Rickettsial eschars are necrotic lesions that occur at the site of tick or mite bites and represent locations of primary inoculation of spotted fever group Rickettsia and Orientia species. In the United States, eschars are hallmarks of less severe spotted fever diseases, including those caused by endemic agents such as Rickettsia parkeri (1) and Rickettsia species 364D (2), as well as several imported agents, including Rickettsia africae, Rickettsia conorii, and Orientia tsutsugamushi. Eschars generally do not occur with Rocky Mountain spotted fever (RMSF), a potentially deadly disease caused by Rickettsia rickettsii and have not been associated with Ehrlichia or Anaplasma species. The presence of eschars can help differentiate less severe spotted fever rickettsioses from RMSF and clarify the potential contributions of each within surveillance data. The lone star tick (Amblyomma americanum), the Gulf Coast tick (Amblyomma maculatum), and the Pacific Coast tick (Dermacentor occidentalis) are the three most common species of tick vectors that spread eschar-associated pathogens in the United States (1–4). Lone star and Gulf Coast ticks are vectors of R. parkeri, and Pacific Coast ticks are vectors of Rickettsia species 364D. In commonly available serologic assays, spotted fever group Rickettsia antigens cross-react, which presents a challenge when differentiating RMSF from other spotted fever rickettsioses. Incidence of spotted fever rickettsiosis continues to rise, with few cases providing species-specific laboratory evidence; therefore, the proportion of spotted fever rickettsioses caused by R. rickettsii remains unclear (5). This analysis serves as the first summary of eschar-associated rickettsial disease surveillance data in the United States. During 2010–2016, the presence or absence of eschars was reported in <20% of tickborne rickettsial disease (TBRD) cases. Eschar-associated illnesses represented a small percentage (1.1%) of TBRD cases. Among the 484 reported eschar-associated cases, 97 (20%) were classified as ehrlichiosis or anaplasmosis. Further research is needed to determine whether eschars associated with ehrlichiosis or anaplasmosis reflect a reporting error, a new finding, or the result of coinfection with another eschar-associated rickettsial pathogen.

Notifiable rickettsial diseases are reported to CDC through the National Notifiable Diseases Surveillance System, which also collects basic demographic information. Supplementary information is recorded through submission of TBRD supplemental case report forms, or extractions from state surveillance systems, and includes clinical details, diagnostic criteria, and patient outcomes. Since 2010, the CDC supplemental case report form* has requested information on eschars.

For this report, supplementary surveillance data collected by state and local health departments for illness with onset during 2010–2016 that were received and entered by CDC as of November 13, 2018, were summarized. TBRDs are not reportable conditions in Alaska and Hawaii, so no data from these states were included in this report. Case classifications were made according to the Council of State and Territorial Epidemiologists definitions (6,7). Confirmed cases were clinically compatible and had confirmatory diagnostic evidence obtained by seroconversion (fourfold change) in anti-Ehrlichia, -Anaplasma, or -Rickettsia immunoglobulin (Ig)G antibody titers by indirect immunofluorescence antibody assay or tested positive by polymerase chain reaction (PCR), immunohistochemistry, or culture. Probable cases were clinically compatible and included supportive laboratory evidence from serologic assays (including IgG- or IgM-positive antibodies reactive to Ehrlichia, Anaplasma, or Rickettsia species using immunofluorescence antibody assay or other serologic methods) or reported the presence of morulae (intracellular inclusion bodies in leukocytes) (7). Data were analyzed using SAS software (version 9.4; SAS Institute).

A rickettsial eschar begins as a small, painless papule that appears within a few days after the bite of an infected vector. The papule grows, becomes vesicular or pustular, and ulcerates forming a brown-to-black crust surrounded by a red annular halo (Figure 1). During 2010–2016, a total of 44,099 cases of TBRD with supplemental case report forms were reported to CDC, including 484 (1.1%) reported as eschar-associated TBRD; however, most case reports (35,749, 81.1%) were missing information on eschars altogether. Among reported eschar-associated cases, 387 (80.0%) were classified as spotted fever rickettsioses, 64 (13.2%) as Ehrlichia chaffeensis ehrlichiosis, 30 (6.2%) as Anaplasma phagocytophilum anaplasmosis, one (0.2%) as Ehrlichia ewingii ehrlichiosis, and two (0.4%) as undetermined ehrlichiosis/anaplasmosis. Notation of suspected spotted fever species is not required but was listed for 16 (4.1%) cases, including R. africae (11 cases), R. parkeri (two) and R. conorii (one), Rickettsia species 364D (one), and Rickettsia akari (one). No eschar-associated cases were associated with R. rickettsii.

Patients reporting eschar-associated illnesses were predominantly male (290, 59.9%), white (331, 68.4%), and non-Hispanic (402, 83.1%) (Table). Hospitalization (90, 18.6%) and death (1, 0.2%) occurred among a smaller proportion of patients with eschar-associated illness than among those with illness not associated with eschar (2,120, 27.0% and 21, 0.3%), respectively. Race and sex distributions were similar among patients with and without eschars. All but seven jurisdictions in which TBRD are reportable submitted information on the presence and absence of eschars during this period. Most eschar-associated cases (74.6%, 361) were reported from the South, compared with 60.3% (4,738) of cases not associated with eschar (Table). Most eschar-associated cases (462, 95.5%) were reported from states where ticks that transmit eschar-associated pathogens were present (Figure 2). A large proportion of all TBRD cases were missing travel history (30,455, 69.1%).

Only 42 (8.7%) of 484 eschar-associated cases were confirmed, compared with 1,093 (13.9%) TBRD cases not associated with eschar (Table). Thirty-four (7.0%) reported eschar-associated cases were tested by PCR, one report described visualization of morulae, and 447 (92.0%) cases met confirmed or supportive laboratory criteria using serologic evidence; techniques were not mutually exclusive.