The Agriculture Department is re-emphasizing the importance of biosecurity on dairy farms in light of new reports that identify a variety of ways the H5N1 virus can infect dairy herds and nearby poultry operations, including the movement of animals and people, and the sharing of vehicles and equipment.

“The spread of H5N1 between states is linked to cattle movements (versus independent wild bird introduction) with further local spread between dairy farms in some states,” the Animal and Plant Health Inspection Service said in a national epidemiologic brief released Thursday.

There is “no genomic or epidemiologic evidence that wild birds are spreading H5N1 to cattle,” APHIS said, “but [that] cannot be ruled out.”

A separate paper that looked at 15 affected dairy herds and eight poultry flocks identified “shared employment, housing, or movement of employees” as a potential transmission route from the dairies to the flocks.

“Approximately 22 employees of three poultry flocks worked weekend shifts at two different dairy premises,” the paper said. “Shared housing between dairy and poultry workers was identified between three poultry premises and two dairy premises. It is also possible that dairy employees have social contact with poultry premises employees.”

Testing has confirmed infections in 94 herds in 12 states since the virus was first identified in Texas in late March, and on April 24, USDA required lactating dairy cows being moved interstate to be tested. The department also has announced a voluntary herd status program that aims to encourage surveillance testing in states.

USDA’s National Incident Coordinator in APHIS Veterinary Services, Mark Lyons, said APHIS is having conversations with about 20 states seeking to join the program.

Based on a questionnaire from about half of the affected premises, APHIS identified three “key factors” that put farms at risk, Kammy Johnson, an APHIS veterinary epidemiologist, said on a call with reporters today.

First is “the use of shared equipment and vehicles that aren't always properly cleaned prior to use,” she said. 

Another potential pathway includes “people who have inadvertently carried a virus on their clothing or boots, similar to the way the equipment and machinery carry the virus," she said. "That includes shared personnel between farms, as well as frequent visitors on and off the farm during business practices, who have direct contact with livestock.” 

Third, she identified “animal movement itself,” stressing that “enhanced biosecurity at this time is really critical to stop the spread of H5N1 between dairies and that subsequent spillover to domestic poultry” and other animals.                                   

The brief goes into more detail, noting that more than 60% of affected dairy farms continued to ship cattle off-site even after clinical signs of infection were detected, including “abnormal lactation” or decreased feed consumption.

The presence of other species on farms is another potential route of transmission. More than 80% of affected farms have cats, and at more than half of those, sick or dead cats were observed.

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In addition, more than 50% of the farms “used trucks and trailers that are shared with other farms to transport livestock within 30 days prior to onset of clinical signs; and [more than] 50% of farms that used shared vehicles do not clean vehicles prior to use,” the brief said.

In the Michigan paper, APHIS said H5N1 arrived initially via cows shipped from Texas. After that, the spread of the virus within the state “is likely due to indirect epidemiological links related to normal business operations such as numerous people, vehicles, and other conveyances frequently moving on and off the affected dairy premises, with many of these indirect links shared between premises.”

APHIS continued, “Importantly, disease spread due to independent introduction of the virus onto dairy or poultry premises from migratory waterfowl is not supported based on both genomic and epidemiological data analysis.”

“Despite no genomic evidence that migratory birds are spreading HPAI genotype B3.13 within Michigan, the potential for resident wild birds or peri-domestic species to move and transmit the virus between dairy herds cannot be ruled out, especially for dairy premises located in close proximity,” the paper said. 

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