Tuberculosis in Cattle - It's Not Totally Gone

Bovine tuberculosis is still found worldwide, even though many countries have greatly reduced or even eliminated the disease from their cattle herds.
Tuberculosis in Cattle - It's Not Totally Gone - Articles
Tuberculosis in Cattle - It's Not Totally Gone

Source: OIE World Animal Health Information System adapted by FAO EMPRES-i. Number of bovine TB cases reported by country in 2011.

Bovine tuberculosis is a cattle disease caused by the organism, Mycobacterium bovis (M bovis). Tuberculosis (TB) has been a major disease of humans for centuries (human strain, M tuberculosis). That bacteria is spread primarily between people. The clinical condition caused by the two tuberculosis strains are similar. This short summary is focused on the zoonotic strain found in cattle, M bovis. This bacteria can be a significant disease risk and can be transmitted to people from cattle by the inhalation of the bacteria or by drinking unpasteurized milk. In rare circumstances people can acquire the disease by eating undercooked meat from infected animals.

This bacterial strain has a thick waxy capsule and causes chronic debilitating infections in many animals not just cattle. Spill over into other species such as deer, pigs, sheep, goats, cats, dogs, opossum, raccoons, foxes, and coyotes is possible. Wildlife reservoirs make control programs very challenging. A few upper mid-Western states have endemic infections in deer and this is complicating eradication efforts in beef and dairy herds. (Armadillos are occasionally found positive for a cousin bacterial strain, M leprae, leprosy, along the border between the US and Mexico.) In Europe some badger populations are infected, and this wildlife reservoir is greatly slowing their TB control and eradication efforts.

Bovine tuberculosis is still found worldwide but many countries have greatly reduced or even eliminated the disease from their cattle herds. For many years cattle in Pennsylvania were routinely tested. Today our state and nearly all the US is free from this disease. In disease-free states, testing usually only occurs for sales or shows that cross state borders. Most cattlemen in the Mid-Atlantic region have gotten more lax regarding TB as the Eastern States have been free of this disease for a number of years. Unfortunately, despite getting very close to totally eliminating this disease in US cattle, we never quite got there in the early 1990's. Since that time there has been a steady increase in the geographic locations where the disease can be found. Just a few weeks ago a steer was diagnosed with TB at slaughter at a PA facility. Trace backs led to a farm in Indiana. Further testing revealed several more positive cases and that entire herd was humanely destroyed. Other major cattle states currently trying to eliminate this disease include: Michigan, Minnesota, California, New Mexico, and Texas. PA producers and veterinarians need to be vigilant and maintain high biosecurity standards or this complex and challenging disease may creep back into our herds.

Cattle primarily shed the bacteria from nasal secretions, feces, or milk. On occasion, the bacteria can be found in urine, semen or vaginal secretions. The bacteria can survive for months in the environment under favorable conditions. Cold, dark, and moist conditions promote the organism's survival while dry, warm, sunny conditions promote the organism's inactivation. The disease is a chronic debilitating disease, characterized by weight loss, weakness and a low grade fever. The lymph nodes, especially in the head and upper air way, become enlarged, caseous, or at times calcified. In the US most animals are asymptotic and are found via testing and culling of test positive animals or control programs in exposed herds. The other common detection method is via surveillance by federal inspectors from slaughter house specimens.

In the field the screening test is a tuberculin skin test. Typically in cattle this is a caudal tail fold skin test. An accredited veterinarian oversees and reads the test. 0.1 ml of tuberculin is injected intradermally. Animals that are sensitized to the bacteria develop a delayed hypersensitive reaction or swelling. The accredited veterinarian would check tested animals in 72 + 6 hours and report all significant swellings as a suspect. A state or federal veterinarian would then be notified and conduct more specialized tests (typically the Comparative Cervical Tuberculin or a limited number of specific blood based tests) to determine if the reaction is a true positive or a hypersensitivity reaction due to a similar but different, non-pathogenic bacteria. It is very important that all suspects be carefully tested with another test to accurately rule out M bovis. A few newer ELISA and blood tests are being developed but are not widely available nor as valid as the skin test.

At this time there is no effective vaccine and no cost effective, practical way to successfully treat infected animals. Animals known to be infected are humanely euthanized and their remains must be disposed of properly. Accurate records and traceable animal ID's are needed as a positive test triggers an investigation back to the home farm. It is extremely important to all Ag stakeholders that the source of infected animals is found and controlled as quickly as possible. Quick, accurate response and trace-backs are necessary for public health, access to domestic and foreign markets, and herd profitability.

In the past, test and slaughter as well as test and segregation strategies were successful in virtually eliminating the disease. These same strategies if enforced vigorously should also be effective in limiting the advance of M bovis back into more of the US herd. Excellent biosecurity as well as strategic testing and eradication programs will also help in the longer term goal to ultimately eliminate the disease from US cattle.



David R. Wolfgang, VMD, MPH DABVP-Dairy