Surveillance for Lyme Disease After Implementation of a Revised Case Definition — United States, 2022

Lyme disease, a tickborne zoonosis caused by certain species of Borrelia spirochetes, is the most common vectorborne disease in the United States. Approximately 90% of all cases are reported from 15 high-incidence jurisdictions in the Northeast, mid-Atlantic, and upper-Midwest regions. After the implementation of a revised surveillance case definition in 2022, high-incidence jurisdictions report cases based on laboratory evidence alone, without need for additional clinical information. In 2022, 62,551 Lyme disease cases were reported to CDC, 1.7 times the annual average of 37,118 cases reported during 2017-2019. Annual incidence increased most in older age groups, with incidence among adults aged ≥65 years approximately double that during 2017-2019. The sharp increase in reported Lyme disease cases in 2022 likely reflects changes in surveillance methods rather than change in disease risk. Although these changes improve standardization of surveillance across jurisdictions, they preclude detailed comparison with historical data.


Introduction
Lyme disease is a tickborne infection caused by spirochetes in the Borrelia burgdorferi sensu lato complex (1,2).Signs and symptoms of early disease include erythema migrans, a red, expanding rash often with central clearing, as well as fever and fatigue.Untreated infection can disseminate, affecting the heart, joints, and nervous system (1).National surveillance for Lyme disease in the United States began in 1991 and has documented a steady increase in incidence and geographic range.A majority of cases of Lyme disease are reported from 15 high-incidence jurisdictions (those reporting at least 10 confirmed cases per 100,000 population for 3 years) located in the Northeast, mid-Atlantic, and upper-Midwest regions* (3) After implementation of a revised Lyme disease case definition, a total of 62,551 Lyme disease cases were reported to CDC in 2022 (including 59,734 from high-incidence jurisdictions and 2,817 from low-incidence jurisdictions).† † This finding represented an overall 68.5% increase from the annual average of 37,118 cases reported during 2017-2019, including a 72.9% increase in high-incidence jurisdictions and a 10.0% increase in low-incidence jurisdictions (Table ).During 2022, 95.5% of reported cases were reported from high-incidence jurisdictions, compared with an average of 93.1% during 2017-2019.Lyme disease incidence in 2022 (18.9 cases per 100,000 population) was 68.8% higher than that during 2017-2019 (11.2).In 2022, median incidence among highincidence jurisdictions (68.3 cases per 100,000) was 58% higher than that during 2017-2019 (43.3), although median incidence among low-incidence jurisdictions (0.52 cases per 100,000) was 24% lower than during 2017-2019 (0.68).

Illness Onset and Other Available Dates
Illness onset date was available for more than two thirds (67.8% [75,491 of 111,354]) of cases reported during 2017-2019, but only 4.8% (2,987 of 62,551) of cases in 2022.Illness onset peaked during calendar week 26 during both 2017-2019 and 2022; however, in 2022, the diagnosis, laboratory test, and reporting dates peaked 2 weeks later (week 28) (Figure 2).

Discussion
After implementation of a revised surveillance case definition in 2022, the number of reported Lyme disease cases in the United States increased 68.5% over the average reported during 2017-2019; in high-incidence jurisdictions, the number of cases increased 72.9%, whereas in low-incidence jurisdictions, the number of cases increased 10.0%.This change reflects a large increase in the number of cases reported from high-incidence jurisdictions on the basis of laboratory evidence alone.Before 2022, many of these cases would have been excluded, either because health departments were unable to obtain the necessary clinical information or because available clinical data were inconsistent with the objective criteria specified in the case definition.The increases in incidence in 2022 compared with 2017-2019 are particularly large among high-incidence jurisdictions that had previously modified Lyme disease surveillance practice to minimize the case investigation workload.The total number of cases in many low-incidence jurisdictions decreased, presumably because of changes in the 2022 case definition requiring objective signs and symptoms of Lyme disease for the probable case classification in these areas with lower disease risk.
The relative increase in Lyme disease incidence in 2022 was larger among older age groups, with age-specific incidences more than doubling among adults aged ≥65 years relative to those during 2017-2019.The differential increase in incidence might reflect 1) more frequent laboratory testing among older age groups, 2) proportionally more disseminated illness in older age groups, and 3) proportionally more positive laboratory test results related to previous exposure to B. burgdorferi rather than a current illness.
Date of illness onset is rarely available in high-incidence jurisdictions given reliance on laboratory-based reporting without case investigation to ascertain clinical information.Alternative dates related to laboratory testing or reporting still demonstrate summer seasonality, but are shifted 2 weeks later, reflecting the expected time lag required after symptom onset to mount a detectable immune response to B. burgdorferi (1).

Limitations
The findings in this report are subject to at least two limitations.First, surveillance for Lyme disease is subject to underand overreporting.Despite an increase in reported cases in 2022, it is likely that current surveillance does not capture all cases of Lyme disease, specifically cases of early disease for which diagnosis is based on clinical findings alone, including presence of erythema migrans rash, and laboratory evidence is lacking because of insufficient elapsed time to mount a detectable antibody response.Previous case definitions relied on direct clinician report to identify such cases; however, the frequency of such reporting was highly variable among highincidence jurisdictions (6).Conversely, reporting based solely on serologic testing might result in the inclusion of clinically incompatible or nonincident cases (i.e., a positive laboratory test result based on previous infection).Antibody titers remain elevated for months to years after treatment for Lyme disease, and asymptomatic seroconversion is also known to occur (1).In these instances, testing for Lyme disease when another etiology is responsible for the current illness might generate an erroneous case report.Second, changes in laboratory testing between the two analysis periods might have influenced Lyme disease incidence.The Food and Drug Administration cleared the first modified two-tier test (MTTT) serologic assays for Lyme disease in 2019 § § (9).These assays have higher sensitivity in early illness than do standard algorithms and might have § § https://www.aphl.org/aboutAPHL/publications/Documents/ID-2021-Lyme-Disease-Serologic-Testing-Reporting.pdf

FIGURE 1 .FIGURE 2 .*
FIGURE 1. Reported Lyme disease incidence (A) and the ratio of the 2022 incidence to the average 2017-2019 incidence (B), by sex and 5-year age group -United States, 2017-2019 and 2022 (8)aboratory diagnosisMethodsLyme disease cases are classified by state and local health departments according to CSTE surveillance case definitions and reported to CDC through the Nationally Notifiable Diseases Surveillance System.§Because of reporting anomalies related to the COVID-19 pandemic (2020-2021)(8), cases reported in 2022 were compared with those reported during 2017-2019.2020 U.S. Census Bureau data were used as population denominators for incidence calculations.¶ Several reporting dates were used to compare trends in seasonality.For the years 2017-2019, illness onset date was used, whereas for 2022, illness onset date, diagnosis date, laboratory test date, and date of laboratory report to health department were used.Data were analyzed using SAS software (version 9.4; SAS Institute).This activity was § https://www.cdc.gov/nndss/index.html ¶ https://data.census.gov/table?q=Age+and+Sex&t=Populations+and+ People&g=010XX00US&d=DEC+Demographic+and+Housing+ Characteristics&tid=DECENNIALDHC2020.P12; https://data.census.gov/table?g=010XX00US$0400000&tid=PEPPOP2019.PEPANNRES reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.**Results Overall: 2022 Versus 2017-2019

TABLE . Number of reported Lyme disease cases and Lyme disease incidence, by jurisdiction and incidence category* -United States, 2017-2019 and 2022
See table footnotes on the next page.