Infectious but nonpathogenic isolate of Borrelia burgdorferi.

We document for the first time an infectious but nonarthritogenic variant of Borrelia burgdorferi. Strain 25015, previously isolated from an Ixodes dammini larva collected in upstate New York, was infectious but failed to produce arthritis or carditis in laboratory rats and mice. By contrast, pathogenic strain N40 invariably caused arthritis. This nonarthritogenic variant, with proteins with molecular weights different from those of the standard B31 strain, was frequently isolated from normal joint tissues of experimentally infected rats. Outer surface proteins A and B of strain 25015 have molecular weights of about 32,500 and 35,500, respectively, in contrast to molecular weights of approximately 31,000 and 34,000, respectively, for outer surface proteins A and B of strains B31 and N40. A prominent low-molecular-weight protein of about 23,500 also characterizes strain 25015. Test animals infected for 30 to 60 days had relatively high antibody titers (greater than or equal to 1:1,280). The nonarthritogenic variant will be useful, along with pathogenic strains, in providing comparative insight into the pathogenesis of Lyme borreliosis. Homologous immunoblotting of sera from rats and mice inoculated with both the arthritogenic and nonarthritogenic strains revealed antibody reactivities to proteins of B. burgdorferi different from those revealed in the heterologous tests.