A micro-challenge technique for the study of resistance to Dermatophilus congolensis in Merino sheep

Abstract

The clinical response (erythema, oedema, scab formation) of sheep to a micro-challenge with zoospores of Dermatophilus congolenis is described. The effect of site preparation (wax removal), dose of zoospores applied (0, 1 x 106, 1 x 107, 1 x 109, 1 x 1010. zoospores/mL), and day of inspection after challenge (0, 2, 4, 7, 9, 13, 17, 20, 23 days) were the major factors accounting for differences in responses to challenge. At all doses, large differences were observed between sheep in the prevalence, severity, and persistence of infection. These differences were not due to the effects of flock, or to footrot resistant and susceptible phenotypes. Based on results from this work, modifications were proposed to allow large numbers of sheep to be tested to estimate genetic differences between sheep in their resistance to challenge with zoospores from D. congolensis.