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The most common cause of tuberculosis in cattle is Mycobacterium bovis. Most often the cattle may be infected with the organism and not exhibit clinical signs of disease. In other cases, the clinical signs may be very non-specific and therefore, difficult to diagnose. Mild respiratory signs are reasonably common, as this is the most frequent route of infection.
Although most cattle with tuberculosis do not exhibit clinical signs, they pose a serious health hazard to other livestock as well as to humans. The organism may be spread via exhaled droplets, sputum, feces, milk, urine, vaginal discharge, semen and draining lymph nodes.
There are other Mycobacterium spp such as M. avium and M. tuberculosis which may cause infections, however, their primary importance is in interfering with diagnostic testing in eradication programs caused by cross-reactions. Tuberculosis has essentially been eradicated in most areas of the United States and many other countries by implementing testing programs and using less than perfect testing modalities. The first test that is used is called the Caudal Fold Test (CFT). If the test result is either a suspect (reactor) or positive, the CFT is followed-up with the short interval comparative tuberculin test (SICTT) or sometimes referred to as the comparative cervical test. Following the SICTT, if the test is positive, the animal is slaughtered and tissues are collected for culture of the organism and histopathology. Follow-up testing of suspect or positive animals with further testing is called Testing in Series.
In series testing, tests are conducted consecutively, based on the results of the previous test. In essence, only the animals that test positive receive further testing which maximizes specificity and the positive predictive value. However, the down side is that in most cases, when you increase specificity, you decrease sensitivity and therefore, reduce the predictive value of a negative test. Series testing places much more credence in the value of a positive test, but it is possible that more disease will be missed. Another common expression used to explain the reasons for using series tests is "asking the animal to 'prove' it is affected". Series testing is an important part of disease eradication programs as it helps to prevent unnecessary removal of false positive animals (Thrusfield, M, 1995).
Currently, the prevalence of tuberculosis in the United States is 0.2%. The sensitivity of the caudal fold test (CFT) ranges between 68 and 95%, but for the purpose of this example we will use 72% as the sensitivity (Francis et al, 1978). The specificity of the CFT ranges between 96 and 98.8%, but for purposes of this example we will use 96% (Francis et al, 1978). The total number of animals tested in this example is 10,000.
|Disease +||Disease -||Total|
|Test + CFT||16||399||415|
|Test - CFT||4||9,581||9,585|
If you need a review of sensitivity, specificity and predictive values, please refer to the introductory section of this Web site. In this above instance, the predictive value of the positive test is 16/415= 3.86%. If the cattle were removed from the farms and destroyed, it would result in greater than 96% of the cattle being removed unnecessarily. Therefore, a second test is performed on the positive animals only. This second test is utilized to enhance the positive predicitive value by increasing the specificity and to reduce the number of false positive animals.
In order to increase the specificity of the testing process, the cattle are subjected to a second test procedure called the short interval comparative tuberculin test (SICTT). The SICTT has a sensitivity of between 77 and 95% and a specificity of greater than 99% (Francis, J et al, 1978; Monaghan, ML et al, 1994). For the purpose of this example, we will use a sensitivity of 86% and a specificity of 99% (Francis, J et al, 1978; Monaghan, ML et al, 1994). We will thus perform the second test on the positive animals from the results of the first test. We use this test to increase both sensitivity and specificity. However, due to the difficulty of performing this test, it would not be easily applied as a first line screening test.
|Disease +||Disease -||Total|
|Test + SICTT||14||4||18|
|Test - SICTT||2||395||397|
After the results of the second test, 18 animals would be removed from the population. Of the 18 animals, 14/18 (77.8%) would be true positives and 22.2% (4/18) would be false positive animals. However, instead of removing 399 animals unnecessarily, we would only remove 4 unnecessarily. There is a caveat in that we have now reduced our overall sensitivity to 70% (14/20), but we have increased our specificity to 9,976/9,980 (99.96%). The above 2 examples are the essence of Serial Testing.
Although application of these tests are the basis for the Tuberculosis eradication program in the U.S., as you can see, they are not perfect. However, the tests have worked well in combination to produce a very successful program. These examples are a good illustration of testing in series, and the principles can be applied in the next example used in the detection of Johne's disease in the next section of this Web site (Example 4).