Featured Links
Information About Bovine Tuberculosis
What is TB?
Tuberculosis (TB) is a serious bacterial disease caused by Mycobacterium species that usually affects the respiratory system. Animals infected with TB may not show signs for years, and animals that appear healthy may be capable of transmitting infection to other animals.
Three main types of TB occur:
- Human, most often caused by M. tuberculosis
- Avian, caused by M. avium
- Bovine, caused by M. bovis
Human TB is rarely transmitted to non–humans, 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.
TB's Current Status in California
California regained our Bovine Tuberculosis Accredited–Free status on April 15, 2005 two years after losing our status due to an outbreak in Kings and Tulare counties. Following the detection of bovine tuberculosis in 2003, over 876,000 cattle from 688 herds were tested in California to make sure the disease had not spread beyond the three detected dairy herds. New rules were also put in place requiring that dairy breeding cattle entering California be tested to prevent reintroduction of the disease – this rule remains in effect.
Please see our TB Producer 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.
California TB 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 would be 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, CA abandoned 'area testing' for TB, because the prevalence of TB in CA 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 surveillance as the primary surveillance method. Slaughter surveillance includes examining every bovine killed 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 look for Bovine TB.
If one of these animals does test positve for Bovine TB, CDFA’s Animal Health Branch investigates where this animal came from and what animals may have been exposed.
The Significance of Bovine TB
California's cattle industry 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 leads to loss of consumer confidence in milk and beef products, reduced production in affected cattle and trade restrictions imposed due to TB.
TB in Humans
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. Airborne infection continues to occur among meat industry and slaughterhouse workers, in regions where the infection remains prevalent in cattle, including Mexico.
Humans can also transmit disease back to cattle; unlike M. tuberculosis, it is rare that M. bovis is spread from human–to–human.
How is TB Transmitted?
Bovine TB is most commonly spread 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% of the cases of bovine TB are transmitted through direct contact between cattle. Only 1–5% 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., although theoretically possible. Whitetail deer in Michigan are thus far the only wildlife reservoir for bovine TB identified in the U.S.
Signs of the Disease
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. Clinical signs consist of: cough, production loss, rough hair coat, chronic weight loss, variable appetite and fluctuating fevers.
Lesions
In approximately 90% 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.
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