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High infection risk areas identified for Lyme disease

Diuk-Wasser MA. Am J Trop Med. Hyg. 2012;doi:10.4269/ajtmh.2012.11-0395.

Researchers have identified the Northeast, Mid-Atlantic and Upper Midwest of the United States as areas of high infection risk for Lyme disease, according to study results published in the American Journal of Tropical Medicine and Hygiene.

As well as identifying the South as a low infection risk area for Lyme disease, researchers from the Yale School of Public Health in collaboration with Michigan State University, University of Illinois and University of California, Irvine, developed a map illustrating areas where Lyme disease poses a public health risk.

“A better understanding of where Lyme disease is likely to be endemic is a significant factor in improving prevention, diagnosis and treatment,” researcher Maria A. Diuk-Wasser, PhD, said in a press release. “People need to know where to take precautions to avoid tick bites. Also, doctors may be less likely to suspect and test for Lyme disease if they are unaware a patient was in a risky area and, conversely, they may act too aggressively and prescribe unneeded and potentially dangerous treatments if they incorrectly believe their patient was exposed to the pathogen.”

From 2004 to 2007, researchers sampled 304 sites within the continental United States east of the 100th meridian, an area that encompasses the known distribution of the blacklegged tick Ixodes scapularis, considered the main carrier of the Lyme disease pathogen Borrelia burgdorferi. Researchers employed a spatially stratified random design by overlaying a two-degree sampling grid across the study area, with state parks or other publicly accessible forested areas randomly selected within each grid.

Visiting sites a median of five times during late spring and summer, when I. scapularis actively seek hosts, researchers measured relative density by drag sampling the closed-canopy deciduous forest along five 200-m transects using a 1-m corduroy cloth.

At the close of the study, 5,332 I. scapularis specimen were collected at 94 of the 304 sites (30.1%) sampled from 2004 to 2006, with yearly specimen totals in positive sites ranging from one to 506. Researchers tested 5,328 I. scapularis specimen from 92 of the sites for the presence of B. burgdorferi DNA and identified 1,044 pathogen-positive specimen — an overall infection prevalence of 0.2.

Using the collected data, researchers were able to expand the Lyme disease map to extrapolate areas of clear risk for Lyme disease, including eastern Pennsylvania, Maine, Maryland, northern Virginia, most of Wisconsin, large swaths of northern Minnesota and the northern areas of Illinois. The model was also able to outline areas of “emerging risk” as a result of environmental changes — reforestation, suburbanization and reintroduction of deer — including the Illinois/Indiana border, the New York/Vermont border, southwestern Michigan and eastern North Dakota.

Collected data from the study also indicate an absence of host-seeking I. scapularis infected with B. burgdorferi in the South. Although this species is present in this region, observations have revealed an altered feeding behavior adapted to lizards and skinks, rather than deer species, and do not represent Lyme disease carriers.

“There has been a lot of discussion of whether Lyme disease exists outside of the Northeast and the Upper Midwest, but our sampling of tick populations at hundreds of sites suggests that any diagnosis of Lyme disease in most of the South should be put in serious doubt, unless it involves someone who has traveled to an area where the disease is common,” Diuk-Wasser said. “We can't completely rule out the existence of Lyme disease in the South, but it appears highly unlikely.”

Disclosure: The researchers report financial support from the CDC-Division of Vector-Borne Infectious Diseases Cooperative Agreement No. CI00171-01.

This is an excellent study conducted by experts in this field. There is considerable overdiagnosis of Lyme disease in humans, including in non-endemic areas in which it is virtually impossible to acquire this infection since either the vector Ixodes ticks are not present or, if present, the proportion infected with Borrelia burgdorferi, the cause of Lyme disease, is very low. The purpose of this study was to define the relative degree risk of acquiring Lyme disease in different geographic areas in the Eastern and Central U.S. based on the density  of infected, nymphal-stage Ixodes scapularis (deer) ticks, the vector for Lyme disease in these areas.

The investigators constructed maps based on sampling, at 94 different sites duringa two-year period, for density of ticks and results of testing them for infection by detection of DNA of Borrelia burgdorferi by the polymerase chain reaction assay. This provides useful information about overall risk of acquiring Lyme disease in different areas. If the density of infected ticks is low, the risk of Lyme disease is low, and vice versa. While this doesn't provide definitive data for any individual patient, it is very unlikely that a person in an area with a very low risk of Lyme disease based on the density of infected ticks would acquire Lyme disease there. These data also provide a useful baseline for subsequent similar studies to track progression of risk of Lyme disease in different areas over time.

Eugene Shapiro, MD
Infectious Diseases in Children Editorial Board member

Lyme disease creates havoc in many primary care offices. As I practice in northern Vermont, with a very recent increase in disease activity, some of the mayhem can be attributed to newness: however, much is intrinsic to the nature of the illness and its diagnosis. By developing an extremely detailed acarlogical risk map for Lyme disease the authors provide a tool to direct surveillance, control and prevention efforts. As the disease has associated with it a highly vocal and extremely visible support network that advocates a much wider clinical and diagnostic metric; issues of case definition are charged topics.

The detailed, multivariate statistical analysis provides a ground-level approach to the public health quandaries of Lyme disease, specifically by identifying regions where disease is expected or not. Improving on our clinical knowledge base for risk analysis is a crucial advancement, particularly in those regions of increasing disease prevalence.

William T. Gerson, MD
Infectious Diseases in Children Editorial Board member

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