ARTHRITIS SEVERITY AND SPIROCHETE BURDEN ARE DETERMINED BY SEROTYPE IN THE BORRELIA-TURICATAE MOUSE MODEL OF LYME-DISEASE

Immunodeficient mice infected with Borrelia turicatae, a relapsing fev er agent, have a disorder that resembles disseminated Lyme disease, Tw o serotypes, A and B, differed in their arthritogenicity in both CB-17 SCID and C3H SCID mice. In CB-17 SCID mice infected with serotype A o r B, arthritis was assessed by measurement of tibiotarsal diameter, fu nctional ability on a beam walk test, and microscopic assessment of jo int inflammation, Serotype B-infected mice had greater joint swelling, functional disability, and leukocytic infiltration in the joints than serotype A-infected mice. Joint swelling and disability peaked at 2 w eeks of infection and then decreased, while leukocyte infiltration in the joints persisted, To investigate the basis for the differences in arthritogenicity of serotypes A and B, spirochete burdens in infected mice were measured by quantitative PCR of spirochete DNA in joints, di rect immunofluorescence of spirochetes in joints, and counts of spiroc hetes in the blood, At 2 weeks of infection there were seven times mor e spirochetes in the joints of serotype B-infected mice than in those of serotype A-infected mice, measured by both quantitative PCR and dir ect enumeration, Although serotypes A and B had the same infectivity a nd growth rate in vivo, serotype B spirochetes were eightfold more abu ndant in the blood than serotype A spirochetes and produced greater fa tality in newborn mice. These findings indicate that differences in di sease severity in mice infected with serotype A or B are attributable to differences in the spirochete burden in the joints and blood.