In addition to humans, a broad range of animal species is susceptible to TB, usually resultant from infection with M. bovis. Though more rarely isolated in humans, the bacterium M. bovis is responsible for tuberculosis in domestic ungulates (e.g., cattle, goats, sheep, etc.) as well as wild animals (e.g., deer, elk, badgers, possums) and captive exotic animals (e.g., elephants, giraffes, and camels, among others). Humans infected with M. bovis who subsequently go on to develop TB exhibit clinical symptoms indistinguishable from M. tuberculosis-infected TB, the pathogeneses of which are identical. While animal-to-human M. bovis infection is well documented, evidence of human-to-human transmission of M. bovis is limited and largely anecdotal. In infected populations, M. bovis shows a high degree of virulence for both humans and animals.
Humans can and do get infected with TB from animals, a type of trans-species communication known as zoonosis. In general, the infection of M. bovis in humans is higher in rural areas where there are higher rates of infected herd animals. 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.
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.
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. 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. 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.
Veterinary TB Resources:
- Elephant Care International – www.elephantcare.org
- Elephant News www.elephant-news.com
- Michigan Department of Natural Resources – www.michigan.gov/dnr
- Ontario Ministry of Agriculture, Food and Rural Affairs – www.omafra.gov.on.ca
- United States Animal Health Association – www.usaha.org
- United States Department of Agriculture Animal and Plant Health Inspection Service – www.aphis.usda.gov
- Veterinary Association – www.worldvet.org
- World Organization for Animal Health – www.oie.int
- World Animal Health & Nutrition News – www.animalpharmnews.com