Five of the six dairy herds infected with the H5N1 avian flu virus this month are in California, all in the past week and all in the Central Valley. State officials said those herds were part of a group of farms targeted for testing because of outbreaks in three other herds at the end of August.
California, the No. 1 milk-producing state, now accounts for eight of the U.S. total of 203 herds confirmed with bird flu in 14 states since the disease was first identified in dairy cattle in late March. Colorado has the largest number of infected herds, with 64; no outbreaks have been reported in the state since mid-August.
The California Department of Food and Agriculture (CDFA) said it selected a group of Central Valley farms for testing “due to elevated risks from their recent connections with the initial [three] affected premises.”
“These recent findings are not unexpected and support CDFA’s implementation of current surveillance strategies with the goal of finding affected farms as early as possible. This provides the opportunity to work with farms to quickly implement enhanced biosecurity, cow care, and employee protection,” said the CDFA in a Bovine Health Alert. “All affected dairies have been placed under quarantine and enhanced biosecurity measures are in place to prevent the spread of the virus. Sick cows are isolated and are being treated at the dairies.”
The risk of bird flu to the general population remains low, said the Centers for Disease Control. It recommends that people wear protective gear, such as masks, goggles, and gloves, if they deal with infected or potentially infected animals.
No human cases of bird flu have been reported in California related to the infected dairy herds. Nationwide, 14 human infections have been confirmed since late March, all but one of them among livestock workers. The exception was a Missourian with no known exposure to animals, who was treated with antiviral medications and who recovered earlier this month. The source of the infection was unknown.
“One household contact of the patient became ill with similar symptoms on the same day as the case, was not tested, and has since recovered,” said the CDC. “The simultaneous development of symptoms does not support person-to-person spread but suggests a common exposure.” A healthcare worker involved in the case developed symptoms but tested negative for flu.
To date, more than 4,800 people have been monitored following exposure to infected or potentially infected animals and at least 240 have been tested after they developed flu-like symptoms, said the CDC.
The H5N1 avian flu virus first appeared in the United States in poultry flocks in February 2022. Some 100.7 million birds in domestic flocks have died of highly pathogenic avian influenza, most of them egg-laying hens and turkeys being raised for meat. The last outbreak in a commercial flock was July 19, at a Colorado farm raising young chickens for egg production.