In December 2007, a tuberculosis-positive cow was identified when a TB lesion was discovered on a lymph node during routine slaughterhouse surveillance in California. To prevent the spread of TB, an epidemiological investigation was instigated and the case was traced back to a dairy herd in Fresno County. The affected herd was purchased by the federal government and depopulated, preventing California from losing its accredited-free status. However, on September 18, 2008, a second infected herd was discovered, and California's bovine TB status was downgraded from accredited-free to modified-accredited advanced, limiting trade and movement of cattle from the state.

Bovine tuberculosis is an acid-fast bacillus bacteria caused by Mycobacterium bovis. Primarily known as a respiratory disease, M. bovis can infect many vertebrates (cattle, sheep, goats, deer, pigs, bison, buffalo, camelids, and humans). The disease is spread by direct contact, inhalation of aerosols, and ingestion of contaminated milk or feed. Clinical signs may be difficult to detect but include lethargy, emaciation, anorexia, a chronic intermittent moist cough, dyspnea or tachypnea, and a low-grade fluctuating fever.

Since the 2007 case of bovine TB, 395,000 cattle have been tested for TB within California. Diagnostic tests include the caudal fold tuberculin test (CFT), comparative cervical tuberculin test (CCT), gamma interferon test, histopathology, PCR, DNA fingerprinting, culture, and necropsy. Today, a total of 8 cases have been confirmed, 2 herds depopulated, and over 8,000 cattle killed. Although over $20 million has been spent on the California investigation, the source of TB infection remains unknown.

TB has been an ongoing problem since the early 1900s when it was the leading cause of death in the US. In 1917, the Cooperative State-Federal Bovine Tuberculosis Eradication Program was created as a test-and-slaughter program in the US. Today, 49 states are considered accredited-free. However, the challenge remains to eradicate M. bovis from within the US.