Monitoring human babesiosis emergence through vector surveillance New England, USA

Abstract

Human babesiosis is an emerging tick-borne disease caused by the intraerythrocytic protozoan Babesia microti. Its geographic distribution is more limited than that of Lyme disease, despite sharing the same tick vector and reservoir hosts. The geographic range of babesiosis is expanding, but knowledge of its range is incomplete and relies exclusively on reports of human cases. We evaluated the utility of tick-based surveillance for monitoring disease expansion by comparing the ratios of the 2 infections in humans and ticks in areas with varying B. microti endemicity. We found a close association between human disease and tick infection ratios in long-established babesiosis-endemic areas but a lower than expected incidence of human babesiosis on the basis of tick infection rates in new disease-endemic areas. This finding suggests that babesiosis at emerging sites is underreported. Vector-based surveillance can provide an early warning system for the emergence of human babesiosis.

Keywords: Babesia microti; Borrelia burgdorferi; Connecticut; Ixodes scapularis; Lyme disease; Massachusetts; New England; emergence; human babesiosis; incidence ratio; infection prevalence; parasites; piroplasm; tick-borne pathogens.

Figure 1

Borrelia burgdorferi and Babesia microti infection prevalence among humans and Ixodes scapularis ticks, eastern Connecticut (CT) and Nantucket, Massachusetts (MA). Shading indicates human babesiosis incidence in study towns by year in which the disease became endemic in the town (defined as the first year babesiosis cases were reported for 2 consecutive years). I. scapularis nymphal infection prevalence is shown for B. microti and B. burgdorferi in Lyme/Old Lyme in 2007 and for Nantucket and northeastern CT in 2010, represented as a pie chart for each sampled location. RI, Rhode Island.

Figure 2

Linear regression model of the human ratio (disease incidence rate ratio) and the tick ratio (tick infection prevalence ratio). The regression model includes state-reported case data from disease-endemic sites and case diagnoses from both medical practices. The human ratio derived from state-reported case data in the emerging area (northeastern Connecticut) is also displayed.