Detection of Johne's disease in an Iowa (United States) dairy herd : comparisons of the milk ELISA, serum ELISA, Gamma-Interferon and fecal culture tests and the effect of a skin-test using a cell-free sonicate of Mycobacterium avium subsp. paratuberculosis (19698) on the production of Gamma-Interferon
Analysis using kappa returned a value of -0.116 indicated a very low percentage of agreement between the two testing means. With the SELISA being the regulatory US standard it is not recommended to use the MELISA for purposes of culling decisions. Results of the skin testing portion of the study indicates that inoculation of animals with 19698 MpS significantly increased production of [Gamma]-IFN in cattle declared to have positive (MpS-P) baseline [Gamma]-IFN responses for Johne's disease. Peak production of [Gamma]-IFN occurred at 144 hr following skin testing and was statistically significant P = 0.0005 as compared to day 0. Levels of [Gamma]-IFN dropped on day 9, but remained at levels that were significantly higher than day 0 (P = 0.0325). Levels of [Gamma]-IFN again significantly increased on day 27 when compared to day 15 (P = 0.0362), and was significantly higher than day 0 (P = 0.0004). Further research is warranted to determine if the addition of skin testing to the [Gamma]-IFN assay could increase test sensitivity or durability.Cattle from the Iowa State University, Ames dairy herd were characterized for Johne's disease using results from the milk ELISA (MELISA) (Dairy Lab Services; Dubuque, IA), serum ELISA (SELISA) (IDEXX Laboratories, Inc., Westbrook, ME), [Gamma]-IFN assay (CSL Limited Parkville Victoria Australia) and fecal culture. Using the results of the initial [Gamma]-IFN assay and the SELISA determination, all herd members were divided into four groups consisting of: a negative SELISA and [Gamma]-IFN, suspect [Gamma]-IFN, positive [Gamma]-IFN and SELISA positive individuals. The first three groups were enrolled in the skin testing trial and were randomly assigned within group to one of two treatments. This consisted of an injection of either 100[Mu]g (0.1m1) of a M. paratuberculosis strain 19698 cell free sonicate (19698 MpS) or 0.1 ml of 0.9% saline solution as an intradermal injection. Blood samples were obtained on days 0, 3, 6, 9, 15, and 27 days post-injection for [Gamma]-IFN assay, while skin lesion measurements were made on days 0, 3, 6 and 9. The MELISA test returned the greatest number of test positive individuals at 12.96%, The SELISA was 7.41% test positive, [Gamma]-IFN assay was 4.40%, and fecal culture yielded 0% positive. Further evaluation indicated the SELISA had a significantly lower age (37.2 vs. 50.5 and 48.3 months respectively) at positive test (P> [T] = 0.0192) than did MELISA or [Gamma]-IFN. Due to the low number of fecal culture and [Gamma]-IFN positive individuals, and the significantly lower age at positive test it could be suspect that the SELISA had an inordinate number of false positive individuals. MELISA and SELISA results were compared using an XY plot. No common individuals were identified as test positive by both tests.