Biological characteristics of Katahdin sheep selected for footrot resistance
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The objectives of this study were to select sheep divergent for a footrot resistant genotype by screening with the DQA2 gene marker and to investigate the host resistant immune response, neutrophil phagocytic efficiency, and damage caused by pathogenic bacteria in stratum corneum of hoof tissue. Sixty-eight Katahdin and Katahdin x Dorper crossbred ewes were stratified by body weight and footrot gene marker test scores into control (C) and treatment (D) groups. Cultures of D. nodosus were inoculated to the inter-digital skin of the treatment group. White and red blood cell count, packed cell volume, WBC differential, immune response measurements, body weight, body condition scores, and footrot scores were recorded. The protease thermostability of D. nodosus was tested by using a gelatin test. Neutrophil phagocytic efficiency was tested in different marker groups. Damage caused by D. nodosus on white and black hoof tissue samples was observed by an Environmental Scanning Electron Microscope. The data were analyzed by using mixed model procedures of SAS, and P < 0.05 was considered as significant. At the end of trial, C group had a footrot score of 0, but D group had footrot scores of 0 to 2. The IgG level in sera (mg/ml) was greater in D than C groups (P< 0.0001). The neutrophils in high resistant genotypes of sheep had significantly more efficient phagocytic activity vs. low resistant genotypes. Sinus formation was observed in both white and black hoof tissues, but there was no relationship between the tissue pigment and the ultrastractural changes in tissue. The gelatin test determined the enzyme activity of inoculated D. nodosus and since no virulent cases of footrot were observed during footrot pathogenic challenge, it can be used as evidence of footrot resistance in the Katahdin sheep breed.