Failure of immunisation against sporidesmin or a structurally related compound to protect ewes against facial eczema

Sixty ewes, divided randomly into four groups of 15, were immunised subcutaneously against sporidesmin (sdm) — bovine thyroglobulin (BTG) or 2-amino-5-chIoro-3,4-dimethoxy benzyl alcohol (ACDMBA) coupled to heat killed staphylococci or to bovine gamma globulin. Fifteen ewes served as untreated controls. Approximately 10 weeks after inoculation ewes were dosed orally with sdm at a rate of 0.1 mg/kg body weight/day for three consecutive days. Sdm antibody binding values, in plasma collected before dosing were higher in ewes immunised with sdm-BTG than ewes given the ACDMB A-complexes. Levels in the 15 untreated ewes were all very low. However, despite the presence of antibodies, the immunised ewes were not protected against sdm challenge; and cholesterol and bilirubin levels in serum and liver and urinary bladder damage scores, at slaughter, were all significantly higher (P<0.05) in the immunised compared to the control ewes six weeks after dosing. It is concluded from these results that subcutaneous immunisation against sdm or the structurally related substance used did not protect sheep against sdm dosing.