“Half the truth is often a great lie:” testing for tick-borne diseases and implications for surveillance

Abstract

Importance: Tick–borne diseases (TBD) including Spotted Fever Group Rickettsiosis (SFGR), ehrlichiosis, and increasingly Lyme disease represent a substantial public health concern throughout much of the Southeastern United States. Yet, there is uncertainty about the epidemiology of these diseases due to pitfalls in existing diagnostic test methodologies. Objective: To examine patterns of diagnostic testing and incidence of TBD in a large, academic healthcare system. Design: Cross–sectional study of diagnostic test results from UNC Health for the period January 1st, 2017 to November 30th, 2020. Setting: Large, academic healthcare system in central North Carolina including inpatient and outpatient facilities. Participants: All Individuals seeking routine care at UNC Health facilities who had testing for SFGR, ehrlichiosis or Lyme disease performed during the study period Measurements: Rates of test positivity, testing completeness, and incidence of TBD Results: Among the 20,528 diagnostic tests performed, we identified 47 laboratory–confirmed, incident cases of SFGR, 27 of ehrlichiosis, and 76 of Lyme, representing incidence rates of 4.7%, 7.1%, and 0.7% respectively. However, 79.3% of SFGR tests and 74.3% Ehrlichia tests lacked a paired convalescent sample. The total number of tests for Lyme disease was more than SFGR and ehrlichiosis combined, despite the relatively low incidence of disease in region. Most striking, testing for ehrlichiosis was performed in only half of patients in whom SFGR was ordered, suggesting that this disease remains underrecognized. Overall, we estimate that there were 187 incident cases of SFGR and 309 of ehrlichiosis that were not identified due to incomplete testing; a number that would drastically increase – and in the case of ehrlichiosis, nearly double – the total number of cases reported. Conclusions and Relevance: A majority of patients suspected of having TBD did not have testing performed in accordance with established guidelines, substantially limiting our understanding of TBD epidemiology. Furthermore, there appears to be a large discrepancy between the local burden of disease and the testing that is performed. These findings underscore the need to pursue more robust, active surveillance strategies to estimate the burden of TBDs and distribution of causative pathogens.