After a new study showed that bird flu has spread to dairy farm workers more often than previously known, the US Centers for Disease Control and Prevention is expanding its recommendations for testing and treatment of workers on farms where animals have tested positive.

Following a CDC study showing bird flu exposure among dairy workers, health officials recommend increased testing and preventative measures for farmworkers, even if they show no symptoms, to prevent virus spread and mutation.

The Centers for Disease Control and Prevention (CDC) is updating its recommendations for testing, treatment and protection for workers who may be exposed to animals infected with bird flu. The update comes after the agency found evidence of dairy workers with positive antibodies,

New CDC research suggests that a small but significant percentage of dairy farmers working near infected cows have recently contracted H5N1, often without knowing.

The CDC study provides the largest window to date into how the bird virus first detected in March in dairy cows may be spreading to people. It suggests that the virus has infected more humans than the 46 farmworkers identified in the U.S. as of Thursday. Nearly all were in contact with infected dairy cows or infected poultry.

Canada has detected its first presumptive case of H5 bird flu in a person, a teenager in the western province of British Columbia, health officials said Saturday. The teenager likely caught the virus from a bird or animal and was receiving care at a children's hospital, the province said in a statement.