Evaluation of cell mediated immune diagnostic tests to detect Mycobacterium avium subspecies paratuberculosis

Abstract

Mycobacterium avium subspecies paratuberculosis (MAP) causes Johne’s disease, a chronic progressive fatal disease of ruminants. Similar to tuberculosis, diagnostic tests based on detecting the organism or antibodies are not capable of detecting infected animals in the early stages. Consequently, diagnostic tests based upon the cell mediated immune response, such as the skin test and gamma interferon enzyme-linked immuno absorbent assay (ELISA), which have been successful at detecting tuberculosis need to be investigated further for paratuberculosis. The first paper evaluated the skin test in sheep along with two antibody based diagnostic tests using Bayesian statistical methods that estimate the sensitivity and specificity of diagnostic tests in multiple populations without a gold standard. The second paper used receiver operating characteristic (ROC) analysis to evaluate the accuracy of the gamma interferon (IFN-γ) ELISA in subclinically infected sheep. These 2 studies similarly concluded the skin test and IFN-γ ELISA had an estimated sensitivity of around 70% and a specificity of 98%. The next paper evaluated the environmental parameters necessary for handling whole blood for the IFN-γ ELISA and assessed the validity of positive controls. The conclusion was blood should be maintained at room temperature, stimulated within 8 hours and phytohemaglutinin A (PHA) was the most accurate positive control tested in cattle. Finally the last paper attempts to further characterize the meaning of a positive response to the skin test or the IFN-γ ELISA by assessing the consequences of environmental exposure to dead MAP. This study found that inhaled or ingested dead MAP does not stimulate a detectable response on the skin test or IFN-γ ELISA.