Most of us live in Tuberculosis-free states. However, under the USDA-APHIS regulations on tuberculosis, all cattle two years of age and older must be periodically tested by an accredited veterinarian. That interval is determined by your state’s TB status and incidence of the disease. A complete herd test is required at least every six years — even in TB-free states.
Tuberculosis is a contagious disease in both animals and humans. It is caused by three specific types of bacteria that are part of the Mycobacterium group: Mycobacterium bovis, M. avium, and M. tuberculosis.
Generally, these mycobacteria can live only a few weeks outside the host because they cannot tolerate prolonged exposure to heat, direct sunlight, or dry conditions. Under cold, dark, and moist conditions, the organisms can survive longer. In cattle feces, M. bovis can survive for up to eight weeks.
Bovine TB is a chronic disease. Infected animals seldom show signs until it has reached an advanced stage. Some infected livestock appear to be in prime condition until slaughter, when their carcasses must be condemned.
Bovine TB can be transmitted from animals to humans and vice versa. Although young animals and humans can contract the disease by drinking raw milk from infected dams, the most common means of transmission is through respiration. Inhalation of aerosols — especially in enclosed spaces — is the most common route of infection for anyone who works with diseased livestock. Livestock can be infected when they share a common watering place contaminated with saliva and other discharges from infected animals.
Diagnosis and testing
TB lesions can be found in any organ or body cavity in diseased animals. In early stages of the disease, these lesions are difficult to find — even during post-mortem exams. But in later stages, the nodules or lumps become very evident in the lungs and associated lymph nodes, and in the lymph nodes of the head and intestinal tract, abdominal organs, reproductive organs, nervous system, superficial body lymph nodes, and bones.
Humans and animals with TB develop an immune response, which can be detected by the tuberculin skin test. To test cattle, 0.1 ml (5,000 TB units) of USDA PPD bovis is injected in the skin at the caudal fold of the tail. The site is inspected (visual and manually palpated) at 72 + 6 hours for a characteristic swelling reaction. If any cattle react, the farm is placed under quarantine and results reported to the state or federal veterinarian’s office. No animal movement is allowed on or off the farm without permission from the state veterinarian’s office. Nationally, approximately 1.3 percent of cattle tested by this method will show a reaction. But most are false-negative.
Next, a state or federal regulatory veterinarian will perform a comparative cervical tuberculin test, using the bovine and avian PPD. The hair is clipped, and the thickness of the skin at the injection site is measured pre- and post-injection. After the reading at 72 hours + 6 hours, the measurements are plotted on a scattergraph. The animal is then classified as negative, suspect or positive. Suspect animals may be kept in the herd and retested in 60 to 90 days, but the farm remains under quarantine. All positives and any suspects removed with the owner’s consent are sent to slaughter for post-mortem exam and testing.
The animals tested must be identified with official USDA metal ear tags, registered tattoos or brands that show a unique identification of each animal. Herd owners should keep records of the herd origin of all purchased animals and use these official ID’s to aid in traceback situations. Further regulations under the national-animal-ID system should also be followed.
The infection rate of TB in U.S. cattle and bison is less than 0.001 percent. But continuous testing of cattle and the cooperation of livestock producers and cattle veterinarians is needed as we strive to eliminate TB completely.
Jim Brett is a practicing veterinarian in Montezuma, Ga.