Validation of the gamma-interferon test for ovine Johne's disease.

Abstract

In theory, the interferon-γ (IFN) test has potential for improved control of ovine Johne’s disease (OJD) by early detection before contamination of pasture and transmission of infection. To validate the IFN test, a project has been completed for determining specifity and sensitivity. Because of nonspecific IFN responses, raised cut-points were required to achieve high specificity (≥98%). This resulted in reduction of sensitivity to below 50%, limiting its application for early detection or certification from disease freedom. The assay may have application in a test and cull program as a surrogate test for faecal shedding and the removal of sheep with severe disease but this approach to control of OJD will require further confirmation. A major limitation for adoption is the widespread use of vaccination precluding the use immunological tests for diagnosis. Other limitations, apart from relatively low sensitivity, include cost of the test as well as a narrow window of ~6 hours between the commencement of blood sample collection and laboratory initiation of the assay so that test result validity is not compromised. Thus, in its current format, it is unlikely, that the IFN assay, apart from possibly stud flocks, will have wide application in the sheep industry.