Effect of dexamethasone treatment of tuberculous cattle on results of the gamma-interferon test for Mycobacterium bovis

Abstract

The purpose of this project was to determine the effect of dexamethasone (DEX) treatment of tuberculous cows on antigen-stimulated gamma-interferon (γ-IFN) production in a commercial Mycobacterium bovis γ-interferon test (γ-IFN test) developed for diagnosis of bovine tuberculosis. In the γ-IFN test an enzyme-linked immunosorbent assay is used to detect bovine γ-IFN in the plasma from whole blood samples cultured with M. bovis and Mycobacterium avium tuberculin purified protein derivatives (PPDs). DEX is a synthetic glucocorticoid commonly used as a potent anti-inflammatory agent, and experimentally as a model of stress-induced immunosuppression in cattle. DEX treatment has previously been associated with decreased lymphocyte response to mitogens in cattle, which led to the current hypothesis that DEX treatment would suppress stimulated γ-IFN production resulting in false negative results in the γ-IFN test. In replicate studies using naturally infected dairy cows, blood was drawn daily for at least 2 days prior to DEX treatment, during 3 days of DEX treatment, and for at least 9 days post-DEX. Results of the γ-IFN test were evaluated by optical density (OD), and by three OD calculation methods: two different methods suggested by the manufacturer, and a method adapted from the evaluation of a bovine γ-IFN test used in Australia. Prior to DEX treatment all cows had positive γ-IFN tests by each calculation method. As early as 24 h after the first DEX injection a decline in PPD-stimulated γ-IFN production was reflected in OD data for all cows. Calculated γ-IFN test results were negative after DEX treatment for all but one cow, which was known to produce relatively large amounts of γ-IFN as measured by this test. The degree of γ-IFN suppression, and the number of days that γ-IFN test results were negative after DEX treatment (1–8 days), varied by cow and by data calculation method. Treatment with DEX is associated with suppressed PPD-stimulated γ-IFN production, which may be reflected as false negative results in the γ-IFN test depending on the data calculation method applied. The results have implications for the management conditions and medical treatment schedule under which samples for the γ-IFN test are collected.