BackgroundBabesiosis is a globally growing tick-borne disease in humans. Severe babesiosis caused byBabesia divergenshas been reported in two patients from Asturias (Northwestern Spain), suggesting an undetected risk for the disease. To analyze this risk, we retrospectively evaluated the seroprevalence of babesiosis in the Asturian population from 2015 through 2017, a period covering the intermediate years in which these two severe cases occurred.MethodsIndirect fluorescent assay (IFA) and Western blot (WB) were performed to detectB. divergensIgG antibodies in 120 serum samples from Asturian patients infected with the tick-transmitted spirocheteBorrelia burgdorferisensu lato, a condition that indicates exposure to tick bites.ResultsThis retrospective study confirmed aB. divergensseroprevalence rate of 39.2% according to IFA results.B. divergensincidence was 7.14 cases/100,000 population, exceeding previously reported seroprevalence rates. No differences in epidemiology and risk factors were found between patients infected solely withB. burgdorferis.l. and those infected withB. burgdorferis.l. and with IgG antibodies againstB. divergens. This last group of patients lived in Central Asturias, had a milder clinical course and, according to WB results, developed different humoral responses againstB. divergens.ConclusionsBabesia divergensparasites have circulated for several years in Asturias. Epidemiological evidence of babesiosis makes Asturias an emerging risk area for this zoonosis. Human babesiosis could also be relevant in other Spanish and European regions affected by borreliosis. Hence, the potential risk of babesiosis on human health in Asturias and other European forest regions needs to be addressed by the health authorities.Graphical Abstract