Comparison of the γ-Interferon ELISA and the Skin Test for the Detection of Sub-clinical Johne's Disease in Cattle

Abstract

The diagnosis of Johne's disease in cattle can be difficult. Cattle are often infected for years before they begin shedding Mycobacterium avium subsp. paratuberculosis in their feces or mount a detectable humoral immune response. Current diagnostic tests, serology and fecal culture, are effective diagnostic tests to detect animals in advanced stages of disease, but fail to reliably detect infection in young replacement animals.
 However, it is thought that the majority of animals elicit a detectable cell mediated immune (CMI) response before fecal shedding occurs, and a detectable humoral immune response after fecal shedding. Therefore, it is important to evaluate CMI diagnostic tests as potential tools to use in the detection of subclinical infection, especially in replacement populations. There are two common methods for evaluating the CMI response to M. paratuberculosis, skin testing and the γ-interferon (IFN-γ) test.
 In the past, skin testing has not been viewed favorably in the literature. This may be in part due to the use of a gold standard (either fecal/tissue culture) to determine Johne's-positive status of individual animals. If the CMI response occurs before lesions are detected or organisms are at high enough concentrations to culture, then the skin test would be viewed as lacking specificity. In addition, no studies are published that carefully evaluate the skin test in known negative populations.
 Although the IFN-γ has worked well in research settings, field studies must be done to evaluate the usefulness of this test in production settings. Experience and research with the skin test and IFN-γ ELISA for the detection of Mycobacterium bovis has shown there is high agreement between the two tests. Therefore, our objectives in this study were to evaluate the agreement between the IFN-γ and skin test for the detection of Johne's disease, and to see if these tests detect the disease status of young animal populations.