Human risk of infection with Borrelia burgdorferi, the Lyme disease agent, in eastern United States

Abstract

The geographic pattern of human risk for infection with Borrelia burgdorferi sensu stricto, the tick-borne pathogen that causes Lyme disease, was mapped for the eastern United States. The map is based on standardized field sampling in 304 sites of the density of Ixodes scapularis host-seeking nymphs infected with B. burgdorferi, which is closely associated with human infection risk. Risk factors for the presence and density of infected nymphs were used to model a continuous 8 km×8 km resolution predictive surface of human risk, including confidence intervals for each pixel. Discontinuous Lyme disease risk foci were identified in the Northeast and upper Midwest, with a transitional zone including sites with uninfected I. scapularis populations. Given frequent under- and over-diagnoses of Lyme disease, this map could act as a tool to guide surveillance, control, and prevention efforts and act as a baseline for studies tracking the spread of infection.

Figure 1.

Predicted and observed density of infected host-seeking Ixodes scapularis nymphs (DIN)/1,000 m².

Figure 2.

Statistically significant high- and low-risk areas. High risk: 95% probability that at least 0.3 infected nymphs will be collected per 1,000 m²; low risk: 95% probability that < 0.3 infected nymphs will be collected per 1,000 m²; transitional area: risk cannot be ascertained with 95% confidence (confidence interval includes 0.3); true high risk: > 0.3 infected nymphs collected in a predicted high-risk area; true low risk: < 0.3 infected nymphs collected in a predicted low-risk area; false high risk: < 0.3 infected nymphs collected in a predicted high-risk area; false low risk: > 0.3 infected nymphs collected in a predicted low-risk area.

Figure 3.

Infection status of sites where at least one I. scapularis nymph was collected. Sites where at least 14 nymphs were collected between 2004 and 2006 are highlighted. With this sample size and assuming infection prevalence of 0.20, it is estimated with 95% confidence that at least one infected nymph would have been collected were the site positive.

Figure 4.

Borrelia burgdorferi prevalence estimates in nymphal I scapularis for sites repeatedly sampled with a minimum of 100 nymphs collected in each of the sampled years. We used a binomial log-likelihood probability function to estimate 95% confidence intervals for each prevalence estimate.