High seroprevalence of Babesia antibodies among Borrelia burgdorferi-infected humans in Sweden.

In northern Europe, tick-borne diseases such as Lyme borreliosis (LB) and tick-borne encephalitis (TBE) are well known. The actual incidence of Babesia infections, however, has remained elusive. In this study, the prevalence of antibodies against two Babesia spp. was investigated in a cohort of patients that were seropositive for Borrelia (B.) burgdorferi sensu lato (s.l.). Data were compared to a control group of healthy individuals. Sera were collected from 283 individuals residing in the southernmost region of Sweden, Skåne County. Almost one third of the sera were from patients with a confirmed seropositive reaction against B. burgdorferi s.l. All sera samples were assessed for IgG antibodies against Babesia (Ba.) microti and Ba. divergens by indirect fluorescent antibody (IFA) assays. Seropositive IgG titers for at least one of the Babesia spp. was significantly more common (p < 0.05) in individuals seropositive for Borrelia (16.3%) compared to the healthy control group (2.5%). Our findings suggest that Babesia infections may indeed be quite common among individuals who have been exposed to tick bites. Furthermore, the results indicate that human babesiosis should be considered in patients that show relevant symptoms; particularly for splenectomized and other immunocompromised individuals. Finally, the data challenges current blood transfusion procedures and highlights the current lack of awareness of the parasite in northern Europe.