Medscape

Abstract

We evaluated human risk for infection with Babesia microti at a site in eastern Switzerland where several B. microti-infected nymphal Ixodes ricinus ticks had been found. DNA from pooled nymphal ticks amplified by polymerase chain reaction was highly homologous to published B. microti sequences. More ticks carried babesial infection in the lower portion of the rectangular 0.7-ha grid than in the upper (11% vs. 0.8%). In addition, we measured seroprevalence of immunoglobulin (Ig) G antibodies against B. microti antigen in nearby residents. Serum from 1.5% of the 396 human residents of the region reacted to B. microti antigen (>/=1:64), as determined by indirect immunofluorescence assay (IgG). These observations constitute the first report demonstrating B. microti in a human-biting vector, associated with evidence of human exposure to this agent in a European site.

Introduction

A malaria-like syndrome due to Babesia microti infection has been recognized in parts of the northeastern United States for more than three decades.^[1,2] This protozoon pathogen was first isolated more than half a century earlier from a Portuguese vole^[3]; the pathogen has since been detected in small mammals and ticks throughout Eurasia.^[3,4]

Despite its broad geographic distribution, B. microti has not been implicated as a cause of human illness in Europe. A host-specific, rodent-feeding tick, Ixodes trianguliceps, is widely regarded as the main enzootic vector on that continent. I. ricinus, the most common human-biting tick of Europe, transmits the Lyme borreliosis spirochete, tick-borne encephalitis virus, the agent of human granulocytic ehrlichiosis, and B. divergens, but I. ricinus was believed to be infected only occasionally with B. microti.^[5] This vector-pathogen association may account for the absence of human disease due to B. microti.^[3,6] However, subadult I. ricinus ticks feed abundantly on the reservoirs of B. microti, such as voles and mice, and appear to be competent vectors for B. microti.^[7] In fact, recent studies indicate that Swiss residents may have concurrent infection with the Lyme disease spirochete and B. microti^[8] and that the human population of certain parts of Germany is exposed to B. microti.^[9]

Human exposure to B. microti may occur more often in Europe than has been recognized. Accordingly, we assessed the potential of zoonotic transmission in eastern Switzerland, where other I. ricinus-transmitted infections are present. In particular, we determined how frequently B. microti parasites infect I. ricinus ticks locally, how infection in ticks is spatially distributed, and how frequently the sera of nearby residents react to B. microti antigen.