highly significantP = 0.000
A highly significant Standardised Index of Association (I S A ) (0.45, P = 0.000) in cattle respiratory isolates indicated the presence of linkage disequilibrium within this population of P. multocida isolates and the results of SplitsTree analysis corroborated this, showing a tree-like, rather than a network, structure (Additional file 1 , Figure S1).