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Incidence of Lyme disease in Maryland is on the rise

Since the first Lyme Disease (LD) case was reported in Maryland in 1979, the number of reported cases has steadily increased. From 1993 to 1996, the incidence rate (IR) of LD increased by 51 percent, from 3.9 to 8.3 cases per 100,000 Maryland residents. These are the findings of a study supported by the Agency for Health Care Policy and Research (HS07813).

The overall IR for the State during the study period was 7 cases per 100,000 inhabitants, with the IR varying by region, sex, and age. The IR ranged from 0.7 in Allegheny County in Western Maryland to 88.3 in Kent County on Maryland's Upper Eastern Shore. In fact, counties in the Upper Eastern Shore had the highest IR per age group, sex, and jurisdiction, while the IR was lowest in Maryland's Western counties, where no LD cases were reported for patients younger than 10 or older than 59 years of age.

This pattern of LD distribution in Maryland has not changed since 1989, when LD became a reportable disease in the State, and is supported by a State-wide survey of ticks from white-tailed deer. It showed that the percentage of ticks infected with Borrelia burgdorferi, the infectious agent that causes LD, ranged from 0 percent in Western Maryland counties to 15 percent in regions of the Upper and Lower Eastern Shore.

The current study also found that males were 1.2 times more likely to contract LD than females (IR of 7.7 vs. 6.2). The IR peaked in the 10 to 19 age group (IR of 8.5), 50 to 59 age group (IR of 8.9), and the 20 to 29 age group (IR 3.9). Outdoor activities (for example, hunting), type of clothing, and use of tick repellents may play an important role in the different LD IR between males and females and by age group. Also, women are more apt to seek medical attention after a tick bite or the first signs of the disease, which would make a difference in the IR. The bimodal age distribution also may be explained by the type of activities in which the youngest and oldest age groups are involved. However, the authors were unable to determine why the 20 to 29 year age group had the lowest IR.

These findings are based on a State-wide surveillance for LD in 24 Maryland jurisdictions from 1993 to 1996, using the Maryland Lyme Disease Registry. The study was carried out by Cesar A. Pena, D.V.M., M.H.S., and George T. Strickland, M.D., Ph.D., of the University of Maryland School of Medicine.

More details are in "Incidence rates of Lyme disease in Maryland: 1993 through 1996," by Drs. Pena and Strickland, in the March/April 1999 Maryland Medical Journal 48(2), pp. 68-73.

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