Discussion

This report updates available information on the epidemiology of reported Lyme disease cases. For many states with high incidence, the number of case reports appears to have stabilized or declined recently. The decrease in reported cases among many states with high incidence could be attributable to several different factors, including actual stabilization of disease incidence or an artifact from changes in case verification practices designed to minimize the resource demands of conducting Lyme disease surveillance. In contrast, during 2008–2015 the number of cases reported from many of the neighboring states increased. Geographic expansion of areas with substantial occurrence of human Lyme disease is supported by a documented increase in the number of counties in the United States with established I. scapularis tick populations (13,14).

Although the overall demographic and clinical characteristics among reported cases are similar to those detailed in previous reports (7,15), this report reveals distinct differences in the demographics associated with confirmed and probable cases from states in all surveillance categories. Confirmed cases in states with high incidence and neighboring states occurred most commonly among males and with a modal age in young children. In contrast, confirmed cases from states with low incidence were associated with a substantially higher modal age and occurred more commonly among females. Overall, probable cases reflected an older patient population than that of confirmed cases. Although probable cases from states with high incidence still occurred more commonly among males, probable cases from neighboring states and states with low incidence occurred more commonly among females. Demographic differences among cases reported from states with high and low incidence have been previously documented (16,17) and might reflect lack of specificity of erythema migrans in locations where southern tick-associated rash illness occurs (18) as well as higher potential for false positive serologic results stemming from lower positive predictive value of those tests in settings with low incidence (19,20). Many confirmed cases in states with low incidence likely reflect travel to states with high incidence (16). In contrast, probable cases reported from neighboring states and states with low incidence appear to reflect a different patient population, thereby suggesting decreased specificity of the probable case definition in those states.

Approximately 75% of all confirmed case reports that included clinical data had indication of erythema migrans. Infections with illness onset outside the peak season of transmission (spring and summer) might sometimes be considered a result of infection that was acquired during the spring and summer months but did not clinically manifest until months later. Arthritis, the most common disseminated manifestation of Lyme disease, was the most common clinical finding among patients with reported illness onset during the coldest months in the temperate United States. Nevertheless, erythema migrans was the most commonly reported sign of infection among patients with illness onset not only in the peak spring and summer months but for two thirds of the year (April–November) (data not shown). This seasonal pattern underscores that adult ticks that seek blood meal hosts during the fall months have a proportional role in human illness and that prevention messages should not be focused only during the spring and summer season when nymphal ticks seek hosts.

The usefulness of Lyme disease surveillance differs across jurisdictions. High numbers of possible Lyme disease cases that require clinical follow-up have taxed public health resources in states with high incidence to an unsustainable level (21). Solutions vary, with some states investigating cases as resources allow, which at times means curtailing surveillance activities. Other states have begun to employ statistical methodology to estimate the number of cases each year. For example, several counties in New York have implemented a system in which 20% of positive laboratory reports are sampled and investigated to determine what proportion can be confirmed; these results are extrapolated to the remaining unsampled laboratory reports to arrive at an estimate of Lyme disease case counts in those counties (22). Several states are considering adopting similar methodologies to better manage public health surveillance for Lyme disease (23). In line with historical case-based surveillance systems, case estimates are not reported to CDC through NNDSS, one of several factors that contribute to underreporting of cases nationally. In areas where Lyme disease incidence has remained high for years, expensive, ongoing surveillance does not yield new information about the magnitude or geographic distribution of the disease and potentially diverts limited public health resources that might be spent on prevention. Taken together, these points suggest the need for a paradigm shift in states with high incidence that would minimize personnel and resource costs while still maintaining awareness of the disease. In contrast, public health surveillance in states where Lyme disease is emerging can serve to increase knowledge of local disease incidence and spread, which can in turn be used to target educational measures for health care providers and the public.

To improve specificity of reported cases in areas with low incidence and areas where Lyme disease is emerging, CSTE voted to modify the Lyme disease surveillance case definition effective in 2017 (24). Confirmation of infection acquired in states outside those with high incidence now requires laboratory evidence of infection. As Lyme disease emerges in neighboring states, clinical suspicion of Lyme disease in a patient should be based on local experience rather than incidence cutoffs used for surveillance purposes.

Identification of effective methods to prevent Lyme disease has proven challenging. Measures aimed at reducing tick populations on residential properties have not proven effective in decreasing the number of human Lyme disease infections (25). Long recommended behavioral interventions, such as wearing permethrin-treated clothing or using repellent containing DEET, have not been adequate to control Lyme disease on a population scale (26,27). In addition, adherence to recommendations aimed at preventing Lyme disease has been poor, even in areas of high risk (28,29). New approaches are needed to reduce the incidence and spread of Lyme disease, including exploration of a second-generation human vaccine (28).