Producers urged to be cautious with commingling to reduce tuberculosis risk

Recreational cattle imported from Mexico may be one of the biggest culprits of bovine tuberculosis (TB) infections in the U.S. While the disease isn’t widespread in this country, it can be devastating for ranches with infected cattle. 

“The risk, as a whole, is low in the U.S., but it is still possible to buy cattle from an infected herd – and not just from Mexico. We still have sporadic outbreaks,” says Wyoming Assistant State Veterinarian Bob Meyer. 

Pre-purchase testing

Cattle producers are encouraged to test breeding cattle before purchasing them. 

“We can also reduce our risk by keeping a closed herd as much as possible,” he says. “If we are buying cattle and putting groups together, know the source.” 

“If we don’t, we are increasing our risk,” he adds.

Meyer tells producers they should be cautious when commingling cattle. 

“The U.S. brings in about a million steers each year from Mexico for feeding,” he explains. “Most of those go out for grazing. We also get about 10,000 steers a year from Mexico that are used for roping. These cattle come to the U.S. with a negative TB test but not all cattle infected with TB respond to the test.” 

“They start out as a roping steer, then they are a dogging steer, and then they are fed out in a feedlot,” he says.

Rodeo risk

Bucking bulls can also become infected from being commingled with the Mexican roping cattle. 

“Rodeo cattle can be a risk. That is why some states require additional TB tests on recreational animals,” he says. 

Most states require a health certificate, a permit and a negative TB test within the last 12 months to bring recreational cattle into a state.

Meyer tells producers that if they have a few roping cattle, they should be segregated from the beef breeding herd. 

“I have heard about people throwing a couple of old roping steers in with their replacement heifers, and then, they end up with a TB problem,” he states. 

Historic presence

TB has been in the U.S. since the early 1900s. The industry, state and federal government got together to develop a plan for totally eradicating TB, but more than 100 years later, total eradication has been elusive in this country. 

“TB is a public health issue,” Meyer says. 

People can contract TB from drinking raw or unpasteurized milk. It is also zoonotic, meaning it can be spread from animals to humans and vice versa. 

Inside the disease

“TB is a chronic bacterial disease than infects humans, the avian population and bovine,” Meyer explains. “It has a long incubation period, and those infected may never actually show clinical signs.”

Without clinical signs, Meyer says cattle can easily spread the virus to other animals through the air when they sneeze or cough or orally through feed, water or milk. Cows can also spread TB to their calf through their milk. 

“TB is primarily a respiratory disease, but it can impact the lungs, bone and other organs,” he says. 

In children in Central America, TB can cause them to become hunchbacks because it can impact the spinal cord.

Humans are vaccinated in countries where the disease has high prevalence. 

“Vaccination doesn’t really do a good job preventing TB, but it does do a good job preventing the clinical symptoms,” he says. 

In cattle, there isn’t a vaccine because it causes the animals to react to future TB tests, Meyer adds. 

Spread

Meyer reassures ranchers the disease doesn’t spread through casual contact but through repeated, continuous contact or commingling. 

“It is more likely in conditions of crowding and stress, like in the larger dairies,” he says. 

TB can be in the lymph nodes or lungs, and when the cattle cough, it disperses millions of organisms that can impact the herd, Meyer says.

“It impacts the respiratory tract and lymph nodes and can progress over several years. Lateral infections are possible, and may become active if the immune system is compromised by stress,” he explains.

Diagnosis

Diagnosis in live cattle is possible with a skin test, although Meyer admits it isn’t 100 percent accurate. 

“At best, the skin test is only 85 percent effective finding infected animals. About 15 percent can be infected, but it doesn’t show up on the test. We repeat the tests to catch the cows that are incubating the disease. Some cows can also be anergy, meaning they have a lot of disease, but they also don’t respond to the test. It may not be the best test, but it is the best we have today,” he says. 

During a skin test, tuberculin is injected at the base of the tail. Three days later, the veterinarian will come back to read it. 

“They feel for a response or some type of lump,” Meyer explains. “Some cows will have large responses and a lot of disease, while others have a minimal response and a lot of disease.”

“Often, there are no signs. I’ve seen very fleshy first-calf heifers with the disease,” Meyer says. “Many times, we don’t see the symptoms until the advanced stage of the disease.” 

Without clinical signs, TB may not be diagnosed until slaughter, and the meat will be condemned. 

“The main way we find TB is when cull cows go to slaughter. The federal inspectors find suspect lymph nodes and send them to a lab for more evaluation,” he explains. 

When the disease is diagnosed in a live herd, the only cure is complete total eradication.

“We have to depopulate all the animals, so we don’t miss one,” Meyer says. 

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.