Dairy herds in the USA are tested at least once a year to be certified brucellosis-free. with the "Brucella" milk ring test. Cows confirmed to be infected are often killed. In the United States, veterinarians are required to vaccinate all young stock, to further reduce the chance of zoonotic transmission. This vaccination is usually referred to as a "calfhood" vaccination. Most cattle receive a tattoo in one of their ears, serving as proof of their vaccination status. This tattoo also includes the last digit of the year they were born.