We report a role for
Borrelia microti
as a cause of relapsing fever in Iran supported by robust epidemiological evidence. The molecular identity of this spirochete and its relation with other relapsing fever borreliae have, until now, been poorly delineated. We analyzed an isolate of
B. microti
, obtained from
Ornithodoros erraticus
ticks, by sequencing four loci (16S rRNA,
flaB
,
glpQ
, intragenic spacer [IGS]) and comparing these sequences with those of other relapsing fever borreliae. Phylogenetic analysis using concatenated sequences of 16S rRNA,
flaB
, and
glpQ
grouped
B. microti
alongside three members of the African group,
B. duttonii
,
B. recurrentis
, and
B. crocidurae
, which are distinct from
B. persica
, the most prevalent established cause of tick-borne relapsing fever in Iran. The similarity values for 10 concatenated sequences totaling 2,437 nucleotides ranged from 92.11% to 99.84%, with the highest homologies being between
B. duttonii
and
B. microti
and between
B. duttonii
and
B. recurrentis
. Furthermore, the more discriminatory IGS sequence analysis corroborated the close similarity (97.76% to 99.56%) between
B. microti
and
B. duttonii
. These findings raise the possibility that both species may indeed be the same and further dispel the one-species, one-vector theory that has been the basis for classification of relapsing fever
Borrelia
for the last 100 years.