Editor's Note: Each year DTN publishes our choices for the Top 10 ag news stories of the year as selected by DTN analysts, editors and reporters. This year, we're counting them down from Dec. 18 to Dec. 31. On Jan. 1, we will look at some of the runners-up for this year. Today, we continue the countdown with No. 5: The detection of H5N1 in dairy cattle for the first time and how it has affected dairy cows differently than avian influenza in poultry.

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REDFIELD, Iowa (DTN) -- Highly pathogenic avian influenza (HPAI) has been in the poultry industry for a few years, but in March, the virus jumped to dairy cows and spread across the country, causing a decrease in milk production and illness in affected cows. The dairy strain was identified as H5N1.

Cows were tested for HPAI in March when a mysterious illness slowed their milk production and showed mastitis-like symptoms; veterinarians were baffled as to how a virus affecting poultry could jump to a mammal, but it did.

Confirmation of the illness has led to extensive research of the virus.

With birds being the source of the virus to cows, animal health professionals originally suggested keeping birds away from where cows are located. Migratory birds continue to be the source of spreading avian influenza among poultry facilities across the country. While the exact way the virus spread hasn't completely been identified, it seems to have spread early by farms that were the source of dairy breeding stock to other farms.

The herd in Texas was blamed for some of the spread, although it hasn't been certainly confirmed. The Texas Animal Health Commission responded, "The TAHC, USDA, and other federal, state, university, and industry partners and subject matter experts are all actively working to collect and share epidemiology information, discuss diagnostic information updates, and work together to further characterize the full scope of the incident."

VIRUS AFFECTS ANIMALS DIFFERENTLY

HPAI is considered lethal in birds, and the facilities are depopulated. The dairy cows affected by the later identified HPAI H5N1 clade 2.3.4.4b, genotype B3.13 show symptoms of a drop in milk production, loss of appetite and changes in manure consistency. Cows should be sorted into the sick pen to be treated with fluids and any other medicine prescribed by the herd veterinarian.

Producers report the decrease in milk production and say some cows will quit producing milk. According to the American Veterinary Medical Association, there has been a low mortality/culling rate of 2% or less, although California has seen a higher percentage than other states.

Testing of lactating dairy cows going to expositions and fairs or being transported across state lines began midsummer to help prevent the spread of the virus by live animals. This testing and concern led to fewer dairy cows being exhibited at summer fairs.

Jamie Jonker, chief science officer for the National Milk Producers Federation, said there continues to be no concern about the safety of the milk supply or that this circumstance poses a risk to consumer health because products are pasteurized before entering the market, per the Food and Drug Administration.

"Pasteurization has continually proven to inactivate bacteria and viruses, including influenza in milk," he said. "All dairy cattle are also subject to the Federal Meat Inspection Act and must be slaughtered and processed under inspection by USDA's Food Safety and Inspection Service, ensuring that all meat entering the food supply has been inspected and approved for human consumption."

FURTHER TESTING TO FIND VIRUS

Currently, testing is being conducted on Grade A milk intended for pasteurization as part of the National Milk Testing Strategy. Tests will be done at milk plants to find any problems in herds that haven't yet seen the virus. Patrick Gorden is a professor at Iowa State University's College of Veterinary Medicine who specializes in dairy production medicine and has been involved with the team that developed the national milk testing strategy.

"This testing strategy will allow us to determine individual state status with the goal of having freedom of H5N1 in the U.S. dairy population," he said. "I think dairy producers can control their own destiny. They need to make sure biosecurity is in place on farms to protect against this virus."

In addition to the virus affecting dairy cows, it has been found in pigs and goats, which have been on farms where cases were also present in cattle and/or poultry.

More concerning is the number of cases found in humans, usually workers at dairy farms. Human cases started back in April and led to the recommendation of more personal protective equipment for these workers. Federal funding is available for dairy farms to afford the proper safety equipment and biosecurity on the operations.

The Center for Disease Control (CDC) has suggested farmworkers get the flu vaccine to reduce the risk of "co-infection" of seasonal flu viruses with the H5N1 virus. Livestock workers are at risk of infection from seasonal flu just like anyone else, but they are also at risk of exposure to H5N1 from infected livestock.

Such dual infections are rare but could potentially lead to an exchange of genetic material between the two viruses. More recently, the cases among humans have been more severe and led to hospitalization.

As of Dec. 24, H5N1 confirmed cases in dairy cows since the outbreak began total 880 cases in 16 states. In the past 30 days, there has been an increase in HPAI cases in poultry due to the seasonal migration of wild birds. Since Nov. 23, 101 flocks have been tested and were confirmed to have HPAI, with 14.52 million birds affected. According to the CDC, since April, there have been a total of 61 reported human cases of bird flu in the U.S.

See more DTN coverage:

-- "Sick Dairy Cows Recovering from HPAI," https://www.dtnpf.com/…

-- "Is Cattle Market's Reaction to HPAI Justified?" https://www.dtnpf.com/…

-- "Dairy HPAI Cases Now in 5 States," https://www.dtnpf.com/…

-- "Researchers Continue to Look for Transmission Source of Influenza in Dairy Cows,"

https://www.dtnpf.com/…

-- "Fewer Cows at Fairs Due to H5N1 Testing,"

https://www.dtnpf.com/…

-- "Oregon Pig Tests Positive for H5N1," https://www.dtnpf.com/…

-- "Patient in Texas Contracts Bird Flu After Close Contact with Dairy Cattle,"

https://www.dtnpf.com/…

-- "H5N1 Aid, Incentives Offered to Dairies,"

https://www.dtnpf.com/…

-- "CDC Seeks Livestock Workers to Vaccinate Against Seasonal Flu Following H5N1 Outbreak," https://www.dtnpf.com/…

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More countdown stories:

-- See No. 10 story, "2024 Was Year of Labor Unrest at US and Canada Rail and Shipping Ports," at https://www.dtnpf.com/…

-- See No. 9 story, "'Over-the-Top' Dicamba Product Registrations Vacated," https://www.dtnpf.com/…

-- See No. 8 story, "Record-High Prices Rippled Through the Cattle Market in 2024," https://www.dtnpf.com/…

-- See No. 7 story," EPA Advanced Plans in 2024 to Meet Endangered Species Obligations for Pesticides," https://www.dtnpf.com/….

-- See No. 6 story, "Active Weather Pattern's Massive Impact: Megafires to Flooded Fields, Flash Drought to Hurricane Fatalities," https://www.dtnpf.com/…

Jennifer Carrico can be reached at jennifer.carrico@dtn.com

Follow her on social platform X @JennCattleGal

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