potential threat of a new pandemic, we will continue to build a knowledge base through fundamental virus research that enables proactive preparedness for infectious disease crises facing humanity.”

The results of this study were published on September 27 in Science Advances (Impact Factor 12.5), a leading multidisciplinary journal published by the American Association for the Advancement of Science (AAAS).