|Robbe Austerman, Suelee|
Submitted to: American Association of Bovine Practitioners Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/18/2003
Publication Date: N/A
Technical Abstract: The diagnosis of Johne's disease in Cattle can be difficult. Cattle are often infected for years before they begin fecal shedding or mount a detectable humoral immune response. Current diagnostic test, 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 sub-clinical infection, especially in replacement populations. There are two common methods for evaluating the CMI response, skin testing and gamma interferon (IFN-gamma). In the past skin testing has not been viewed favorably in the literature. This may be impart due to the use of a gold standard (either fecal/tissue culture) to determine Johne's positive status. 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. NO studies are published that carefully evaluate the skin test in known negative populations. Although the IFN-gamma 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-gamma ELISA for the detection of Mycobacterium bovis has shown there is high agreement between the two tests. Therefore, our objectives in this study are to evaluate the agreement between the IFN-gamma and skin test for the detection of Johne's disease and to see if these tests detect the disease status of young animal populations and if so, to what level. Known negative and positive beef herds were identified. On day one, 0.1 ml of Johnin was injected intradermally in a shaved area of the neck. Both a serum sample and heparinized whole blood sample was obtained as well as ~50g of fecal material. On day two the heparinized whole blood was set up for the IFN-gamma test. One ml each of whole blood was exposed to pokeweed mitogen (PWM), johnin, M. Avium and one well was left as a control. The blood was then incubated for 18 hours and the plasma was then used to measure gamma-IFN production by an ELISA (Bovigam, Biocor; Omaha, NE). Seventy-two hours post injection; the skin test site was read and measured. Preliminary results indicate that in one Johne's infected beef herd 14% (64/452) were positive on skin testing and 10% of animals (9/90) between the ages of 7-9 months of age were positive. In this herd every other animal was IFN-gamma tested. The initial run of IFN-gamma tests were all negative. It was felt that the cells were cold shocked enough they were unable to respond to the johnin or avium PPD. The samples were taken in December with an average temperature of 33f. Every 20 minutes samples were taken from chute side and put in a heated vehicle. The herd was located 2.25 hours from the lab. Interestingly, the positive control, PWM, had an average absorbance reading of 1.3, which would be considered adequate. One month later, the herd was revisited 3/35 samples were gamma-IFN positive. The average PWM response across all 35 samples was 1.8. It appears that PWM may not be an acceptable positive control for the IFN-gamma test. Further results will be discussed.