January 20, 2016
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Lyme disease vectors spread to nearly half of continental US counties

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New surveillance data indicated the presence of two tick species identified as the primary vectors of Lyme disease in nearly half the counties in the continental United States, a 44.7% increase in distribution since the most recent data were collected in 1998.

Furthermore, the number of counties in which Ixodes scapularis the vector found primarily in the eastern U.S. — is considered to be established has more than doubled since publication of the previous distribution map, according to Rebecca J. Eisen, PhD, research biologist in the vector-borne disease division at the CDC, and colleagues.

“Rising case counts and geographical expansion of Lyme disease endemic areas have been attributed to range expansion of I. scapularis in the eastern United States,” they wrote. “However, because of a lack of systematic surveillance of I. scapularis and [I. pacificus], national trends in the geographic distribution of these medically important ticks are difficult to document.”

I. scapularis expands across Northeast

To update the 1998 distribution data of I. scapularis and its West Coast relative I. pacificus, Eisen and colleagues reviewed literature published from Jan. 1, 1996 to Aug. 25, 2015 for studies and articles related to the two species. These, along with data from state health department websites and public health officials and investigators, were used to update county estimates after the same classifications used in the 1998 publication. Counties were classified as “reported” if at least one tick of any life stage was identified there, or if a county’s records did not specify either the number or life stage of the identified ticks. Counties where a tick species was considered “established” were those reporting at least six individual ticks or the identification of two out of three host-seeking life stages during a single year.

Credit: Gary Alpert, Harvard University, Bugwood.org

Figure 1. This is a blacklegged tick (Ixodes scapularis), one of the most common vectors of Lyme disease.

Source: Gary Alpert, Harvard University, Bugwood.org

According to Eisen and colleagues, I. scapularis were collected from 37 states and 45.7% of continental U.S. counties, while I. pacificus was found in six states and 3.6% of counties. Combined, these Lyme vectors have been identified in 49.2% of continental counties and 43 states, a distribution that the researchers wrote is a 44.7% increase over the previously reported surveillance report.

The number of counties in which I. scapularis was considered to be established increased from 12.7% of counties in 1998 to 27.1% of counties, and was distributed across 35 states. While this increase was largely driven by expanding focus points in the Northeast and North-Central U.S., the researchers wrote, the number of established counties in South and South-Central states remained relatively stable. Meanwhile, the portion of U.S. counties with established I. pacificus increased only modestly, rising from 2.9% to 3.1% among the Western states from 1998 to 2015.

“This study shows that the distribution of Lyme disease vectors has changed substantially over the last nearly 2 decades and highlights areas where risk for human exposure to ticks has changed during that time,” Eisen said in a press release. “The observed range expansion of the ticks highlights a need for continuing and enhancing vector surveillance efforts, particularly along the leading edges of range expansion.”

Cases of Lyme disease remain underreported

The spread of these ticks increases the opportunities for new cases of Lyme disease, the vast majority of which may never be reported to health care providers or public health.

In a recent retrospective analysis of medical claims data, Christina A. Nelson, MD, MPH, of the CDC in Fort Collins, Colorado, and colleagues assessed claims information collected from 2005 to 2010. The dataset included more than 103 million person-years of observation; the median age of the population was 37 years, and 51.9% were women.

During this period, there were 45,430 clinician-diagnosed cases of Lyme disease; 985 were inpatient diagnoses, and 44,445 were outpatient diagnoses. Average annual incidence was 44.8 diagnoses in 100,000 people, which peaked in 2009 at 56.3 events per 100,000.

After applying their data to the full U.S. population, the researchers estimated that approximately 329,000 cases occurred annually, the majority of which went unreported.

“Our findings underscore that [Lyme disease] is a considerable public health problem, both in terms of number of cases and overall health care use,” Nelson and colleagues wrote in Emerging Infectious Diseases. “Furthermore, as with other conditions, underreporting in the national surveillance system remains a challenge. Continued research and education are necessary to enhance prevention efforts and improve diagnostic accuracy to reduce the effects of this disease.” – by Dave Muoio

Disclosure: The researchers report no relevant financial disclosures.