Tuberculosis is a globally distributed zoonotic disease. In cattle, a bacillus known as Mycobacterium bovis causes the disease.

All animal species are susceptible to Mycobacterium bovis. Cattle, pigs and goats are especially prone to this disease.

Infected animals are the main source of transmission of the disease. The most common form of transmission is through inhalation of the tuberculosis bacteria exhaled by an infected animal. Transmission by milk, urine, uterine & vaginal discharge, and faeces are also sources of the disease.

Tuberculosis infects a host in two stages. The first stage begins with an injury at the entry point, the oropharyngeal cavity, and in the local lymph node, spreading to regional lymph nodes. Lesions characterized by calcifications are generated, with great presence of granulation tissue, monocytes and plasma cells that form the pathognomonic granuloma (tubercle). The second stage, miliary tuberculosis, is the dispersion of the bacilli into the caudal lobe lung, causing a series of discrete nodular lesions.

Even during the advanced stages, the infected animals do not manifest evident clinical signs. An intradermal test with tuberculin is commonly used to diagnose the disease. This test involves the injection of bovine tuberculin PPD (Purified Protein Derivative) from cattle type, in the dermis of the animal, registering the inflammatory reaction at the injection site. The intradermal injection is usually performed in the mid-area of the neck or in the caudal fold of the tail.

Results are interpreted in conjunction with guidelines prescribed by the official veterinary service for each country. They often perform a comparative intradermal test that also involves the injection of avian tuberculin PPD into an animal’s dermis. This test distinguishes animals infected with Mycobacterium bovis from those sensitive to the tuberculin due to the exposure to other organisms (for example Mycobacterium avium).

The potency or activity of the tuberculin is expressed in International Units (IU).