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Lyme disease in the Twin Cities area

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Lyme disease is a bacterial infection transmitted to humans by deer ticks (Ixodes scapularis- official common name is the black legged tick). A Lyme infection can most notably affect the joints, nervous system, and heart. The Metropolitan Mosquito Control District's Lyme Disease Program identifies and monitors the distribution of deer ticks within the seven-county Twin Cities metropolitan area. The deer tick occurs in northern and eastern Anoka County, northern Ramsey County, and most of Washington County. Isolated populations have also been found in Hennepin, Dakota, Scott, and Carver Counties.

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Both wood ticks and deer ticks can be found in the spring and early summer, but wood ticks are generally not disease transmitters in Minnesota. Adult wood ticks have white markings and are mainly out in the spring. Adult deer ticks do not have white markings and are out in both spring and fall. Infected deer tick females can transmit disease as they feed. Male deer ticks do not transmit disease because they generally don’t feed. Sometimes the males do briefly attach to their host, but their attachment time is too short for disease transmission to occur. Although less likely to be infected, the immature deer tick nymph transmits Lyme disease more often than adult ticks because their small size keeps them undetected. Nymphs are most common May – July.

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Prevention: Deer ticks are found in brushy and/or wooded habitat. Check yourself for ticks after being in wooded areas. The best disease prevention tool is you! Ticks transmit disease during the feeding process, but simply finding an attached tick does not mean you will get a tick-borne illness. Not all deer ticks are infected and at least 24 - 48 hours of attachment time must occur before an infected tick could infect you.

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Cases: Lyme disease is the most commonly diagnosed tick-borne illness in Minnesota each year. Since 2000 the Minnesota Department of Health (MDH) has been consistently tabulating record-setting human tick-borne disease case totals (they have counted > 463 human Lyme cases per year since 2000). Their all-time high statewide tabulation occurred in 2007 with 1239 Lyme cases.

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District deer tick surveillance results: The numbers of deer ticks collected and our total number of positive collection sites in our Lyme tick distribution study have been higher than average since 2000, suggesting an upward trend in the local deer tick population. This means greater potential for you to come into contact with deer ticks.

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Other Diseases

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Although Lyme disease is more common, deer ticks can simultaneously carry more than one disease.

Human granulocytic anaplasmosis (HGA) is a bacterial disease that is potentially fatal, though the vast majority of people recover. People are exposed to HGA most frequently in central Minnesota, but a few exposures have been documented in the Twin Cities area since 2000. HGA had been detected in small mammals in MMCD's collaborative research beginning in 1995.

Babesiosis is a rare disease that is caused by protozoa, not bacteria. Both diseases, like Lyme disease, begin with flu-like symptoms.

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Cases: Infection with HGA occurs at a lower rate than Lyme disease in Minnesota each year and it is the second most common Minnesota tick-borne illness. A new HGA case record was set in 2007 when the MDH counted 322 cases. HGA case numbers had ranged from 78 to 186 from 2000 to 2006 compared with an average of roughly 15 cases per year before the year 2000. Babesiosis cases used to average fewer than 10 cases per year but in recent years the MDH has tabulated up to 30 cases.

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Other Ticks/Other Diseases

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Deer ticks are rarely found in the core cities of the metro area, but in the fall of 2008 MMCD received two independent credible reports of deer ticks being found along the Mississippi River in the cities of Minneapolis and Saint Paul. In each case the tick was found on a dog. Both areas were intensely surveyed by MMCD, but no additional ticks were found.

Lone star ticks
Lone star ticks are aggressive human biters and have been found by us or submitted to us from the public on a rare, sporadic basis since we began our tick work in 1990. In spring of 2009 the Minnesota Department of Health (MDH) informed MMCD of an Amblyomma americanum (Lone star tick) submission to MDH, most likely from Theodore Wirth Park in Minneapolis. The tick was found on a dog. Subsequent sampling by MMCD in Theodore Wirth Park did not turn up additional Lone star ticks. In summer 2009 a tick mailed to MMCD for tick identification was another Lone star tick. It had been picked up in the Circle Pines area. It was very unusual to receive two Lone star tick reports in one year. Lone star ticks can transmit human monocytic ehrlichiosis (HME) and although both the Lone star tick and HME are more common to the southern U.S. the Lone star tick range is known to be moving northward.

If you find an unusual tick or collect a tick in an unusual place, MMCD is interested in knowing about it. Please mail the tick so we can track these unusual tick collections.

Deer ticks and Lone star ticks are found in brushy and/or wooded habitat. Again, check yourself for ticks after being in wooded areas!

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