Dramatic Increase in Lyme Disease and Other Tick-borne Diseases, 2004

May/June 2005: Volume 33, Number 3

Record numbers of Lyme disease cases were reported to the Minnesota Department of Health (MDH) in 2004, including substantial numbers of case-patients exposed to infected Ixodes scapularis ticks (deer ticks or black legged ticks) in some western and central Minnesota counties not previously considered high risk areas. Other diseases transmitted by I. scapularis, including human anaplasmosis
(HA, formerly known as human granulocytic ehrlichiosis or HGE) and babesiosis, were also reported at record or near-record levels. This article will summarize the 2004 tick-borne disease season, focusing on epidemiologic characteristics pertinent to assisting medical providers with clinical assessment.

Lyme disease

Since MDH began Lyme disease surveillance in 1983, 5,833 cases of Lyme disease (agent, Borrelia burgdorferi) have been reported among Minnesota residents. In 2004, a record 1,023 Lyme disease cases were reported; this represents a 116% increase from the 473 cases reported in 2003, and an 18% increase from the prior high of 867 cases in 2002 (Figure 1). As recently as 1999, Lyme disease incidence was 6.0 per 100,000 Minnesota residents, versus the 20.0 per 100,000 observed in 2004.

During 2004, 644 (63%) Lyme disease case-patients were male. The median age of case-patients was 39 years (range, 1-94 years). Two hundred fifty-four (25%) cases occurred in children under the age of 12 years. During 2004, 880 case-patients (86%) had a history of erythema migrans (EM), the bullseye-like rash consistent with early-stage Lyme disease. However, 200 (20%) case-patients had at least one disseminated or late manifestation of Lyme disease before they were diagnosed and began treatment. Of those 200 case-patients, 132 (66%) had a history of objective joint swelling, and 54 (27%) reported cranial neuritis. Fifty-seven (29%) of the disseminated or late manifestation case-patients recalled a history of possible EM. For case-patients with a known disease onset date (n=865), illness onset peaked in July (301 cases, 35%), corresponding to the mid-May through mid-July peak activity of I. scapularis nymphs; 77% (663 cases) had onset from May through August. Case-patients with a late manifestation were more likely to be diagnosed during September-April than early stage cases (112 [56%] of 200 disseminated/ late stage cases, vs. 188 [23%] of 823 early stage cases).

Four hundred forty-two (43%) of the Lyme disease case-patients in 2004 resided in the seven-county Twin Cities Metropolitan area. However, of the 736 (72%) case-patients with location of tick exposure that could be identified at least to the county level, only 40 (5%) exposures occurred in the Metro area. Most of the Metro area exposures (31 cases, 77%) occurred in Anoka and Washington Counties. As in past years, the majority of Lyme disease case-patients in Minnesota were exposed in the east-central region of the state (Figure 2) with Crow Wing, Cass, Aitkin, and Pine Counties accounting for 309 (42%) known exposures. One hundred and forty- two (19%) case-patients were likely exposed to infected ticks in Wisconsin. Four northern counties (Becker, Beltrami, Hubbard, and Itasca) with a history of sporadic Lyme disease case exposure had higher numbers of exposures in 2004 (21, 16, 25, and 26 case-patient exposures, respectively) than in previous years, suggesting that I. scapularis and/or B. burgdorferi are becoming established or more common in areas not previously considered to be high risk for Lyme disease. These data stress the importance of ascertaining travel history for patients when determining the likelihood of Lyme disease.

Human Anaplasmosis

Reported cases of human anaplasmosis (HA, agent Anaplasma phagocytophilum) also increased in 2004, with 139 reported cases (incidence of 2.8 per 100,000 Minnesota residents) (Figure 1). This represented a 78% increase from the 78 cases reported in 2003. Reported HA cases peaked in 2002 at 149 cases (incidence of 3.0 per 100,000). In 2004, two (1.4%) HA case-patients had objective evidence of a co-infection with Lyme disease. However, co-infection with Lyme disease has been more striking as recently as 2002, when 20 of 149 (13%) HA patients also had objective evidence of Lyme disease. Incidence of HA cases and HA/Lyme disease coinfections is likely under-estimated in Minnesota, because many of the reported HA case-patients with clinically compatible illness were treated empirically without sufficient laboratory evidence (positive antibody or blood smear test) to meet the surveillance case definition.

Of the 139 HA case-patients in 2004, 81 (58%) were male. The median age of case-patients was 59 years (20 years older than the median age of Lyme disease case-patients [Figure 3]) and ranged from 1-89 years; 120 (86%) of the case-patients were over 40 years of age. Of the 139 HA case-patients, 33 (24%) lived in the seven-county Twin Cities Metropolitan area, and 106 (76.3%) lived in Greater Minnesota.

In 2004, 56% of HA case-patients (77 of 137 with known onset) had onset of illness in June or July, indicating exposure to infected I. scapularis in early summer. However, lower numbers
of cases occurred until December, likely due to continued exposure risk through the fall months from adult female I. scapularis.

Location of I. scapularis exposure was known for 118 HA case-patients. Most exposures occurred in the same Minnesota counties as the majority of Lyme disease exposures (111 cases, 94%); the remaining seven (6%) were likely exposed in Wisconsin. Eighty-four HA case-patients (76%) were exposed in the following four counties: Aitkin (10 cases), Cass (23), Crow Wing (40), or Hubbard (11). The recent rise in exposures in Hubbard County (only two known exposures during 1998-2003) suggests a westward expansion of HA risk, similar to that of Lyme disease. Unlike tick exposures for Lyme disease, a majority of HA case-patients were exposed in their county of residence (76 HA cases [64%], vs. 323 [44%] of Lyme disease cases with known exposure). Notably, however, nearly all of the Twin Cities case-patients were exposed outside of the metro area in Minnesota (22 of 26 known exposures, 85%) or in Wisconsin (three cases, 12%). Metro area clinicians should therefore suspect HA in patients with compatible symptoms who report outdoor activity in wooded or brushy tick habitat within HA-endemic areas.


During 2004, a record nine babesiosis (agent, Babesia microti) cases were reported to MDH. Six (67%) of these case-patients were male. The median case-patient age was 57 years (range 48-79 years). During 2002-2004, 13 of 17 (76%) babesiosis case-patients were diagnosed during July or August. Of the 17 case-patients identified during 2002-2004, eight (47%) were most likely exposed to infected I.scapularis ticks in their county of residence. Most babesiosis cases were exposed in the same counties that are high risk for Lyme disease and HA.

Key Points

  • A dramatic increase in Lyme disease and other tick-borne diseases occurred in 2004.
  • Risk for exposure to Ixodes scapularis ticks has expanded north and west into counties which were not previously considered high risk areas.
  • Lyme disease risk peaks from May to July.
  • Most cases of human anaplasmosis (HA) occur in June and July, but cases occur throughout the fall months. The majority of case-patients are over 40 years of age.
  • Obtaining a travel history for tick-borne disease patients is important, since many patients are infected outside of their county of residence.

Updated Tuesday, 16-Nov-2010 14:04:23 CST