If you graduated from veterinary school within the last 30 or so years, you may have thought of tuberculosis as an ancient, retired disease that posed no threat to your clients’ cattle operations or the health status of your state. Until recently, that is.
Bovine tuberculosis has mounted a mild resurgence in recent years. While by no means an epidemic, it requires the attention and respect of bovine veterinarians and producers throughout the United States, and not just in areas known to be battling the disease.
History and hot spots
Tuberculosis (TB) is caused by three species of Mycobacterium: Mycobacterium bovis, Mycobacterium avium and Mycobacterium tuberculosis. M. bovis is by far the most prevalent of the three and infects a much wider range of animal species than the other two organisms. While “bovis” indicates that it infects cattle, it also can infect all other warm-blooded vertebrates, including humans. M. avium can affect all species of birds, as well as hogs and cattle. M. tuberculosis is primarily a human organism but also can infect hogs, cattle and dogs.
In the early 1900s, bovine TB was the most common infectious disease of cattle and hogs in the United States. According to USDA Veterinary Services, it caused more losses among U.S. farm animals in the early part of the century than all other infectious diseases combined.
Because of its zoonotic potential, bovine TB also was the main impetus for pasteurization of milk for human consumption and the establishment of the federal meat inspection system, according to Bob Meyer, DVM, MS, USDA Animal and Plant Health Inspection Service (APHIS), Veterinary Services, Ft. Collins, Colo. “When the Cooperative State-Federal Tuberculosis Eradication Program was begun in 1917, 5 percent of all cattle tested for TB reacted to the test,” says Meyer. “By 1942, that number had been reduced to less than one-half of 1% and remains similarly low today.”
The incidence of TB in humans (in the U.S. and other industrialized nations) also has dropped dramatically in the last century, due to a combination of the livestock eradication program, improvements in sanitation and hygiene, the discovery of effective antibiotics to treat humans and the pasteurization of milk.
Epidemiological surveillance at slaughter plants was begun in the 1960s and continues today, involving federal meat inspectors evaluating the lungs and lymph glands of cattle. If they suspect an animal is infected with TB, tissue samples are sent to APHIS’ National Veterinary Services Laboratory in Ames, Iowa, for confirmation, and the carcass is held until results are received. If the laboratory confirms that the suspect lesions were caused by bovine TB, the infected animal’s carcass is condemned, and its history is traced back through market channels to the originating herd, which is then tested for tuberculosis.
In addition, USDA officials perform a “trace-in, trace-out” assessment of such herds, tracking down all of the herds that have sold cattle to the positive herd and all herds that have received cattle from it during the lifetime of the positive animal. Those herds usually are quarantined and tested as well.
When Arnie Hentschl, DVM, started practicing more than 50 years ago at Harbor Beach Veterinary Service, Harbor Beach, Mich., TB was a fundamental part of his duties. “Practitioners would perform the caudal fold test on all animals in a herd,” Hentschl explains. Any animal showing a sign of reaction to the caudal fold test was considered to be a reactor, branded with a “T” and required to go to slaughter within 30 days. If visible lesions confirmed TB at slaughter, the carcasses were condemned. Livestock farmers were so informed and were paid an indemnity price from the government for all animals sent to slaughter, but if they were found to have no visible lesions (NVL), farmers were understandably upset. “NVLs were tough for the prac-titioner to justify to the producer,” recalls Hentschl. “One time we had to remove 30 cows from a 60-cow herd, and only one turned out to have visible lesions. The current methodology of conducting a confirmatory test, such as the comparative cervical test, certainly helps to minimize ‘false positive’ productive bovines from being removed from the herd.”
Hentschl has witnessed the near-complete eradication of cattle TB in Michigan, and the state eventually was granted tuberculosis Accredited Free status. That began to change about 10 years ago, when Michigan Department of Natural Resources (DNR) officials began detecting TB in the state’s whitetail deer population. Feeding practices of wild deer and high deer numbers encouraged its spread in that population and eventually to cattle in Michigan’s northeastern Lower Peninsula (see TB case study, page 28). Michigan lost its Accredited Free status and has not regained it today. “We think the cattle initially gave it to the deer, and they’ve passed it back to the cattle,” Hentschl postulates. “We’re not exactly back to square one, but we’re dealing with more testing, confined animal movement and loss of income for some cattle producers.”
Minnesota, declared tuberculosis Accredited Free for 34 years, dealt with a TB resurgence in 2005 that eventually led to a drop in USDA status from Accredited Free to Modified Accredited Advanced (MAA). Five cattle herds in the state have been declared infected with TB since July 2005. As of January 1, 2006, 65 additional herds had been quarantined due to selling animals to, or purchasing them from, infected herds or fenceline contact with infected herds. More than half of the quarantined herds already had been released from quarantine after whole-herd testing produced no positive animals.
Minnesota officials now are working with producers and the USDA to depopulate and disinfect the premises where known TB-infected animals originated. The owners of infected herds are eligible for an indemnity payment from the U.S. government, the amount of which is based on an independent herd appraisal. The same scenario applies to owners of quarantined herds who sacrifice potentially exposed animals for testing.
As a result of Minnesota’s new MAA status, the USDA requires that all sexually intact cattle be tested for bovine TB within 60 days of shipment. Cattle are exempt from testing if they are moved interstate directly to slaughter or an approved feedlot or if the animals are from a TB-accredited herd. Cattle shipped across state lines also must comply with the testing requirements of the receiving state, which can be more stringent than USDA standards.
As additional measures of safety, the Minnesota DNR is evaluating the deer population in the regions of the positive herds, and the Minnesota Board of Animal Health is offering testing, free of charge, to herd owners within a 15-mile radius of the TB-positive herds.
New Mexico and Texas are also not completely TB-Accredited Free. New Mexico has a Modified Accredited Advanced zone in the southeast part of the state; the remainder of the state is Accredited Free. The entire state of Texas has been classified as Modified Accredited Advanced since June 2002, due largely to the detection of several TB-positive herds in the El Paso milkshed. Both states are working on TB surveillance, testing and prevention, with the goal of regaining complete Accredited Free status. According to Meyer, as a part of this effort, all of the dairies in the El Paso area were depopulated as of March 2006.
|TB lesions in the lung.|
|Lungs with a mild case of TB.|
Etiology and pathology
Understanding the characteristics of bovine TB and its modes of transmission are critical to limiting its spread. Mitch Palmer, DVM, PhD, Lead Scientist for Bovine Tuberculosis Research for the USDA Agricultural Research Service’s National Animal Disease Center in Ames, Iowa, explains that animal density is a major factor in the transmission of M. bovis.
“Inhalation of airborne droplets carrying the bacteria from the respiratory tracts of infected animals is the primary route of new infections,” says Palmer. “Thus, dairy herds tend to be more susceptible to transmission, because their housing situations generally place many animals in close proximity to one another.” This is especially true in poorly ventilated barns. Livestock also are more likely to infect one another when they share common feeding and watering areas contaminated with saliva from infected animals.
Palmer says the bacteria can remain airborne for several hours and, when inhaled, infect the deepest regions of the host’s lung tissue because they are small enough to bypass the protective actions of the trachea and cilia in the respiratory tract.
Oral transmission also is possible, and Palmer notes that it probably is the more likely route of TB transmission in deer and other wildlife. In laboratory experiments, various feedstuffs have been found to contain M. bovis up to eight weeks when stored at 75˚F and 14 weeks when stored at 32˚F. The organism does not survive well when exposed to heat, direct sunlight or dry conditions, which is probably the reason that pasture transmission has not been found to be a major issue. M. bovis can survive longer under cold, dark and moist conditions. Feeding on the carcasses of infected animals is a likely source of infection for non-cervid, carnivorous wildlife, including black bear, bobcat, coyote, opossum, raccoon and red fox.
Transmission from dam to calf also is possible in cattle if the disease has advanced to the stage that it infects the lymph nodes associated with the mammary glands. Bovine tuberculosis is not realistically treatable in cattle, and experimental vaccine results indicate only about 50% protection.
Palmer says the zoonotic potential of bovine TB is quite limited today. “Although there have been cases documented of farmers contracting TB from their cattle, 90% of humans who are exposed to M. bovis are not affected,” he notes. Pasteurization of milk kills the organism, as does thoroughly cooking meat. All cattle carcasses identified as TB-infected are condemned, so the likelihood of humans consuming beef from infected animals is low, although a slightly greater possibility exists that humans could consume venison from infected deer. There has not been a single, documented case of a deer hunter being infected with TB from eating venison, and Michigan harvests about 500,000 deer annually.
Bovine tuberculosis is a chronic disease that takes years to develop. Many infected animals never show clinical signs, and Meyer notes that cattle feedlot operations have less trouble with the disease because the population of relatively young animals turns over quickly enough that clinical disease, and its transmission to other animals, seldom occurs there.
Older beef and dairy cows, how-ever, can become clinical. Palmer outlines the “classic” model of tuberculosis progression in cattle:
1. Exposed animals become infected, usually via the respiratory tract.
2. The animals’ defenses could clear the infection, or
3. Lesions could develop, first in the lymph nodes of the head, then the lungs, then the mammary system.
4. The infection could stay in a quiescent state unless/until the animal is stressed.
5. Clinical signs could develop, including emaciation, depression, intolerance to exercise, coughing, nasal discharge and difficulty breathing. In severe cases, superficial lymph nodes of the neck can develop large abscesses that may rupture and drain through the skin.
Testing the problem away
With one exception, the testing performed to detect and eradicate bovine TB has changed little since the USDA/APHIS began its eradication program in 1917.
The common, two-step testing process consists of:
1. A licensed, USDA-accredited veterinarian performs a caudal fold tuberculin (CFT) test to screen animals and identify those that need to undergo further testing. A small amount of non-infectious bovine TB protein is injected in the layers of skin underneath the base of the tail. After 72 hours, the same veterinarian examines the injected animals to check for swelling at the injection site. Animals that test negative need no further testing; positive responders must be tested further.
2. For animals testing positive on the CFT test, a state or federal veterinarian performs a comparative cervical tuberculin (CCT) test. The CCT test is performed within 10 days of the CFT injection date; otherwise, a waiting period of at least 60 days must be observed to negate any residual effects of the previous test. This test is performed by injecting two different, non-infectious TB proteins into the neck. The same veterinarian measures the skin thickness around the injection site and declares the status of the animal 72 hours later.
In 2003, another confirmatory test option was made available and has been used in parallel or as a replacement of the CCT test. The BOVIGAM® Bovine Gamma Interferon Test kit is a monoclonal antibody-based sandwich enzyme immunoassay formulated to evaluate the reaction of lymphocytes in whole blood cultures to the introduction of specific mycobacterium antigens present in bovine tuberculin purified protein derivative. In general, animals infected with M. bovis have lymphocytes that will recognize the antigens, while non-infected animals will not. The BOVIGAM test is used widely throughout the world for bovine tuberculosis testing and is marketed in the United States by Prionics USA Inc.
Tom Kellner, DVM, Professional Veterinary Services, Inc., recommends the use of BOVIGAM as an ancillary test to the CFT skin test. Kellner says the sensitivity of the BOVIGAM test is between 82-100% while the specificity ranges from 94-100%. When combined with the initial screening of the caudal fold test, “we maximize the accuracy and sensitivity of our test results, which allows us to step up our efforts in eradicating this disease,” states Kellner.
The use of BOVIGAM in each state is determined by the designated TB epidemiologist, the Area Veterinarian in Charge, the Regional TB Epidemiologist and the state veterinarian’s office.
What’s at stake?
|TB lesions in an opened lymph gland.|
Despite tremendous advancements in reducing its prevalence, the reality is that bovine tuberculosis is not gone for good in the U.S. If the incidence of bovine TB increased even further in this country, at stake would be:
Increased costs to cattle producers, who would bear the financial and physical burden of more testing, restrictions on cattle movement, potential loss of animals and genetic progress, and time and labor devoted to rectifying the problem. Management systems, like contract heifer rearing, also could be at risk of regulatory
More bureaucratic chores for veterinarians, cattle producers, livestock haulers, state and federal officials, as testing efforts would have to be stepped up, and animal movements monitored more strictly.
Possible restrictions on international exports of U.S. live cattle, beef and dairy products.
Potential risks to human health, as a higher prevalence of M. bovis could increase the possibility of TB transmission via aerosol inhalation, or quality assurance failures in milk pasteurization or meat inspection. “It is important to remember that, worldwide, tuberculosis is still one of the leading causes
of death associated with infectiousdiseases in humans and that several million people die from it each year,” cautions Kellner. “In industrialized nations, the incidence of human TB has declined steadily, but if we are not careful, it can be back on the rise as we experienced in the late 1980s with HIV.”
However, the experts are optimistic about eventual bovine TB eradication in the United States. Palmer says on-going research -- much of it being performed in conjunction with human TB research -- in both diagnostic testing and vaccine development could accelerate the eradication process. “An ideal tool would be an oral bait vaccine that could be provided to wildlife,” he says, “but it is probably more likely that we will see more testing options first.”
Hentschl says that coming nearly full circle with the disease has taught him a valuable lesson in consistency and persistence. “It’s one of those things you just can’t let up on, and it’s something all veterinarians, regardless of their location, need to become familiar with,” he advises. “TB is a regulatory disease, so, like it or not, it’s going to be part of every bovine veterinary practice. If you’re not certified to perform TB testing, go out and get that training, and keep yourself up-to-date on developments with the disease.”
“Mandatory, permanent animal identification is one of the best things that could happen to the TB eradication effort, because it will help us pinpoint, without question, the herd of origin and interim location record of every TB-positive animal,” Meyer says. “It would make containment and control of the cases that do surface much more efficient.”
Kellner believes that the recent bovine TB events in the U.S. actually have generated renewed energy for the TB eradication effort. “We’ve come a long way, and maybe this is what we needed to energize our efforts in research and muster the collective effort to cross the finish line,” he states. “If we find a way to eliminate the reservoirs, we can finish off the disease.”
|Arnie Hentschl||Mitch Palmer||Bob Meyer||Tom Kellner|