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H5N1 Avian Influenza is Spreading to Mammals

Avian influenza refers to disease in birds caused by infection with avian influenza Type A viruses. These viruses occur naturally among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species. Wild aquatic birds include ducks, geese, swans, gulls, terns, storks, plovers, and sandpipers.

Avian influenza A viruses are classified into two categories: low pathogenicity avian influenza (LPAI) A viruses, and highly pathogenic avian influenza (HPAI) A viruses. Most avian influenza A viruses are low pathogenic and cause few signs of disease in infected wild birds. Highly pathogenic avian influenza viruses cause severe disease and high mortality in infected poultry. Only some avian influenza A(H5) and A(H7) viruses are classified as HPAI A viruses.

Poultry may become infected with avian influenza A viruses through direct contact with infected wild birds or through contact with surfaces that have been contaminated with the viruses. H5N1 was first detected at a goose farm in Guangdong China in 1996 and then spread to poultry farms across China and Hong Kong in 1997. Crucially, 860 people became infected and half died.

Around 2005, the virus spilled over into migratory birds, which have since spread it across the world in several big waves. A new variant (H5N1 clade emerged in October 2020 in the Netherlands that was better adapted to infect all birds and spread faster and farther than any predecessor.  The European Food Safety Authority reported that more than 58 million birds had died or been culled in 37 European countries since October 2021.

Avian influenza H5N1 arrived in North America by December 2021. Since then, more than 58.3 million wild and domestic birds have been infected across 47 states in the United States. This is the first avian influenza outbreak in the U.S. since 2016. The previous avian flu pandemic lasted 6 months. The current outbreak has already lasted twice that time and killed many more animals.

While avian influenza has primarily infected birds, recent outbreaks have been documented in at least 26 mammalian species including cats, farmed mink, seals, sea lions, farmed blue foxes, wild foxes, and polish cats. Mammalian outbreaks have occurred in 10 countries across three continents.

The UN Food and Agriculture Organization (FAO), the World Health Organization (WHO), and the World Organization for Animal Health (WOAH) have issued a joint statement warning that increased infections of mammals suggested that the virus could adapt to more easily infect humans. So far, human cases have remained sporadic and linked to people with close contact with infected birds. Of 10 reported cases, some have been severe or fatal. The H5N1 viruses isolated from minks have a mutation in the PB2 gene that allows better replication in mammal cells, and samples from minks—and some birds—have extra mutations more commonly seen in human viruses.

Most people have minimal immunity to the hemagglutinin antigen of the current H5N1 virus. So far, the limited number of human cases have shown no mutations linked to resistance to antiviral medications such as oseltamivir or baloxavir. 

To combat the spread of avian influenza, the FAO, WHO, and WOAH recommended preventive measures including vaccination of poultry; enhanced biosecurity; rapid detection and response to outbreaks; improved surveillance in animals and humans; sharing of genetic sequencing data in public databases; collaboration between animal and human health sectors, and effective communication of risks.


Abbasi J. Bird Flu has begun to spread in mammals – Here’s what’s important to know. JAMA Published online February 8, 2023. doi:10.1001/jama.2023.1317

Johns Hopkins Center for Health Security, July 13, 2023

Lisa Schnirring, CIDRAP, July 12, 2023

Lisa Schnirring, CIDRAP, July 13, 2023

Lisa Schnirring, CIDRAP, July 14, 2023

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