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What is TB?

Tuberculosis (TB) is a serious bacterial disease caused by Mycobacterium, a group of bacteria that usually affects the respiratory system. Animals infected with TB may not show signs for years, but animals that appear healthy may still be capable of transmitting infection to other animals.

Tuberculosis is classifed into three types:

• Human, most often caused by M. tuberculosis
• Avian, caused by M. avium
• Bovine, caused by M. bovis

Human TB, or M. tuberculosis is rarely transmitted to animals, although M. tuberculosis is often the type of TB that affects elephants. Avian TB is typically restricted to birds (pigs and, occasionally, other animals are affected). Bovine TB is a chronic disease that has affected animal health throughout recorded history and is capable of infecting most mammals.

For more information see our Bovine Tuberculosis brochure (PDF 139 KB).

TB's Current Status in California

The U.S. Department of Agriculture maintains a monitoring system that classifies states based on the presence of bovine TB within a state’s cattle population. States are classified in five categories (Accredited Free, Modified Accredited Advanced, Modified Accredited, Accreditation Preparatory and Non-Accredited). The categories are based on the number of cases of bovine TB that have been identified through surveillance programs. A state’s status can be downgraded if new cases of bovine TB are identified.

In 1998, the California Department of Food and Agriculture (CDFA) implemented an aggressive TB action plan with a special focus on the state’s large dairy industry. California achieved Accredited Free status in 1999; however, an outbreak in 2002 and 2003 in three Central Valley dairy herds required a downgrade in status. After depopulating the infected herds, extensive tracing and tested of cattle associated with the herds and testing over 700,000 cattle in Tulare, Kings and Fresno counties, California regained its Accredited Free status in April 2005.

In June 2008 confirmations of new cases of bovine tuberculosis in two dairy herds in Fresno County mean USDA will officially downgrade California’s status from Accredited Free to Modified Accredited Advanced through a notice in the Federal Register. Bovine TB was detected in a single herd in January 2008, bringing the state’s current number of affected herds to three.

The downgrade in status is required by USDA’s Code of Federal Regulations following the detection of at least two tuberculosis positive herds within a 48-month period. The new status will impose additional testing requirements on California’s cattle industry when shipping animals out of state.

CDFA animal disease control experts are working with the USDA to evaluate regionalizing the affected counties, so that the remainder of California can return to an Accredited Free status. This would allow unaffected areas to conduct business as usual, while intensive disease eradication efforts could continue in the affected area of California. Breeding cattle would need official identification and a TB test to move from the lower status region to the higher status region of California. Efforts for a regionalization plan are ongoing and updates can be obtained from the California Department of Agriculture.

Please see our California Bovine TB Update for information on the latest bovine TB investigations in California and across the United States.

When moving cattle out of California, always check with the state of destination for their TB testing requirements.

Contact your local CDFA Animal Health Branch district office if you have any questions.

The Significance of Bovine TB

The Department is concerned about the affects of TB on the industry for several reasons. First, there is a zoonotic risk for individuals consuming raw milk from TB–infected animals. Second, there is concern about the establishment of infection in a wide host range including free–ranging wild species. Additionally, infection with Bovine TB reduces production in affected cattle and imposes trade restrictions. While the risk of humans contracting bovine TB is extremely low, positive cases in the state’s cattle herd can negatively affect consumer confidence in milk and beef products. It is important for consumers and public health officials to understand that the TB bacteria is killed when meat is cooked and milk is pasteurized—steps that provide a final barrier to protect public health.

CDFA’s objectives for the bovine tuberculosis program are to

1. Minimize the risks to public health.
2. Work in partnership with United States Department of Agriculture and industry, to fully recognizing their roles and responsibilities in disease control and prevention.
3. Use sound independent science to guide disease surveillance and control strategies.

California TB Eradication Program History

In the early 1900's the TB eradication program in the U.S. began because of the impact Bovine TB has on animal and human health. Initially, the TB program consisted of area testing, in which approximately 15% of a state's cattle population was tested each year. All herds in the state were tested every six years. This method was the primary tool used in the eradication effort until national prevalence levels of TB were greatly reduced. In 1994, California abandoned 'area testing' for TB, because the prevalence of TB in California and the rest of the U.S. had reached such low levels that area testing was no longer an efficient method to detect the disease.

Tuberculosis surveillance had shifted to slaughterhouse inspection as the primary surveillance method. Slaughter surveillance includes examining every bovine processed in California for characteristic signs of disease. Any animals with lesions suspicious for TB are not used for human consumption and undergo further testing to determine if bovine TB is present. If a carcass is found to be positive, a traceback investigation is initiated by the California Department of Food and Agriculture to determine the origin of the animal.

TB and Public Health

Bovine tuberculosis is a zoonotic disease, meaning humans can be infected by exposure to affected animals. As the prevalence of bovine TB has decreased in the U.S. and pasteurization has become routine, fewer infections are traced to drinking milk. However, humans can still become infected by drinking raw milk or eating unpasteurized cheese from TB–infected animals.

Bovine tuberculosis does not threaten the quality and safety of milk and meat products produced in California. Almost all milk sold in California is pasteurized, which destroys organisms that could be harmful to humans, including tuberculosis organisms. The state’s two raw milk dairies are regularly tested for tuberculosis. All cattle processed for meat are inspected for signs of tuberculosis infection and rejected for consumption if they show signs of the disease.

A few cases of airborne infection continue to occur among meat industry and slaughterhouse workers, in regions where the infection remains prevalent in cattle, including Mexico. Additionally, humans can also transmit disease back to cattle; unlike M. tuberculosis, however, it is rare that M. bovis is spread from human–to–human.

How is TB Transmitted?

Bovine TB is most commonly spread among cattle through respiration. Invisible droplets containing TB bacteria may be exhaled or coughed out by infected animals and then inhaled by another animal in direct contact with the infected animal. Over 95 percent of the cases of bovine TB are transmitted through direct contact between cattle. Only 1 to 5 percent of infected cattle shed the organism in the milk. (FD Menzies and SD Neill, " Cattle–to–Cattle Transmission of Bovine Tuberculosis" The Veterinary Journal, 2000,160,92–106). Contaminated feed and water account for a very small percentage of bovine TB cases. Transmission of TB through the contamination of the environment has not been documented in the U.S. Whitetail deer in Michigan are thus far the only wildlife reservoir for bovine TB identified in the U.S.

Signs of the Disease

Clinical signs consist of: cough, production loss, rough hair coat, chronic weight loss, variable appetite and fluctuating fevers. Ninety percent of animals infected with tuberculosis DO NOT develop clinical signs. Animals infected with bovine TB may not show any outward signs of illness, but may eventually exhibit weight loss and a gradual decline in general health.

Lesions

In approximately 90 percent of tuberculosis cattle, lesions primarily involved the lymph nodes of the respiratory system. Lesions can also be found in thoracic cavity, head, and mesenteric lymph nodes. Approximately half of the lung lesions were located in the distal part of the diaphragmatic lobes. (FD Menzies and SD Neill, " Cattle–to–Cattle Transmission of Bovine Tuberculosis" The Veterinary Journal, 2000,160, 92–106).

Formation of the TB Tubercule

The tubercule is a granuloma consisting of inflammatory cells. When an animal's immune system identifies M. bovis, it releases inflammatory cells, chiefly macrophages. However, M. bovis is resistant to the destructive action of the macrophage, and once within the macrophage it has the ability to replicate and destroy the macrophage itself. The animal's immune system still recognizes the TB invader and releases additional macrophages. These macrophages encapsulate the area to wall it off from other tissue and a tubercule is formed. Tubercules are usually located in the respiratory lymph nodes and the lungs. The preliminary diagnosis of tuberculosis in slaughter establishments is the identification of these tubercules. Any granulomas suspected of being M. bovis granulomas are sent to an approved laboratory for further evaluation.

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Contact us for more information:
California Department of Food and Agriculture
Animal Health and Food Safety Services, Animal Health Branch
1220 N Street, Room A–107
Sacramento, California 95814
Telephone: (916) 654–1447
Fax: (916) 653–2215
or send an email to: ahbfeedback@cdfa.ca.gov