Distribution ofBorrelia burgdorferi inHostMiceinPennsylvania

Hostmice(Peromyscus leucopusandPeromyscus maniculatus) were sampledthroughout thestateof Pennsylvania todetermine thegeographical andecological distribution oftheLymedisease spirochete Borrelia burgdorferi. All67counties ofthestate were sampled. A total of1,619 micewere captured froma total of157 sites during theperiod 1990to1993for an overall capture rateof29.69%o. A total of112(6.92%) isolations of B.burgdorferi weremade.Thedistribution ofisolations revealed thereason forthecorrelated distribution of humancasesofLymedisease inthestate. Significantly more micewere captured andsignificantly more isolations were madefromhemlock (Tsuga canadensis) habitat thanfromdeciduous species forest. Nevertheless, highisolation ratesfromcounties ofthesoutheastern corner ofthestateillustrate wellthathemlock habitat isnotessential. Evidence suggests thatinsome areas,transmission between miceisoccurring insome way other thanthrough ticks as vectors. Hostmiceproveduseful fordetermining thegeographical and ecological distribution ofB.burgdorferi. Lymeborreliosis isa humandisease caused bythespirochete Borrelia burgdorferi. Thereservoir hosts arewild mammals, primarily miceofthegenusPeromyscus. Theprincipal vectors areticks ofthegenusIxodes. Deer(Odocoileus virginianus) areimportant formaintaining tick density. Surveillance todetermine thegeographical distribution of zoonotic diseases traditionally hasbeenbased on reports of humanordomestic-animal cases.Dataonhumansanddomesticanimals aremore readily obtained thanaredatafromthe definitive sylvatic host. Infact, whenthepathogen isvector borne, thevectorismore often examined thanthevertebrate hosts. Toobtain information on thepresenceofan infectious agentinwildvertebrate hosts ismore difficult; however, ithas theattraction ofa clearly direct approach todetermine the geographical distribution oftheagentinnature. Lymeborreliosis surveillance that relies onreports ofhuman caseshasthedisadvantage that atleast sometimes thesite of exposureisnotknownbecause ofthegreatmobility ofthe humanpopulation. Surveillance through vectorinvestigations ismore accurate because itallows us toshowregions of potential infectivity forhumans.Nevertheless, thetick may havedetached fromthevertebrate host, primarily rodents such asPeromyscus spp.orlarger animals, fromwhichitreceived its infection andcould havebeenmovedfromthesite whereit becameinfected. Thepurposeofthis study wastodetermine thegeographical distribution ofB.burgdorferi insmall-mammal hosts inPennsylvania. Peromyscus leucopus, thewhite-footed mouse,isthe principal hostinNewEngland (3), andPeromyscus maniculatus,thedeermouse,istheprincipal hostinotherregions (5). Thesetwospecies arewidely distributed inPennsylvania (7), andbothprefer forest habitat. Forest habitat isabundant in thestate, andconsequently Peromyscus spp.arewidely distributedinall67counties. Although thePeromyscus sp.hostsare welldistributed