Submitted to: Journal of Veterinary Diagnostic Investigation
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/29/2006
Publication Date: 1/1/2007
Citation: Robbe-Austerman, S., Stabel, J.R., Morrical, D.G. 2007. Skin test and Gamma Interferon enzyme-linked Immunosorbent assay results in Sheep exposed to dead Mycobacterium avium subspecies paratuberculosis Organisms. Journal of Veterinary Diagnostic Investigation. 19(1):88-90. Interpretive Summary: Johne's disease is a chronic, debilitating intestinal disorder in cattle characterized by diarrhea, reduced feed intake, weight loss and death. Cattle usually become infected as young calves by ingesting feces containing the causative bacteria. However, symptoms of disease do not usually present themselves until the animals reach 3 to 5 years of age or even older. During this time the animal is infected and may be shedding the organism in its feces without showing any clinical signs of disease. In addition to reduced milk production by these animals, they also present a potential infective threat to the rest of the herd. Johne’s disease is difficult to diagnose and therefore to control. Development of accurate and sensitive diagnostic tests is dependent upon understanding the immune responses of the host animal during infection. It is unknown if positive results on diagnostic tests reflect true infectivity or may be influenced by exposure to dead microorganisms. This study demonstrated that the exposure of sheep to dead mycobacterium will not result in false positive reactions in 2 diagnostic tests for JD. It is possible that this information will lead to better detection methods for paratuberculosis, particularly in the early stages of disease.
Technical Abstract: Cell mediated immunity (CMI) diagnostic tests, such as the gamma interferon enzyme-linked immunosorbent assay (IFN-gamma ELISA) and the johnin skin test, have the potential to detect animals infected with Mycobacterium avium subspecies paratuberculosis (MAP) early in the course of the disease. While these CMI tests tend to be relatively specific in non-infected flocks, in MAP infected flocks these tests often identify animals that can not be confirmed infected by any other reference test including necropsy and culture. The aim of this study was to determine if antigen exposure by inhalation or oral ingestion of killed MAP organisms would cause a detectable CMI response in sheep. Forty-eight lambs 4 months of age were randomly divided into a control group, an orally exposed group (dosed with 1 x 10**10 autoclaved MAP organisms three times) and an inhalation exposed group (dosed once with 1 x 10**5 dead organisms). Lambs were skin tested and/or bled pre-exposure, and 1, 2, 3, 4, 12 months post exposure. No significant difference was seen with either the oral or inhalation exposed groups of lambs vs. controls with either the IFN-gamma ELISA or the skin test at any time pre or post exposure. These results suggest that infection/invasion of MAP organism must occur in order to have a positive skin test or IFN-gamma ELISA beyond the false positive rate. Simple exposure is not enough to elicit a detectable CMI response.