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Humans can and do get infected with TB from animals, a type of trans-species communication known as zoonosis. In the case of TB, this zoonotic disease process is caused by the bacterium M. bovis, a member of the M. tuberculosis complex group. The incidence of TB zoonosis shows considerable regional variation depending on the presence and extent of the disease in the animal population, social and economic variables, standards of food hygiene, and the effective implementation of farm management preventative measures. In general, the infection of M. bovis in humans is higher in rural areas where there are higher rates of infected herd animals. The transmission of bovine TB from cattle to humans was substantially reduced in the 1930’s after the introduction of pasteurization of milk. Today, it is believed that the relatively low report rate of M. bovis associated with human TB is due in part to the failure of laboratories to distinguish between M. tuberculosis and M. bovis, and to social reluctance to report positive cases.TB in animal species is generally referred to as “Bovine TB” secondary to the prevalence of the disease in cattle. Bovine TB has affected animal and human health since ancient times, manifesting in lesions of the lung, bone, and other body parts. Although visible symptoms are highly species specific, the disease frequently presents as weight loss and general debilitation, and can be fatal. In highly developed areas of the world (i.e., North America, Australia, New Zealand, Japan, and Western Europe) TB elimination programs have been vigorously instituted into herd populations based on the test and slaughter method, with substantial success. Elsewhere in the world, the disease is widespread and its prevalence and associated risks to public health are closely linked to relative levels of economic development and the practicalities of implementing control measures for infected herd populations.
M. bovis bacteria are frequently excreted by infected animals via exhaled air, sputa discharge, feces, milk, urine, and vaginal and uterine discharges, as well as from peripheral lymph nodes. Housing (in contrast to free-range) and zero-grazing may predispose animals to the disease. For example, the highest incidence of bovine TB is routinely observed where intensive dairy production is most common, notably in the milk sheds. Transmission of bovine TB can also occur through animal contact with infected environmental sources such as soil and water.
Many TB-infected free-range wildlife populations may be visibly asymptomatic, showing no obvious clinical symptoms even when lesions are well developed, thus making timely detection problematic. In addition, even though seemingly asymptomatic, these animals are highly contagious. No practical treatment or preventive measures exist for free-ranging wildlife other than relatively ineffective cull practices, while sylvatic reservoir hosts often complicate domestic herd eradication efforts – for example, the ongoing challenges within the United Kingdom concerning TB management in cattle and badger populations.
Numerous methods are available for the detection of animals infected with TB. To date, skin testing remains the only “reliable” test to detect bovine TB. Among these, the CFT (Caudal-Fold Tuberculin Test) is often used as a frontline skin screening TB test. An accredited veterinarian is generally required to perform and read such skin tests. A positive response to the CFT test indicates that the animal has mounted an immune response capable of recognizing M. bovis. The test hasn’t shown to be as specific or sensitive when used on other animal species aside from Bovids, whether free-range or exotic. Skin testing often produces high rates of both false positive and negative results, particularly when the species tested is not bovid. Exposure to other closely related mycobacteria – such as M. avium (avian tuberculosis) and M. paratuberculosis (Johne’s disease) – represent two examples of other mycobacterium species that can yield a false positive CFT test. A false negative CFT response can occur in an animal that is infected with bovine TB but is also co-morbidly infected with another disease that prevents a proper immune response, is in the very early stages of the disease, or in the event of problematic test administration. It is estimated that the false negative rate of the CFT test may be as high as 15%. In other species, the tuberculin skin test is so unreliable that alternative tests are required in order to confirm TB infection.
TB due to either M. tuberculosis or M. bovis has gained increasing recognition as a serious emerging disease of multiple zoo and other exotic wildlife species, as well as in captive wildlife herds (e.g., Cervid species). Tuberculin skin testing has proven to be unreliable or ineffective for most of these species, necessitating reliance on other diagnostic procedures such as the culturing of trunk washings in elephants, which in turn are proving to be similarly insensitive and nonspecific.
Serological assays have shown promise as a diagnostic alternative to skin testing or culture testing for many of these species. Serological blood based TB assays are appealing not only due to better sensitivity and specificity for captive wildlife, exotic zoo species, and other non-traditional livestock, but also because they require only a single handling event, thereby minimizing capture-associated injuries. The serological test concept is simple, rapid, easy to interpret, inexpensive, and is very useful as a slaughter surveillance test or an effective and efficient trap and cull assay.
Chembio Diagnostic Systems, Inc. has developed a family of novel lateral-flow serological tests for TB for non-human primates (PrimaTB STAT-PAK®), white tail deer, reindeer, and elk (CervidTB STAT-PAK®), cattle (BovidTB STAT-PAK®), badgers (BrockTB STAT-PAK®), camels, llamas, and alpacas (CamelidTB STAT-PAK®) and exotic species such as elephants (ElephantTB STAT-PAK®). These tests are all antibody detection assays that employ unique cocktails of carefully selected recombinant antigens of M. bovis and M. tuberculosis. The tests can use serum, plasma, or whole blood samples and yield a result positive or negative result within 20 minutes.
Chembio Diagnostic Systems, Inc.’s family of TB STAT-PAK® veterinary test assays are currently undergoing validation for USDA licensure and market introduction.
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