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Skin test and gamma interferon enzyme-linked immunosorbent assay results in sheep exposed to dead Mycobacterium avium subspecies paratuberculosis organisms

Correspondence: ¹Corresponding Author: Suelee Robbe-Austerman, VDPAM/ISU, 1717 Vet Med, Ames, IA 50011, e-mail: srobbe{at}iastate.edu

Cell-mediated immunity (CMI) diagnostic tests, such as the gamma interferon enzyme-linked immunosorbent assay (IFN- 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 noninfected flocks, in MAP-infected flocks, these tests often identify animals that cannot 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 10¹⁰ autoclaved MAP organisms 3 times), and an inhalation-exposed group (dosed once with 1 x 10⁵ dead organisms). Lambs were skin tested and/or bled pre-exposure and 1, 2, 3, 4, and 12 months postexposure. No significant difference was seen with either the oral- or inhalation-exposed groups of lambs versus controls with either the IFN- ELISA or the skin test at any time pre- or postexposure. These results suggest that infection/invasion of MAP organisms must occur in order to have a positive skin test or IFN- ELISA beyond the false-positive rate. Simple exposure is not enough to elicit a detectable CMI response.

Key Words: Diagnostic testing • gamma interferon • paratuberculosis Johne's disease • skin test