State officials detected a case of highly pathogenic H5N1 avian influenza in a backyard flock of chickens in Charles County, the Maryland Department of Agriculture said Wednesday.

The affected property has been “thoroughly disinfected” and is under a quarantine, officials said. The birds in the flock have been killed to prevent the spread of the contagious disease. The lab results are being validated by the U.S. Department of Agriculture’s National Veterinary Services Laboratory.

Jessica Hackett, a spokeswoman for the Maryland Department of Agriculture, said there was “increased mortality” among the flock, so the entire flock was tested. One presumptive positive for avian influenza was returned from the department’s health lab in Frederick, she said.

The entire flock was made up of 27 chickens, six ducks and one goose. The Maryland Department of Agriculture does not release the locations of affected premises for privacy reasons, Hackett said.

This is the first case detected in Maryland since November 2023, when the virus was found in Caroline County.

Over the last 30 days, 18 chicken flocks have been affected by avian flu nationwide, according to data from the U.S. Department of Agriculture. The data was last updated Wednesday afternoon.

Avian influenza, or bird flu, “is a highly contagious airborne respiratory virus that spreads easily among birds through nasal and eye secretions, as well as manure,” according to the state agriculture department.

“The latest non-negative result should serve as a reminder for all poultry growers, operators, and backyard flock owners to remain vigilant when it comes to the threat of HPAI [highly pathogenic H5N1 avian influenza], especially as the spring migratory season begins,” Maryland Department of Agriculture Secretary Kevin Atticks said in a statement.

State officials said the risk of transmission between birds and the general public is low.

Biosecurity — steps people can take to protect against the spread of disease — is “the most effective way to prevent the spread of this disease among poultry,” Atticks said in his statement.

Those measures include washing hands before and after handling live poultry and limiting the number of people who come into contact with birds, regardless of the flock size.

To report a possible case of bird flu, call the Maryland Department of Agriculture at 410-841-5810. Commercial chicken growers and backyard flock owners can email questions about the outbreak to MD.Birdflu@maryland.gov.