Results
Overall Analysis
During 1996–2007, CDPH staff processed 87,174 Lyme disease reports, of which 7,278 (8.3%) were duplicate entries and were removed from the database. A total of 79,896 individual reports were analyzed. Of these, 43,767 (54.8%) were reported through mandatory laboratory surveillance, 19,350 (24.2%) through passive physician surveillance, 13,040 (16.3%) through active physician surveillance, and 3,739 (4.7%) through enhanced laboratory surveillance. Overall, 33,457 (41.9%) reports were classified as cases, and 26,318 (32.9%) as not cases; 20,121 (25.2%) were lost to follow-up (Table 1). Except for calculation of PPV, reports classified as lost to follow-up were excluded from further analyses.
During 1996, Connecticut had 5,473 reports of Lyme disease. The number of reports increased with the successive implementation of enhanced and mandatory laboratory surveillance reporting, peaking at 12,947 in 2002 (Figure 1). In 1998, the first year of mandatory laboratory surveillance, the overall number of Lyme disease reports increased by 80.3%, cases increased by 49.4%, and physician-based reporting increased by 26.9% over the previous year; incidence was 104.5 cases per 100,000 population. In 2003, the first year after laboratory reporting ended, the overall number of reports decreased by 82.1%, cases decreased by 69.7%, and physician-based reporting decreased by 37.8% over the previous year; incidence was 41.2 cases per 100,000 population. During 2003–2006, the period with no laboratory surveillance, the number of total reports dropped substantially to an annual mean of 2,411, a 78.5% decrease from the 1999–2002 annual mean. In 2007, laboratory surveillance was reinstated for laboratories with the capability to electronically report results. The total number of reports increased by 228.3%, the number of cases increased by 71.0% over the previous year (Figure 1), and the incidence nearly doubled to 89.8 cases per 100,000 population. An average of 16.0% more cases were reported through physician-based surveillance during years with mandatory laboratory reporting.
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Figure 1.
Number of Lyme disease surveillance reports received and incidence per 100,000 population, Connecticut, 1996–2007. White bar sections, passive surveillance; gold bar sections, active surveillance; blue bar sections, enhanced laboratory surveillance; green bar sections, mandatory laboratory surveillance; line, incidence, determined by using decennial census data encompassing the year data were reported.
The PPV varied across surveillance methods and was highest for physician-based surveillance methods (Table 1). Less than 25% of reports received through mandatory laboratory surveillance were classified as cases. Cases reported through this method accounted for nearly one third (31.9%) of all cases during the study period.
Demographic Characteristics
The median age of case-patients was 38 years (range 34–43 years). Case-patients <20 years of age were more likely to be reported through physician-based surveillance (p<0.001); laboratory-based surveillance was more likely to report case-patients >40 years of age (p<0.001) (Figure 2). Overall, whites accounted for 82.0% of cases, similar to the state's racial distribution, and the distribution did not differ significantly by surveillance method. Ethnicity data were available for approximately one third (32.6%) of case-patients; only 1.2% were reported as Hispanic. Laboratory-based surveillance reported an average of 32 Hispanic case-patients annually, compared with 20 reported through physician-based surveillance (Table 2). On average, 9.6% more male than female case-patients were reported by each surveillance method.
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Figure 2.
Mean annual number of Lyme disease cases, by age group and surveillance method, Connecticut, 1996–2007. Black bar sections, physician-based surveillance; white bar sections, laboratory-based surveillance.
Clinical Features
Of the 33,457 cases, 66.2% were characterized by erythema migrans only, 27.6% by >1 late manifestation and positive laboratory findings, and 6.2% by both (Table 3, Table 4). Overall, symptoms of erythema migrans only were more likely to be reported through physician-based surveillance than through laboratory-based surveillance (75.8% vs. 50.3%; p<0.001). Conversely, late manifestations were more likely to be reported through laboratory-based surveillance than through physician-based surveillance (43.2% vs. 18.1%; p<0.001). Of all case-patients reported through laboratory-based surveillance, 30.5% had Lyme arthritis, compared with 13.4% of those reported through physician-based surveillance. Of cases characterized by late manifestations only, arthritis was most frequently reported (72.1%). Of late manifestation cases for which arthritis was reported, 74.0% were based on physician surveillance and 70.8% on laboratory surveillance (p<0.001). Laboratory-based surveillance was more likely to report case-patients with second- or third-degree atrioventricular block (p = 0.051).
Seasonality
In 72.9% of cases, illness onset occurred during the summer (76.3% physician-based vs. 66.8% laboratory-based cases) (Table 5). Erythema migrans occurred in 84.2% of cases with onset during the summer. Erythema migrans was significantly more likely to be reported during the summer through physician-based surveillance than through laboratory-based surveillance (71.3% vs. 28.7%; p<0.001). Late manifestations were 2× more likely to be reported through laboratory-based surveillance during the summer months (17.5% vs. 8.2%; p<0.001).