New research suggests Lyme disease ticks are endemic in the Northeast and Northern Midwest of the United States, prompting calls for further education programs on prevention and awareness of Lyme disease. In the most extensive and meticulous mapping project to date on Lyme disease, researchers at the Yale School of Public Health, in Connecticut, worked at more than three hundred sites to assess the presence of ticks infected with borrelia bacteria. Their results could influence Lyme disease policies across America.
Unreliable Tick Reports
Led by Maria A. Diuk-Wasser, an assistant professor of epidemiology at Yale, eighty field operatives spent hours traversing the country armed with 1m² flags of corduroy. Flagging for ticks can help identify high-risk areas and warn people accordingly but, up until now, the reports available on tick-endemic areas were largely unreliable. The ticks responsible for spreading Lyme disease, Ixodes scapularis in the US, were trapped by the researchers and then tested for evidence of bacterial infection.
Two Main Foci of Lyme-Infected Ticks
The Yale team developed a map showing that the Northeast and upper-Midwest of the US were the main areas in which ticks infected with Lyme disease bacteria were endemic. Publishing their findings in this month’s issue of the American Journal of Tropical Medicine and Hygiene, the researchers note that they identified areas where infected ticks were present but where there were still not reported cases of Lyme disease. This could demonstrate limited human exposure to these tick populations or it could signify misdiagnosis or asymptomatic infection in people in the tick-endemic area.
Ticks Preferring Non-Human Hosts
One other reason for low reported numbers of Lyme disease infection in humans in areas where infected ticks are now known to be endemic is the preference of these ticks to choose non-human hosts. Ixodes scapularis nymphs appear to choose lizards and skinks as their hosts in the more southern states, unlike in their northern counterparts. Cases reported in the south are thought likely due to misdiagnosis or an infection picked up whilst travelling.
Checking for Ticks
For three years, between 2004 and 2007, field assistants checked for nymph ticks an average of five times at each study site. The checks occurred in late spring and summer, during the most active time of the year for nymphal ticks. Thirty-seven states were included in the study, all east of the 100th meridian, as this is the area in which nymphal ticks have been identified previously. A number of factors appear to influence the presence of ticks in an area, namely low elevation, low humidity, and seasonal temperatures. One specific area in which infected ticks are rife is between southern Maine and northern Virginia; the other main focus is Wisconsin, northern Minnesota, and in part of northern Illinois. Continued migration of deer in these areas, as well as tree-planting schemes and other alterations to the urban and rural landscape will all likely effect the resident populations of ticks infected with Lyme disease bacteria.
Preventing Lyme Disease
A key outcome of this research is the scrutiny applied to current tick awareness procedures. As it stands, tick bite prophylaxis, i.e. use of tick repellents and use of Lyme disease antibiotics after a bite, is recommended only when 20% or more of nymphs in an area are infected with Lyme disease bacteria. The scientists at the Yale School of Public Health conclude that the presence of any nymphs infected with Borrelia burgdorferi is sufficient for postexposure preventative measures to be taken. The knowledge that an area has infected ticks present, even in the absence of reported Lyme disease cases, allows local authorities to improve prevention, diagnosis, and treatment protocols in such Lyme disease-infected tick endemic areas.
Diuk-Wasser MA, Hoen AG, Cislo P, Brinkerhoff R, Hamer SA, Rowland M, Cortinas R, Vourc’h G, Melton F, Hickling GJ, Tsao JI, Bunikis J, Barbour AG, Kitron U, Piesman J, Fish D., Human Risk of Infection with Borrelia burgdorferi, the Lyme Disease Agent, in Eastern United States.
Am J Trop Med Hyg. 2012 Feb;86(2):320-327.
Lyme map. The map shows an elevated risk of Lyme disease from Maine south to Maryland, Northern Virginia, and the Washington DC area. Investigators found a separate region of increased risk in the upper Midwest. The 3 sites marked with red crosses indicate the only sites where investigators collected high numbers of infected tics, but where the statistical model did not accurately predict them as high risk.