EDITORS/NEWS DIRECTORS:

An outbreak of bird flu has spread to more than two dozen herds of dairy cows in several U.S. states. The outbreak, announced March 25, marks the first time that a strain of avian influenza often fatal to wild and commercial poultry has spread to cattle -- and the first time a human has contracted this version of the virus from a mammal, according to federal health and agriculture officials. The virus had been spreading in the U.S. among birds since 2022.

Wild migratory birds are believed to be the original source of the virus in cattle, the agency said. But officials have confirmed some cases where spread was linked to cattle movement between herds or spread from dairy cattle locations back into nearby poultry sites.

Health officials said the risk to human health and the nation's food supply remains low, but there is rising concern about potential spread of the virus -- known as Type A H5N1.

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READ AP'S STORIES

Bird flu is spreading to more farm animals. Are eggs and milk safe? Person is diagnosed with bird flu after being in contact with cows in TexasWhat to know about the latest bird flu outbreak in the U.S.

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FIND YOUR STATE: WHERE HAS BIRD FLU BEEN DETECTED IN DAIRY COWS?

The map and table on this page show in which states bird flu has been detected. You can count the number of sites in a given state in the downloadable table. USDA updates this page weekdays at 4 p.m. ET.

Bird flu has been detected in dairy cows in eight states:

IDAHO

KANSAS

MICHIGAN

NEW MEXICO

NORTH CAROLINA

OHIO

TEXAS

SOUTH DAKOTA

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WHAT IS BIRD FLU?

Some flu viruses mainly affect people, but others chiefly occur in animals. Avian viruses spread naturally in wild aquatic birds like ducks and geese, and then to chickens and other domesticated poultry.

The bird flu virus drawing attention today was first identified in 1959, but emerged in southern China in 1996, causing large poultry outbreaks. Like other viruses, it has evolved over time, spawning newer versions of itself. Since 2020, the virus has been spreading among more animal species -- including dogs, cats, skunks, bears and even seals and porpoises -- in scores of countries.

In the U.S., this version of the bird flu has been detected in wild birds in every state, as well as commercial poultry operations and backyard flocks. Nationwide, tens of millions of chickens have died from the virus or been killed to stop outbreaks from spreading.

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HAVE PEOPLE BEEN AFFECTED?

The first cases of human infection were reported in China in 1997. Another outbreak of the virus in birds resurfaced in 2003, causing sporadic human infections. Since 2003, more than 23 countries have reported more than 880 sporadic human infections with H5N1 bird flu viruses to the World Health Organization. Illness in people ranges in severity from no symptoms to severe disease and death.

In the U.S. two people have contracted bird flu. A Texas worker exposed to sick dairy cows was confirmed to be infected with the virus in April. The worker reported mild eye symptoms and has recovered. In 2022, a prison inmate in a work program caught the virus while killing infected birds at a Colorado poultry farm. His only symptom was fatigue, and he recovered.

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WHERE DID THE CURRENT OUTBREAK START?

Experts confirmed the first outbreak in dairy cattle after samples of unpasteurized milk and nose and throat swabs from sick cows in Texas and Kansas tested positive for the virus. Agriculture officials in Texas had puzzled for several weeks about a mystery illness that caused sharply decreased milk production, lethargy and other symptoms in older milk-producing cows. The discovery in dairy cows followed detection of H5N1 in goats in Minnesota, the first time the virus was found in a domestic ruminant.

The cows don't appear to become seriously ill from the virus and recover within two weeks, experts said. Because spread has been detected among cows, several states have restricted cattle coming from places reporting infection. U.S. agriculture officials are urging caution in moving cattle, plus enhanced biosecurity measures to prevent spread of the virus.

The virus also has been detected recently in commercial poultry flocks in Michigan, Texas and New Mexico, including the largest supplier of eggs in the U.S. Cal-Maine Foods, Inc. said in a statement that approximately 1.6 million laying hens and 337,000 pullets, about 3.6% of its total flock, were destroyed after the infection, avian influenza, was found at a facility in Parmer County, Texas.

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HOW WILL THIS IMPACT MILK AND EGGS?

Health officials said there's little risk to the U.S. commercial milk or egg supply. Milk from sick cows must be diverted or destroyed and commercial milk is heat-treated or pasteurized to kill bacteria and viruses. The risk of eggs from infected hens getting into the food chain is "extremely low," because of federal safeguards, the USDA said. Proper processing, storage and cooking of eggs also prevents transmission of viruses, including bird flu, the agency said. However, federal officials have emphasized longstanding advice not to drink raw or unpasteurized milk and milk products such as cheese.

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HOW IS THE CDC RESPONDING?

To date, scientists with the USDA and the Centers for Disease Control and Prevention say there's no evidence that the virus has changed to become more easily transmitted to humans or to evade existing antiviral treatments. The U.S. Department of Health and Human Services says it has been building a supply of vaccines to protect people against H5N1 since 2005 and that it has stockpiled tens of millions of doses of antiviral medicines. Officials say they also have two vaccines that are well-matched to the version of H5N1 virus that's currently circulating, and could have nearly 10 million doses ready to go within a few months.

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CONSIDER THESE REPORTING THREADS

-- How concerned are farmers in your state about the spread H5N1 to dairy cattle? What steps are individual farmers taking to limit movement of cattle or to bolster biosecurity measures?

-- Backyard birds can be infected with H5N1, too. How are people who keep backyard poultry flocks in your state reacting to the H5N1 outbreak?

-- Raw or unpasteurized milk has become increasingly popular in some states. It's not allowed to be sold across state lines, but how are raw milk producers in your state reacting to the H5N1 news?

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AP medical writer Mike Stobbe and statehouse reporter Sean Murphy contributed reporting.

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