Understanding the molecular epidemiology of the footrot pathogen Dichelobacter nodosus to support control and eradication programs

The Gram-negative anaerobe Dichelobacter nodosus is the primary etiologic agent of ovine footrot. Few studies of the genetic diversity and epidemiology of D. nodosus have been done, despite the economic cost and welfare implications of the disease. This study examined a large collection of Australian isolates; 735 isolates from footrot-infected sheep from 247 farms in Western Australia (WA) were tested by pulsed-field gel electrophoresis (PFGE), and a subset of 616 isolates was tested by infrequent restriction site PCR (IRS-PCR). The genetic diversity of WA isolates was compared to that of 61 isolates from three other Australian states. WA isolates were genetically diverse, with 181 molecular types resolved by PFGE, resulting in a simple diversity ratio (SDR) of 1:4 and a Simpson's index of discrimination value (D) of 0.98. IRS-PCR resolved 77 molecular types (SDR = 1:8 and D = 0.95). The isolates were grouped into 67 clonal groups by PFGE (SDR = 1:11, D = 0.90) and 36 clonal groups by IRS-PCR (SDR = 1:17, D = 0.87). Despite the high genetic diversity, three common clonal groups predominated in WA and were found in other Australian states. On some farms, molecular type was stable over a number of years, whereas on other farms genetically diverse isolates occurred within a flock of sheep or within a hoof. This study provides a large database from which to appropriately interpret molecular types found in epidemiological investigations and to identify common and unknown types that may compromise footrot eradication or control programs.