The first cases of a highly infectious strain of bird flu that has killed millions of birds in the United States this year have been detected in Marin County.

Marin poultry farmers and health officials are on high alert for an outbreak of the virus, known as H5N1, which easily spread among whole flocks of birds through shared food and water, bird droppings and handlers carrying the virus on their clothing. The flu has also infected humans and other mammals in limited cases.

Morgan Giammona, the owner of the Tomales Bay Pastures hen and egg farm in Marshall, said his flock of 4,000 hens is especially at risk given that they feed in open pastures on grasses and bugs, exposing them to wild birds that pass through the area every day.

If only one of his hens is infected, the U.S. Department of Agriculture would require his entire flock to be culled.

“Hopefully this thing passes without wiping out all of us, but it’s not looking too good right now,” Giammona said. “The more I check online with the reports of farms being infected, it scares me every day.”

A near-record 49 million birds across 44 states have been killed by the virus or euthanized, according to data from the U.S. Department of Agriculture and the U.S. Centers for Disease Control and Prevention. This strain of the virus last spread throughout the U.S. and other parts of the world in 2015, but diminished after the summer, unlike the current version of the virus.

The outbreak has driven up the price of poultry products, including turkeys for Thanksgiving.

The first case in Marin was confirmed in late October in a Canada goose. The goose was spotted in mid-September by a crossing guard in Corte Madera who noticed the bird wandering in an intersection and acting confused. The crossing guard reported the goose to WildCare animal hospital in San Rafael, where it was taken to an outdoor triage tent set up to quarantine suspected cases of avian flu.

Upon inspecting the goose, WildCare veterinarian Dr. Juliana Sorem said she noticed it had a cloudy eye, which was a common symptom of H5N1 infection for Canada geese in other areas. The goose was euthanized and sent to a national laboratory in Iowa for testing and was confirmed to have the virus on Oct. 19, Sorem said.

Another 26 birds reported to WildCare since then are now suspected of having contracted the virus.

“Frankly, I don’t think there is a huge reason to test them now that we know they’re here,” Sorem said. “We’re getting a pretty steady stream of birds that we suspect are positive.”

Sorem said it is important for residents to be on the lookout for dead birds or birds exhibiting unusual behaviors and to report them to animal control organizations such as Marin Humane, WildCare or the California Department of Fish and Wildlife. Symptoms include respiratory issues, mucus, poor circulation, particularly in the feet, and neurological issues. Waterfowl have been found to be paddling around in circles, acting dizzy, twitching or holding their necks in an unnatural position, Sorem said.

The virus can spread easily among wild birds, especially in areas where they congregate, such as bodies of water. Raptors and scavenger birds are also particularly vulnerable if they eat infected birds.

Stefan Parnay, the Marin County agriculture commissioner, said information has been sent to all of Marin’s poultry farmers recommending strict biosecurity measures.

“Poultry has been a large producer in our county. Even though we don’t have huge facilities like they do in the Central Valley, we do have small- to medium-sized producers where that is their livelihood,” Parnay said. “Back in other states like Indiana it’s gotten into commercial flocks and they’ve had to euthanize millions of birds. This is extremely serious.”

At Tomales Bay Pastures, Giammona and his staff are under strict rules to prevent introducing the virus to their hens. He uses guard dogs to chase away wild birds that might try to land in his pastures. Before starting work, he and his staff are required to change out of their shoes into boots that remain on the ranch. The boots must be dipped into a disinfectant before entering the trailers where the hens sleep. Hands must be washed regularly. Giammona is also limiting visitors and is no longer sharing equipment.

“Kind of reminds me of the beginning of when COVID had an outbreak and when everyone was trying to be super careful,” Giammona said.

But Giammona said there are only so many precautions he can take. Losing his hens could result from something as simple as wild birds landing in his pasture to drink some of his hens’ water, or a visit to his local feed store.

Sorem recommends that residents raising poultry take similar precautions and keep their animals under cover where possible.

Bird research and conservation groups such as Point Blue Conservation Science are also taking precautions to prevent the spread of the disease as part of their research in western Marin and the Farallon Islands, which is home to the largest seabird breeding colony in the contiguous U.S.

Point Blue avian ecologist Diana Humple said she and other scientists monitoring birds in western Marin have not seen any wild birds showing symptoms so far. As part of their work banding birds, Humple said, researchers are taking more precautions to disinfect equipment and wash hands regularly to prevent the spread.

Avian flu infections among humans are nowhere near as widespread, but have occurred in the past. The U.S. Centers for Disease Control and Prevention reported the first infection of a U.S. resident from the H5N1 strain in Colorado earlier this year. The health risk of avian flu is low, according to the agency, but people working closely with birds are being urged to take precautions to prevent potential spread.