EVIDENCE FOR SERPULINA-HYODYSENTERIAE BEING RECOMBINANT, WITH AN EPIDEMIC POPULATION-STRUCTURE

The population structure of Serpulina hyodysenteriae was investigated using multilocus enzyme electrophoresis. A total of 231 isolates were divided into 50 electrophoretic types (ETs), with a mean genetic diver sity of 0.29 for the number of ETs and 0.23 for the number of isolates . Subsets of isolates from two Australian states (71 isolates from Vic toria and 68 isolates from Queensland) exhibited as much genetic varia tion as the entire collection. The calculated index of association (I- A) for the number of ETs (0.29+/-0.17) was not significantly different from zero, and hence provided evidence for the occurrence of signific ant genetic recombination accounting for the observed variation betwee n strains. In contrast, the I-A for the number of isolates (3.93+/-0.0 3) was significantly different from zero, with seven of the 50 ETs (ET s 4, 6, 13, 14, 20, 33 and 35) containing 51% of all the isolates. Eve n when multiple isolates from the same farm were removed from the anal ysis, the I-A value for the number of isolates remained significantly greater than zero (I-A 9.87+/-0.04), indicating that it was not biased by their inclusion. The results suggest that S. hyodysenteriae has an epidemic population structure.