Avian influenza virus type A (H5N1) in U.S. dairy cattle

Last updated: June 17,  2025
On March 25, 2024, the U.S. Department of Agriculture (USDA), Food and Drug Administration (FDA), and Centers for Disease Control and Prevention (CDC) announced that highly pathogenic avian influenza (HPAI), specifically avian influenza virus type A (H5N1), had been identified in U.S. dairy cattle for the first time. Here are important details on this rapidly evolving situation:

• Avian influenza virus type A (H5N1) has been confirmed in dairy cattle in 17 states: 767 herds in California, 107 in Idaho, 64 in Colorado, 31 in Michigan, 29 in Texas, 13 each in Iowa and Utah, 11 in Nevada, nine each in Minnesota and New Mexico, seven in South Dakota, four each in Kansas and Arizona, two in Oklahoma, and one each in North Carolina, Ohio, and Wyoming. Get updates on detections here and background information here.
• Tests so far indicate that the virus detected in dairy cattle is H5N1, Eurasian lineage goose/Guangdong clade 2.3.4.4b. This is the same clade that has been affecting wild birds and commercial poultry flocks and that has caused sporadic infections in several species of wild and domestic mammals in the United States.
• Until early 2025, all detections of the virus in dairy cattle had been H5N1 clade 2.3.4.4b genotype B3.13. However, a second genotype—D1.1—has since been confirmed by whole genome sequencing in dairy cattle in Nevada and Arizona. The D1.1 genotype represents the main genotype in North American flyways this past fall and winter and has been identified in wild birds, mammals, and spillovers into domestic poultry.
• Common clinical signs in affected cows include low appetite, reduced milk production, and abnormal appearance of milk (thickened, discolored). Lactating cows have been most highly affected.
• While avian influenza virus type A (H5N1) is associated with high morbidity and mortality in birds ("highly pathogenic"), this hasn't been the case for dairy cattle. Most affected animals reportedly recover with supportive treatment, and the mortality/culling rate has been low at 2% or less on average.
• The spread of H5N1 within and among herds indicates that bovine-to-bovine spread occurs. Overall, animal movement is a recognized risk for disease transmission. Evidence also indicates that the virus can spread from dairy cattle premises to nearby poultry facilities. Potential risk factors for local transmission between facilities include shared personnel between premises; shared, uncleaned vehicles and equipment between premises; and frequent visitors on/off premises.
• Biosecurity remains the best defense against H5N1, and farms are encouraged to practice rigorous biosecurity even if the virus has not been detected in their state or vicinity. The USDA also urges veterinarians and producers to monitor for, separate, and test sick animals; minimize cattle movements; and isolate and monitor any newly received dairy cattle for 30 days upon arrival.
• Since April 29, 2024, a federal order has been in effect, requiring testing of lactating dairy cattle for H5N1 prior to interstate movement, and reporting of positive nucleic acid detection and serology results for livestock to the USDA’s Animal and Plant Health Inspection Service (APHIS). While the movement restriction initially applies to lactating dairy cows, this may be adjusted based on an evolving scientific understanding of the disease and risks. Find answers to frequently asked questions about the order here and guidance for producers and veterinarians here.
• A second federal order went into effect December 6, 2024, requiring that raw (unpasteurized) milk samples be collected from milk silos at dairy processing facilities nationwide and shared with the USDA for testing as part of its National Milk Testing Strategy (NMTS). The order is intended to complement the April federal order by facilitating comprehensive H5N1 surveillance of the nation’s milk supply and dairy herds. It also requires that herd owners with positive cattle provide epidemiologic information and that private laboratories and state veterinarians report positive results of NMTS testing to the USDA. Testing, which is being phased in, began on December 16, with 45 states currently participating. The D1.1 outbreaks in Nevada and Arizona were identified through silo testing prior to affected cattle developing clinical signs, providing evidence that the NMTS is working. Find updates on NMTS testing here.
• States also have issued restrictions on the importation of dairy cattle, and dairy cattle marketers are encouraged to check with state agriculture officials for current restrictions. State-specific restrictions on cattle movement must be followed in addition to federal requirements. If and where a particular state’s requirements are more restrictive than the federal order, that state’s requirements must be followed.
• California, which is the nation’s top milk-producing state, declared a state of emergency in December 2024 to bolster the state’s preparedness and accelerate cross-agency response efforts.
• Canada has tightened import requirements on dairy cattle from the United States.
• The USDA is providing financial assistance for producers with H5N1-affected premises to improve on-site biosecurity, as well as compensation for lost milk production. Additional financial help to promote biosecurity and viral testing is available for producers whose herds have not tested positive for H5N1.
• Federal and state agencies continue to conduct testing of clinical samples, including unpasteurized milk, nasal swabs, and tissue samples. They also are performing viral genome sequencing. The USDA and state health officials encourage producers to work with their veterinarians to support sampling and testing.
• Testing thus far has not found changes in the virus that would make it more transmissible to humans.
• The USDA continues to invest heavily in vaccine research and development to help prevent the spread of H5N1 among animals. Vaccines are currently available for poultry in the United States, including a killed H5N2 variant vaccine (Zoetis, recently conditionally approved for chickens), an H5N1 virus-like particle vaccine, and some older H5N1 vaccines that are in the Strategic National Stockpile. No H5N1 vaccines have been approved for dairy cattle in the United States, but vaccines are under development and field trials are underway.
• The American Association of Bovine Practitioners (AABP) has a working group of its members that, together with AVMA, is communicating with federal and state officials. AABP members can find more information about these activities here.
• The CDC recommends monitoring people exposed to infected animals of any species—including people wearing recommended personal protective equipment. Those who develop symptoms of H5N1 infection should be tested for the virus at a state or local public health department.

Impact on other animals and public health

• Dozens of domestic cats have tested positive for H5N1 in states where the virus also has been found in dairy cattle. Big cats (e.g., mountain lions, tigers, leopards, and bobcats) have been affected as well. In cats—a species previously known to contract the virus, illness reportedly has been severe, manifesting as neurological signs, severe depression, copious oculonasal discharge, and other respiratory signs, often progressing rapidly to death. Recent investigations implicate food as a source of infection, most often unpasteurized milk and raw or undercooked meat (e.g., poultry)—including retail pet diets containing raw meat. Cats that share homes with dairy or poultry farm workers in areas where H5N1 is circulating also may be at risk. These and other findings reinforce the importance of keeping pets away from wild birds, raw milk or colostrum, and raw or undercooked meat and taking other measures to prevent H5N1 transmission. The CDC advises veterinary staff to take precautions when working in close contact with cats with confirmed or suspected exposure to the virus. Find more details on H5N1 in cats here.
• Since the outbreak in dairy cattle began, infection with avian influenza virus type A (H5) has been confirmed in 70 people in the United States. Twenty-four of these cases involved poultry farm workers exposed to infected poultry in Washington (11 cases), Colorado (9 cases), and Iowa, Ohio, Oregon, and Wisconsin (1 case each). Forty-one other cases involved dairy farm workers exposed to sick or infected cows—36 in California, two in Michigan, one each in Colorado, Nevada, and Texas. Reported symptoms in farm workers generally have ranged from eye redness or discharge (consistent with conjunctivitis) to more typical flu symptoms, such as fever, chills, coughing, and sore throat/runny nose. However, serious illnesses have occurred, and one infected person with underlying health conditions (exposed to a noncommercial backyard flock and wild birds) has died. There has been no indication of person-to-person transmission, and the CDC continues to consider the risk of HPAI to the general public low.
• Recent infection with avian influenza virus type A (H5) also has been identified in three of 150 bovine veterinarians who participated in a September CDC serosurvey. All three veterinarians were asymptomatic. None worked with dairy cattle with known or suspected H5N1 infection; however, one did work with infected poultry.
• A mutation (PB2 D701N) has been identified in some genetic sequences of the D1.1 genotype of H5N1 from dairy cattle and one human case in the United States. A different mutation (PB2 E627K) has been identified in a single sequence of the B3.13 genotype in a dairy cow and in two human cases (one with the D1.1 genotype and the other with the B3.13 genotype). Both mutations improve replication efficiency in mammalian cells and have been associated with mammalian adaptation; however there has been no evidence of person-to-person transmission. Neither mutation has been reported in D1.1 or B3.13 viruses from wild birds.
• In November 2024, the NVSL also confirmed detection of H5N1 in two of five pigs in an Oregon backyard (noncommercial) farm where poultry and livestock were comingled, representing the first detection of the virus in U.S. swine. One of these pigs met the clinical case definition for HPAI H5N1. Poultry on the farm had been diagnosed with HPAI days earlier and were noted to share water sources, housing, and equipment with livestock. Results of viral genome sequencing suggested the animals likely contracted the virus through migratory birds rather than other livestock. The USDA stated that it has no concern about the safety of the nation’s pork supply. Overall, the swine and alpaca detections underscores the importance of strong biosecurity practices.
• People with close or prolonged, unprotected exposure to infected animals or their environments are at greater risk of infection, and people with weak or compromised immune systems may be at risk for more severe illness. For tips on how to protect yourself, see the CDC's information for workers exposed to the virus and the USDA’s recommendations on personal protective equipment. For tips on how to protect your workers, see the CDC’s interim guidance for employers.

Impact on food safety

• The USDA, FDA, and CDC continue to state they have no concerns about the safety of the commercial milk supply because milk from impacted animals is being diverted or destroyed so it does not enter the human food supply. In addition, products are pasteurized before entering interstate commerce for human consumption, and testing led by the FDA has confirmed that pasteurization is effective in inactivating the virus.
• The FDA, alongside federal and state partners, continues to undertake research activities to help ensure the safety of the milk and milk product supply during the outbreak.
• In December 2024, the FDA began sampling aged raw cow’s milk cheese from warehouses and distribution hubs throughout the country to determine whether these products contain viable H5N1. If the FDA detects viable virus in a sample, the agency will take follow-up action as warranted to protect consumers.
• The USDA’s Food Safety and Inspection Service (FSIS) has tested ground beef samples from stores in states with confirmed-positive dairy cattle herds. The samples tested negative for the virus using polymerase chain reaction (PCR) methods, reaffirming that the meat supply is safe.
• To better understand the implications of H5N1 for beef destined for human consumption, the FSIS in September 2024 began testing dairy cows at slaughter as part of its existing national residue monitoring program. In the event of a positive test result, the USDA will work with industry to ensure the relevant carcass does not enter the food supply.
• The FDA recommends that states that permit the sale of raw milk (for human or animal consumption) within their state use their regulatory authorities to stop that sale if the milk may contain viable H5N1 virus, as may occur when the milk comes from an infected herd and is not pasteurized. In December 2024, all raw whole milk and cream from a California dairy was recalled following detection of the virus in multiple samples from retail stores and dairy storage and bottling sites. While no illnesses were reported, the finding emphasizes the risks of foodborne disease inherent to unpasteurized milk and milk products.
• The FDA strongly encourages that any milk diverted for feeding calves be heat treated to kill harmful bacteria or viruses before feeding.
• People are advised not to drink unpasteurized milk or eat unpasteurized milk-based cheese, and to properly handle and cook meat to an internal temperature of at least 165°F. This includes any meat used to feed pets.

For the most current information and resources on the H5N1 outbreak, visit the relevant webpages of the USDA-APHIS, FDA, and CDC.