A review of footrot in sheep: new approaches for control of virulent footrot.
Footrot caused by the bacterium Dichelobacter nodosus is a contagious disease of small ruminants and in its virulent form, it causes severe economic loss and is a significant animal welfare issue. There are 10 different serogroups of D. nodosus (A-I and M) and immunity is serogroup-specific. Multivalent vaccines of either whole cell or recombinant DNA based pilin vaccines result in reduced protective antibody titres. The use of targeted vaccines against serogroup specific isolates in affected flocks provides an avenue for long-term protection. Long-term control is also feasible through selective breeding. Genetic variation among breeds, strains with breeds and genetic differences within flocks all provide avenues to reduce susceptibility and therefore impact of the disease. The heritability of resistance is in the range of 15-25% and provides a means to identify animals with superior breeding values based on clinical examination of affected individuals or progeny from candidates under selection. The use of DNA markers based on whole genome selection using high density markers is likely to be successful to identify resistant animals in absence of disease, but will be breed and flock specific. Selective breeding for increased resistance is unlikely to directly impact negatively on genetic change in other production or disease traits, but will reduce genetic gain in such traits by the inclusion of footrot in a multi-trait breeding objective. Selective breeding for resistance is unlikely to be of value in case of benign footrot or when eradication is sought. Genetic variation in response to vaccination provides an alternative control strategy to improve response to multivalent vaccines. However, this will be limited by the need to include all serogroups as separate response variables, given the limited common genetic control of vaccine responsiveness.