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."