Faced with the rapid spread of avian flu through California dairies, health officials are now doing weekly testing of every dairy farm in the state — expanding initial efforts amid new evidence that some infections are going undetected and there may be unknown paths of transmission.

Reports that cows have been infected despite farmers' preventive measures indicate there are multiple routes of transmission

California officials have declared a state of emergency over the spread of bird flu, which is tearing through dairy cows in that state and causing sporadic illnesses in people in the U.S.