California is investigating five possible human cases of avian influenza among dairy farm workers, in addition to the six cases previously confirmed in the state, Reuters reported, citing the state health department on Monday.

Avian influenza has spread to 100 dairies in California and 300 nationwide, according to the US Department of Agriculture. There have been 19 confirmed human cases this year among farm workers exposed to sick poultry or dairy, including the six confirmed cases in California this month.

The risk to the general public from bird flu remains low and pasteurized dairy products are safe to consume, federal officials have said.

Specimens from the five possible cases are being sent to the Centers for Disease Control and Prevention for confirmation and are expected to arrive early this week, said the California Department of Public Health.

The possible and confirmed cases originated on nine different dairy farms and the individuals had mild symptoms and were not hospitalised, the state said.