Observations on a skin sensitivity test for ovine enzootic abortion

Abstract

A delayed hypersensitivity skin test for ovine enzootic abortion (OEA) was evaluated using a Chlamydia psittaci (ovis) A22 strain antigen prepared from infected avian yolk sacs. A control antigen was also prepared from uninfected yolk sacs. Twenty-eight ewes and eight of their lambs were tested; 16 of the 28 had lambed three to five months earlier, the remaining 12 had not conceived. They had all been challenged with a field strain of C. psittaci (ovis) by various routes. Two of the 16 pregnant ewes had been clinically affected with C. psittaci (ovis) when they lambed and the chlamydiae they excreted contaminated the environment of the other ewes and the lambs as they were born. Consequently, 23 of the 28 ewes seroconverted in this perinatal period. However, seroconversion was transient and all except three had insignificant titres at the time of the skin test. Fifteen control sheep with no experimental contact with C. psittaci (ovis) were also tested. Twelve of the 16 ewes that had lambed developed positive skin tests, whereas only three of the 12 barren ewes, two of the eight lambs and two of the 15 control sheep were positive. Only three sheep subsequently developed clinical OEA. They were two of the experimental sheep that had lambed and one of the lambs. None of these three had a skin reaction. The results suggest that ewes aborting following C. psittaci (ovis) infection may be those which failed to mount an effective cellular immune response as demonstrated by the delayed hypersensitivity skin reaction. The sheep that had not conceived and the lambs were less able to mount a cellular immune response to chlamydial infection than the ewes that had lambed. Cellular, rather than humoral immune responses were more frequently detectable after infection of mature sheep.