Bovine tuberculosis (TB) no longer poses the significant cattle and human health risks it did in 1917 when the nation's TB eradication program was developed. But recent new TB cases underscore the need for renewed vigilance.

Nebraska found TB in a beef cow herd in June, the state's first case in 17 years. So far, 44 herds have been quarantined (some of those have since been cleared) and approximately 15,000 head of cattle are in the process of being tested.

Texas confirmed TB in a dairy herd in June. Now the state's TB-free status is in jeopardy. Texas gained accredited-free status in 2000 and then lost it two years later when TB was confirmed in dairy and beef herds. The state regained TB-free status again in 2006 after testing 2,014 purebred beef herds and all of the state's 818 dairies.

New Mexico, California, Michigan and Minnesota are also in the midst of dealing with TB.

Most infected cattle have been discovered through USDA's Adult Cattle Slaughter Surveillance program; these cattle comprise the lion's share of 1 million head or so cattle tested for TB annually.

Mexican cattle

“If we're going to get rid of TB in this country, as an industry we have to be willing to stop doing things that increase our risk of exposure,” says Bob Hillman, Texas state veterinarian and head of the Texas Animal Health Commission (TAHC).

Specifically, Hillman says reducing the risk of TB hinges upon identifying cattle with the highest risk for carrying TB and keeping them separate from breeding herds and replacement cattle.

Though there are obviously other sources of exposure, Hillman says, “I'm convinced a significant amount of the problem we encounter with TB in adult cattle in Texas stems from Mexican-origin cattle.”

Keep in mind, breeding cattle from Mexico aren't the problem, Hillman says, because of stringent TB-testing required for import eligibility. Nonbreeding cattle aren't subject to the same standards.

Maybe you buy a few head of Mexican steers to rope on. It could be you end up with more grass than anticipated, so you buy a load of Mexican steers and run them with or next to the cows. Or, Hillman says, “You don't have enough grass so you send your replacements to the feedlot for development, and, lo and behold, guess what's in the pen next door?”

Domestic dairy cattle are also a higher risk population, in part because of their close confinement and how TB is spread. Primarily it's via inhalation of the bacteria from close animal-to-animal (nose-to-nose) contact and through ingestion via feces and milk from infected animals.

“Most of the TB cases we've seen in Texas, the Southwest and out to California have been in dairy herds,” Hillman says. “That's why essentially all states have imposed more strict testing requirements for the interstate movement of breeding-age dairy-breed cattle.”

Wildlife can be the culprit, too, as producers in Michigan and Minnesota have learned.

Reporting on the state's TB eradication project last year, officials with the Michigan Department of Agriculture explained, “Our experience suggests that there is a continued spread of disease between wildlife and livestock. Serious challenges result from the differing views held by members of the agricultural, retail, hunting, residential and tourist communities. The current program is expensive, with $100 million spent on the program over the past 10 years. Funding at this level cannot be maintained.”

Fortunately, Hillman explains, “To our knowledge we don't have wildlife known to be infected with TB in any other states.”

In every case, the same risk-reducing biosecurity measures can decrease TB exposure (see “TB Dos and Don'ts”).

“Don't accept replacement cattle without a negative TB test, and a negative test for brucellosis, too,” Hillman says. “Don't commingle breeding and replacement cattle with high-risk cattle, especially those of Mexican origin.”

Control vs. eradication

Based on the number of herds with infected cattle, the national TB eradication program is successful. Approximately 5% of the nation's herds were infected when the program began, compared to less than 0.2% today. But a variety of folks today argue the cost is now running beyond the gain.

According to the New Mexico Livestock Board, federal indemnity in the state for TB exceeded $20 million in 2007 and destroyed over 10,000 head of cattle to remove 52 infected animals from the supply chain.

That's when there were federal funds for whole-herd depopulation. That money is gone. Producers are still indemnified for cattle that test positive. But the only management option is quarantine, repeated testing of the herd and removal of test-positive animals.

In the meantime, when TB is discovered, a state's TB status can be downgraded, spawning an expensive process to prove the risk level. Yet, the downgrading is based on the number of herds infected, rather than risk relative to a state's cattle population.

Those are some of the issues that state officials like Hillman were eager to address at a national meeting in July, hoping federal rules will be modernized.

“If we're going to get rid of TB in this country, we've got to quit doing things that increase our risk, and expecting USDA to bail us out,” he says.