Louse and tick borne relapsing fevers.

Relapsing fever,caused by spirochaetes belonging to the genus borrelia,was once the cause of worldwide epidemic zoonotic disease. this was largely through infection with the louse-borne form of the disease,caused by borrelia recurrentis [(louse-borne relapsing fever (lbrf)]. another form of replasing fever is caused by borrelia duttonii [(tick-borne relapsing fever (tbrf)]. the introduction and the wide use of ddt in the last century,which is forbidden now,the incidence and prevalence of both lbrf and tbrf markedly decreased. however,with the increase of rapid transportation,crowdedness and poverty,as well as abundance of lice and ticks,replasing fever is still endemic to some countries. besides,with the appearance of others closely related borrelia species that begins to emerge,the threat or the burden of these arthropod-borne zoonotic may be unpredicted or underestimated.