MSK Library Guides: Avian Influenza: H5N1 in Dairy Cattle

Latest Updates

APHIS Confirms D1.1 Genotype in Dairy Cattle in Nevada
H5N1 D1.1 has successfully jumped to cattle. This is the first detection of this virus genotype in dairy cattle (all previous detections in dairy cattle have been HPAI H5N1 clade 2.3.4.4b, genotype B3.13). Genotype D1.1 represents the predominant genotype in the North American flyways this past fall and winter and has been identified in wild birds, mammals, and spillovers into domestic poultry. This genotype is already highly optimized for cross-species adaptation. Selective pressure for mammalian adaptation is intensifying.

Current H5N1 Bird Flu Situation in Cattle

Data Source: USDA-APHIS HPAI Detections in Livestock

U.S. National Milk Testing Strategy

The National Milk Testing Strategy (NMTS) facilitates comprehensive H5N1 surveillance of the Nation’s milk supply and dairy herds. It provides a strategy to identify which States and herds within them are affected with H5N1.

Specifically, the NMTS gives States a five-stage roadmap to demonstrate the elimination of this virus from their dairy herds. It is designed to:

Increase USDA’s and public health partners’ understanding of where the virus is present in the United States,
Support the rapid implementation of enhanced biosecurity measures to decrease the risk of transmission to other livestock, and
Inform critical efforts to protect farmworkers to help lower their risk of exposure.

Current Status by State

Last Updated: February 7, 2025

The map provides State status and testing and surveillance activities for each State in the contiguous United States for the NMTS.

Background of Current Outbreak

A multi-state outbreak of HPAI A(H5N1) bird flu in dairy cows was first reported on March 25, 2024. This is the first time that these bird flu viruses had been found in cows.

On April 1, CDC confirmed one human HPAI A(H5N1) infection in a person with exposure to dairy cows in Texas that were presumed to be infected with the virus. This is thought to be the first instance of likely mammal to human spread of HPAI A(H5N1) virus. In May 2024, CDC began reporting additional, sporadic human cases in people who had exposure to infected dairy cows.

That latest human case counts are available at H5N1 Bird Flu: Current Situation Summary | Avian Influenza (Flu) (cdc.gov).

H5N1 in Mammals

While rare, mammals can be infected with highly pathogenic avian influenza (HPAI) A(H5N1) (“H5N1 bird flu”) viruses. Reports of these sporadic infections in mammals have occurred globally amid widespread outbreaks of bird flu infections in wild birds and poultry.

Mammals can be infected with H5N1 bird flu viruses when they eat infected birds, poultry, or other animals and/or if they are exposed to environments contaminated with virus. Spread of H5N1 bird flu viruses from mammal to mammal is thought to be rare, but possible.

In the United States, since 2022, USDA APHIS has reported HPAI A(H5N1) virus detections in more than 500 mammals.