Lyme Disease -- United States, 1996

Lyme disease (LD) is caused by the tickborne spirochete Borrelia burgdorferi sensu lato and is the most common vectorborne disease in the United States. Surveillance for LD was initiated by CDC in 1982, and the Council of State and Territorial Epidemiologists designated it a nationally notifiable disease in January 1991. For surveillance purposes, LD is defined as the presence of an erythema migrans rash greater than or equal to 5 cm in diameter or laboratory confirmation of infection with evidence of at least one manifestation of musculoskeletal, neurologic, or cardiovascular disease (1). This report summarizes the provisional number of cases of LD reported to CDC during 1996 and indicates that the number of cases reported to CDC was a record high.

In 1996, a total of 16,461 cases of LD were reported to CDC by 45 states and the District of Columbia (overall incidence: 6.2 per 100,000 population *), representing a 41% increase from the 11,700 cases reported in 1995 and a 26% increase from the 13,043 cases reported in 1994 (Figure_1). As in previous years, most cases were reported from the Mid-Atlantic, Northeast, and North Central regions (Table_1). Eight states reported LD incidences that were higher than the overall national rate (Connecticut, 94.8; Rhode Island, 53.9; New York, 29.2; New Jersey, 27.4; Delaware, 23.9; Pennsylvania, 23.3; Maryland, 8.8; and Wisconsin, 7.7); these states accounted for 14,959 (91%) of the nationally reported cases. In 1996, zero cases were reported from five states (Alaska, Arizona, Colorado, Montana, and South Dakota).

Eighty-seven counties each reporting greater than or equal to 20 cases accounted for 89% of all reported cases. Reported incidences were greater than 100 per 100,000 ** in 18 counties in Connecticut, Maryland, Massachusetts, North Carolina, New Jersey, New York, Pennsylvania, Rhode Island, and Wisconsin; the highest reported county-specific incidence (1247.5 per 100,000) was in Nantucket County, Massachusetts (Figure_2). From 1995 to 1996, a total of 28 states reported increases in the number of cases, 16 states reported decreases, and seven states reported no change. Approximately 90% of the total increase in reported cases in 1996 occurred in five states (Connecticut, New Jersey, New York, Pennsylvania, and Rhode Island) where average annual LD incidence rates had exceeded the national average for the previous 5 years combined.

Of 5298 cases for which information was available, 217 (4%) were reported as having been acquired outside of the United States, and 156 (3%) cases were reported as having been acquired in the United States but outside of the reporting state. The highest proportions of cases occurred among persons aged 0-14 years (3784 {23%}) and adults aged 40-79 years (7694 {47%}). Of 16,422 cases for which sex was reported, 8634 (53%) were male.

Reported by: State health depts. Bacterial Zoonoses Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Disease, CDC.

Editorial Note

Editorial Note: LD continues to be an important emerging infection: geographic spread within states with endemic disease and intensified transmission of the LD spirochete in established foci of infection have been associated with increased numbers of reported cases in the United States. In the eastern United States, the patterns of human LD cases reflect the geographic distribution of Ixodes scapularis, also known as the black-legged or deer tick (2,3). Substantial annual fluctuations since 1992 in the number of reported cases in several northeastern states with endemic disease have been attributed, in part, to variations in I. scapularis density (4,5). The principal vector in western coastal states is

1. pacificus (the western black-legged tick). LD also is transmitted by Ixodes spp. in Canada and in temperate areas of Eurasia, including Europe, Russia, northern People's Republic of China, and Japan (6).

Increases in reported LD cases in 1996 were limited to certain counties in some states, consistent with focal differences in the distribution and density of the tick vector. In both Connecticut and Rhode Island, the numbers of reported cases of LD increased statewide, although increases were greatest in coastal counties. In both states, this increase was associated with increased population densities of I. scapularis (K. Stafford, Connecticut Agricultural Experiment Station, and T. Mather, University of Rhode Island, personal communications, 1997). In New York, the greatest increases occurred in Dutchess County, where reported cases of LD nearly doubled from 1995 (918) to 1996 (1832). Because an LD vaccine trial was being conducted in the area, some of this increase may have resulted from heightened awareness and reporting of LD. The number of reported cases was stable in other counties of New York with endemic disease, including Putnam, Suffolk, and Westchester counties. In New Jersey, eight counties with active surveillance reported higher rates than the remaining counties with passive surveillance systems.

Since 1991, state health departments in regions with endemic disease have been expanding their use of laboratory testing for assisting in LD surveillance. A positive laboratory result is required for reporting of persons with systemic manifestations of LD but is not required for persons with an erythema migrans rash greater than or equal to 5 cm in diameter (i.e., early LD). Since August 1995, when CDC published recommendations for standardized two-step (enzyme immunoassay and Western immunoblot) serodiagnostic testing for LD (7), states have reported a shift toward use of the recommended two-step method in diagnostic laboratories. The impact of these changes in laboratory methods on LD surveillance is unknown.

The increase in reported LD cases in 1996 probably represents a combination of increased tick density, enhanced health-care provider awareness and reporting, and improved laboratory surveillance. In addition, case reporting has been enhanced through the availability of CDC resources for LD surveillance in eight states (Connecticut, Michigan, Minnesota, New Jersey, New York, Oregon, Rhode Island, and West Virginia).

Most LD cases respond well to appropriate antibiotic therapy; drugs of choice include amoxicillin, doxycycline, and ceftriaxone (8). Vaccines to prevent LD are under evaluation but are not yet available. Personal protection methods recommended for preventing cases of LD and other tickborne diseases (e.g., babesiosis, ehrlichiosis, and Rocky Mountain spotted fever) include wearing light-colored clothing (to more readily detect ticks), tucking long pants into socks, using insect repellents and acaricides according to label directions, and performing tick checks at least daily. The use of environmental modifications to residential properties (e.g., application of insecticides, use of deer fencing, and removal of leaf litter) also may help prevent LD.

References

1. CDC. Case definitions for infectious conditions under public health surveillance. MMWR 1997;46(no. RR-10):20-1.

2. Mather TN, Nicholson MC, Donnelly EF, Matyas BT. Entomologic index for human risk of Lyme disease. Am J Epid 1996;144:1066-9.

3. Kitron U, Kazmierczak JJ. Spatial analysis of the distribution of Lyme disease in Wisconsin. Am J Epid 1997;145:558-66.

4. CDC. Lyme disease -- United States, 1994. MMWR 1995;44:459-62.

5. CDC. Lyme disease -- United States, 1995. MMWR 1996;45:481-4.

6. Dennis DT. Lyme disease. Dermatologic Clinics 1995;13:537-50.

7. CDC. Recommendations for test performance and interpretation from the Second National Conference on Serologic Diagnosis of Lyme Disease. MMWR 1995;44:590-1.

8. Steere AC. Borrelia burgdorferi (Lyme disease, Lyme borreliosis). In: Mandell GL, Bennett JE, Dolin R, eds. Principles and practices of infectious diseases. New York: Churchill Livingstone, 1995:2143-55.

* State rates are based on 1996 population estimates. 

** County rates are based on 1990 population estimates.

TABLE 1. Number of reported cases of Lyme disease, by state, 1991-1996 *, and rate + of Lyme disease,
1996 -- United States
=========================================================================================================
State                      1991     1992     1993      1994      1995      1996     Total    1996 Rate
---------------------------------------------------------------------------------------------------------
Alabama                      13       10        4         6        12         9        54          0.2
Alaska                        0        0        0         0         0         0         0          0.0
Arizona                       1        0        0         0         1         0         2          0.0
Arkansas                     31       20        8        15        11        27       112          1.1
California                  265      231      134        68        84        80       862          0.3
Colorado                      1        0        0         1         0         0         2          0.0
Connecticut               1,192    1,760    1,350     2,030     1,548     3,104    10,984         94.8
Delaware                     73      219      143       106        56       173       770         23.9
District of Columbia          5        3        2         9         3         3        25          0.6
Florida                      35       24       30        28        17        55       189          0.4
Georgia                      25       48       44       127        14         1       259          0.0
Hawaii                        0        2        1         0         0         1         4          0.1
Idaho                         2        2        2         3         0         2        11          0.2
Illinois                     51       41       19        24        18        10       163          0.1
Indiana                      16       22       32        19        19        32       140          0.5
Iowa                         22       33        8        17        16        19       115          0.7
Kansas                       22       18       54        17        23        36       170          1.4
Kentucky                     44       28       16        24        16        26       154          0.7
Louisiana                     6        7        3         4         9         9        38          0.2
Maine                        15       16       18        33        45        61       188          4.9
Maryland                    282      183      180       341       454       447     1,887          8.8
Massachusetts               265      223      148       247       189       321     1,393          5.3
Michigan                     46       35       23        33         5        20       162          0.2
Minnesota                    84      197      141       208       208       251     1,089          5.4
Mississippi                   8        0        0         0        17        21        46          0.8
Missouri                    207      150      108       102        53        52       672          1.0
Montana                       0        0        0         0         0         0         0          0.0
Nebraska                     25       22        6         3         6         5        67          0.3
Nevada                        5        1        5         1         6         2        20          0.1
New Hampshire                38       44       15        30        28        47       202          4.0
New Jersey                  915      688      786     1,533     1,703     2,190     7,815         27.4
New Mexico                    3        2        2         5         1         1        14          0.1
New York                  3,944    3,448    2,818     5,200     4,438     5,301    25,149         29.2
North Carolina               73       67       86        77        84        66       453          0.9
North Dakota                  2        1        2         0         0         2         7          0.3
Ohio                        112       32       30        45        30        32       281          0.3
Oklahoma                     29       27       19        99        63        45       282          1.4
Oregon                        5       13        8         6        20        19        71          0.6
Pennsylvania                718    1,173    1,085     1,438     1,562     2,814     8,790         23.3
Rhode Island                142      275      272       471       345       534     2,039         53.9
South Carolina               10        2        9         7        17         9        54          0.2
South Dakota                  1        1        0         0         0         0         2          0.0
Tennessee                    35       31       20        13        28        24       151          0.5
Texas                        57      113       48        56        77        97       448          0.5
Utah                          2        6        2         3         1         1        15          0.0
Vermont                       7        9       12        16         9        26        79          4.4
Virginia                    151      123       95       131        55        57       612          0.9
Washington                    7       14        9         4        10        18        62          0.3
West Virginia                43       14       50        29        26        12       174          0.7
Wisconsin                   424      525      401       409       369       396     2,524          7.7
Wyoming                      11        5        9         5         4         3        37          0.6

Total                     9,470    9,908    8,257    13,043    11,700    16,461    68,839          6.2
---------------------------------------------------------------------------------------------------------
* Data for 1996 are provisional.
+ Per 100,000 population.
=========================================================================================================

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