A MICRO-CHALLENGE TECHNIQUE FOR THE STUDY OF RESISTANCE TO DERMATOPHILUS-CONGOLENSIS IN MERINO SHEEP

The clinical response (erythema, oedema, scab formation) of sheep to a micro-challenge with zoospores of Dermatophilus congolensis is descri bed. The effect of site preparation (wax removal), dose of zoospores a pplied (0, 1 x 10(6), 1 x 10(7), 1 x 10(9), 1 x 10(10) zoospores/mL), and day of inspection after challenge (0, 2, 4, 7, 9, 13, 17, 20, 23 d ays) were the major factors accounting for differences in responses to challenge. At all doses, large differences were observed between shee p in the prevalence, severity, and persistence of infection. These dif ferences were not due to the effects of flock, or to footrot resistant and susceptible phenotypes. Based on results from this work, modifica tions were proposed to allow large numbers of sheep to be tested to es timate genetic differences between sheep in their resistance to challe nge with zoospores from D. congolensis.