SUSCEPTIBILITY OF FINNISH LANDRACE, ROMNEY, AND FINN X ROMNEY LAMBS TO A SPORIDESMIN CHALLENGE

Five Finnish Landrace (F) rams and 10 Romney (R) rams were mated with F and R ewes to generate F x F, Fmale x Rfemale, and R x R lambs. Thes e lambs were compared for their susceptibility to an oral challenge wi th sporidesmin, the toxin causing facial eczema. All lambs of 17.0 kg or above were dosed at 4 months of age with sporidesmin (0.15 mg/kg li veweight). Resistance or susceptibility was assessed by taking a blood sample 21 days after challenge, and measuring serum gamma-glutamyltra nsferase (GGT), an enzyme whose concentration is elevated after the li ver injury resulting from a toxic challenge. A total of 199 animals (3 8, 62, and 99 animals respectively per breed group) were challenged, a nd percentages of animals with GGT concentrations below a threshold va lue indicative of subclinical facial eczema (50 i.u./l for F x F, 60 i .u./l for F x R and R x R) were 89, 69, and 33 respectively (P < 0.001 overall). For pairs of breeds, F x F and F x R had higher percentages unaffected than R x R (both P < 0.001), and F x F had higher percenta ges unaffected than F x R lambs sired by rams from different genetic s ources, but the results of significance testing for source varied acco rding to the choice of threshold. This experiment showed that there we re proportionally about 3 times as many F x F as R x R lambs resistant to a high sporidesmin dose, and that F x R lambs were approximately i ntermediate between F x F and R x R in numbers resistant.