What type of test is considered the official tuberculin test used in cattle?

The caudal fold test is the official tuberculin test used in cattle because it specifically evaluates the presence of bovine tuberculosis by injecting a purified protein derivative (PPD) into the skin of the animal's tail fold. The reaction is measured based on the degree of swelling at the injection site, which indicates whether the animal has been exposed to the bacterium that causes tuberculosis (Mycobacterium bovis). This test is well-established and recognized for its effectiveness in screening cattle for TB, thus playing a crucial role in disease control programs.

The other options, while they may involve testing procedures related to diseases, are not specific to tuberculosis in cattle. Blood serum tests, like the enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay, are generally used for various diseases or conditions but do not serve as the official method for diagnosing bovine tuberculosis.