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

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 under- and 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 high-incidence 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 resulted in more persons with positive laboratory evidence of infection (10). In contrast, health departments anecdotally reported challenges in receiving or identifying MTTT assays within their systems because of lack of MTTT-specific Logical Observation and Identifiers Names and Codes (LOINC), which might have resulted in underascertainment of persons with positive laboratory evidence in 2022.

Implications for Public Health Practice

The 69% increase in reported cases of Lyme disease after implementation of the 2022 surveillance case definition, with the largest relative increase occurring among older adults, likely reflects modification of surveillance methods in high-incidence jurisdictions rather than a true change in disease risk. Surveillance in low-incidence jurisdictions still necessitates clinical investigation to ascertain probability of locally acquired infection to accurately guide clinical and public education. The revised approach to surveillance will improve standardization of surveillance data across high-incidence jurisdictions but precludes robust comparison of trends with data collected using earlier case definitions. Specific LOINC codes were created and approved in early 2023.^¶¶ Use of standardized codes by commercial and clinical laboratories is critical to ensuring consistent identification of persons with laboratory evidence of Lyme disease for surveillance purposes. Although the total number of reported cases is higher than in previous years, it still does not approach the estimated 476,000 Lyme disease diagnoses estimated to occur annually in the United States (2), a frequency that highlights the need for effective prevention methods.