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H5N1 Avian Influenza (Bird Flu)

H5N1 avian influenza (bird flu) refers to the disease caused by infection with avian influenza Type A viruses. These viruses naturally spread among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species. Bird flu viruses do not normally infect humans. However, sporadic human infections with bird flu viruses have occurred.

H5N1 bird flu is widespread in wild birds worldwide and is causing outbreaks in poultry, dairy cows, and occasionally other mammals.  A multi-state outbreak of H5N1 in dairy cows was first reported in March 2024. The U.S. Department of Agriculture has confirmed H5N1 in dairy herds in 15 states as of November 2024. This is the first time these bird flu viruses were found in cattle.

There have been 55 human cases of H5 in the United States during 2024:

  • 32 human cases after exposure to dairy cows (Texas [1], Michigan [2], Colorado [1], and California [28])
  • 21 human cases after exposure to poultry (Colorado [9], Oregon [1], and Washington [11])
  • 2 human case with no immediately known animal exposure (California [1] and Missouri [1])

NOTE: 1 additional case was previously detected in a poultry worker in Colorado in 2022.

While the current public health risk is low, the Centers for Disease Control and Prevention is watching the situation carefully and working with states to monitor people with animal exposure.

Wild aquatic birds, especially dabbling ducks, are considered reservoirs (hosts) for avian influenza A viruses. Wild aquatic birds can be infected with avian influenza A viruses in their intestines and respiratory tract, but some species, such as ducks, may not get sick. However, avian influenza A viruses are very contagious among birds, and some of these viruses can sicken and even kill certain domesticated bird species, including chickens, ducks, and turkeys.

In addition to cattle, cats and small birds, such as pigeons and blackbirds, have tested positive on affected dairy farms. While thought to be rare, the recent April 2024 influenza A(H5N1) bird flu virus in a human is the first instance of likely mammal to human transmission.