Lyme Disease and Human Anaplasmosis in Minnesota, 2003Go to full issue: DCN, March/April 2004: Volume 32, Number 2 (PDF: 280KB/12 pages)
While West Nile virus dominated vector-borne disease concerns in Minnesota during 2003, reports of the tick-borne diseases Lyme disease (Borrelia burgdorferi) and human anaplasmosis (HA: formerly known as human granulocytic ehrlichiosis) continued, although in lower numbers than in 2002. This article will summarize decreases in case numbers for both Lyme disease and HA from record levels in 2002 and present relevant updates in tick-borne disease epidemiology. For a more complete review of the epidemiology of these diseases, see the May 2003 (Vol. 31, no. 3) issue of the Disease Control Newsletter.
From 1983, when the Minnesota Department of Health (MDH) began Lyme disease surveillance, through 2003, 4,810 cases of Lyme disease were reported among Minnesota residents. In 2003, Lyme disease cases decreased to 473 from a record 867 in 2002 (Figure 1). As recently as 1999, Lyme disease incidence was 6.0 per 100,000 person-years; the incidence increased to 9.6 in 2001 and to 17.6 in 2002, before dropping back to 9.4 in 2003. During 2003, most casepatients (393 of 473, or 83%) had a history of erythema migrans (EM), the bullseye-like rash consistent with early stage Lyme disease. However, 107 (23%) case-patients had developed a late manifestation of Lyme disease before they were diagnosed and began treatment. This proportion of late manifestation cases was higher than in previous years, but not unexpected given the large number of casepatients reported in 2002. (Many additional cases infected in 2002 likely went undetected until they developed a late manifestation of Lyme disease in 2003.) HA cases also dropped during 2003, from a record 149 cases in 2002 (incidence of 3.0 per 100,000 personyears) to 78 cases (incidence of 1.6) (Figure 1). Five HA case-patients (6.4%) also had objective evidence of a co-infection with Lyme disease. The reasons for the decrease in Lyme disease and HA in 2003 are unclear, but limited tick sampling data and anecdotal reports suggest tick numbers may have decreased from 2002. Most Minnesota residents who develop Lyme disease and/or HA contract these diseases through infected Ixodes scapularis ticks (deer ticks or black-legged ticks) in certain east central Minnesota counties or in western Wisconsin (Figure 2). I. scapularis ticks are especially common in wooded and brushy habitats where humidity at ground level is sufficient to prevent their desiccation. People who engage in activities in wooded or brushy areas from May through July, the feeding period of I. scapularis nymphs, are at most risk. The nymphs are very small and often go unnoticed while feeding on humans. Risk is much lower during fall and early spring when adult I. scapularis ticks are feeding; adult ticks are much easier to find and remove before the 24 to 48 hours of tick attachment time needed for disease transmission to occur. During 2003, Lyme disease onsets peaked in July (39% of cases with known onset dates), just after the peak of the I. scapularis nymph feeding period. Similarly, 55% of HA cases in 2003 had their onsets in June or July.
To prevent tick-borne diseases, MDH continues to stress personal protection measures, such as using tick repellents (containing DEET or permethrin), wearing protective clothing, and checking frequently for ticks. There is no longer a Lyme disease vaccine on the market, and no large-scale tick control methods are available. People should take precautions when they spend significant time in wooded or brushy areas in east-central or southeastern Minnesota from mid-May through mid-July. They should also seek prompt diagnosis and treatment of early-stage Lyme disease to prevent late-stage Lyme manifestations. MDH receives many inquiries from medical providers regarding prophylactic treatment of I. scapularis bites. Nadelman et al. found that a single 200-mg dose of doxycycline given within 72 hours after an I. scapularis tick bite was 87% effective in preventing the development of Lyme disease. While this is treatment may be considered for known I. scapularis bites (especially when engorged ticks are found), MDH does not, in general, recommend it. Although the above study found that doxycycline can be effective, it also showed that in most instances the treatment isn't necessary. Only a low percentage of known tick bites actually result in Lyme disease. This study was conducted in a part of New York that has a much higher incidence of Lyme disease than does Minnesota. Yet in the study's placebo group, only 3.2 percent of tick bite patients went on to develop EM, suggesting that many of them likely found the attached ticks prior to disease transmission, or they were bitten by ticks that were not infected with B. burgdorferi. (Minnesota's tick infection rates have not been well studied, but they are thought to be less than in East Coast areas where up to 50% of I. scapularis ticks may be infected.) The study also documented more adverse effects, mainly nausea and vomiting, in the treatment group than in the placebo group.
Tick identification can be difficult, causing people to mistakenly believe they have been bitten by I. scapularis and thus seek unnecessary antibiotic treatment. Size alone cannot be used to identify I. scapularis, as other ticks also have small immature stages. In Minnesota, the larger adult stage of Dermacentor variabilis ticks (wood ticks) frequently bite people, but ticks of an immature stage of other, less common ticks can also be found on people. The tiny larval stage of I. scapularis (larvae have 6 legs instead of the 8 found on nymphs and adults) sometimes bites people, but such bites are not medically important, as they are not yet infected with B. burgdorferi. People who never see the tick that bit them are at greater risk of disease. In 2002, for example, a tick bite was recalled in only 234 of 867 (27%) Lyme disease cases in Minnesota.