Differential antibody responses to Corynebacterium pseudotuberculosis in sheep with naturally acquired caseous lymphadenitis.
Enzyme-linked immunosorbent assays (ELISA) with Corynebacterium pseudotuberculosis cell wall and bacteria-free supernatant with exotoxin preparations as antigens, and hemolysis inhibition tests were used to detect antibodies in the sera of adult range sheep with naturally acquired caseous lymphadenitis (CL). The extent and severity of lesions were quantitated on the basis of a lesion score, derived from an examination of the carcass (peripheral lymphoid tissue) and viscera (including internal lymphoid tissue) at the time of slaughter. The overall prevalence of C pseudotuberculosis-positive CL lesions in 104 sheep was 31.7%. The cell wall ELISA detected antibodies in 96.9% (32/33) of sheep with C pseudotuberculosis-positive CL lesions. The exotoxin ELISA detected antibodies in 84.8% (28/33) of positive sheep in the same group. Both ELISA resulted in a high number of apparent false-positives, with 64.7% and 49.2%, respectively, positive optical density (OD) values in sheep with no gross CL lesions and no apparent C pseudotuberculosis infection. There was no significant relationship between the extent of lesion development (lesion score) and OD values in both cell wall (r = 0.472) and exotoxin (r = 0.464) ELISA. Similarly, there was no significant relationship between the titer of antitoxin antibodies, as measured by the hemolysis inhibition test, and the extent of disease. These investigations indicate that those ELISA that use crude C pseudotuberculosis antigens are of questionable utility in the field, where C pseudotuberculosis infection is endemic in many sheep populations. Furthermore, these studies suggest that antibodies that are reactive with components of C pseudotuberculosis and that develop in response to infection may have little impact on the recovery of the host.