Does E. ewingii Cause More Cases of Ehrlichiosis Than Previously Thought?

This figure shows the incidence of ehrlichicosis cases by state in 2010 per million persons. Ehrlichiosis was not notifiable in Alaska, Colorado, the District of Columbia, Hawaii, Idaho, Iowa, Nevada, New Mexico, North Dakota or Montana. The incidence rate was zero for Arizona, Connecticut, Indiana, Massachusetts, Oregon, South Dakota, Utah, Vermont, Washington, and Wyoming. Incidence ranged between 0.03 to 1 case per million persons for California, Florida, Louisiana, Michigan, Ohio, Pennsylvania, Rhode Island and Texas. Annual incidence ranged from 1 to 3.3 cases per million persons in Alabama, Georgia, Illinois, Kansas, Maine, Minnesota, Mississippi, Nebraska, New Hampshire, New York, South Carolina and West Virginia. The highest incidence rates, ranging from 3.3 to 26 cases per million persons were found in Arkansas, Delaware, Kentucky, Maryland, Missouri, New Jersey, North Carolina, Oklahoma, Tennessee, Virginia, and Wisconsin.

Image and text credit: The US Centers for Disease Control and Prevention webpage on Statistics and Epidemiology of Ehrlichiosis. Link

Ehrlichiosis is caused by three species of bacteria belonging to the genus Ehrlichia: E. chaffeensis, E. ewingii and the provisionally named E. muris like (EML). A recent article in the CDC’s EID indicates that the proportion of cases attributable to E. ewingii may be higher than what we initially knew it to be.

ResearchBlogging.orgIt is virtually impossible to clinically distinguish between the two infections, because they have similar clinical presentation as well as the fact that there are no serological tests that can tell us which species is behind the disease. The only way to identify the species is to use molecular diagnostic tests which actually diagnose the species of the organism. In addition, because the therapeutic plans for both these infections are similar, there is a major chance that there is a massive under reporting of the other species responsible for ehrlichiosis. The abstract of the article states:

Ehrlichiosis is a bacterial zoonosis, spread through the bites of infected ticks, that is most commonly caused in the United States by infection with the bacterium Ehrlichia chaffeensis. We retrospectively reviewed samples from an 18-month study of ehrlichiosis in the United States and found that E. ewingii was present in 10 (9.2%) of 109 case-patients with ehrlichiosis, a higher rate of infection with this species than had previously been reported. Two patients resided in New Jersey and Indiana, where cases have not been reported. All patients with available case histories recovered. Our study suggests a higher prevalence and wider geographic distribution of E. ewingii in the United States than previous reports have indicated.

Previous estimates reveal around 2% of ehrlichiosis cases to have been caused by E. ewingii, (2) and the current study finds a whopping increase – 10%! The primary vector of all the species of Ehrlichia is the lone star tick – Amblyomma americanum in the United States of America. The CDC has provided the following graphic to better understand the life cycle of this vector:

ehrlichia life cycle

The authors state:

Of 4,177 patients from 41 states who had samples submitted to Associated Regional and University Pathologists Laboratories for detection of Ehrlichiaand Anaplasma species by real-time PCR during an 18-month study period, 99 (2.4%) were positive for E. chaffeensis, 10 (0.2%) for E. ewingii/E. canis, and 0 for E. muris-like pathogen. A total of 179 (4.3%) patients were positive for A. phagocytophilium. Positivity rates were calculated by state (Table 2).

All 10 E. ewingii/E. canis–positive cases were subsequently identified as E. ewingii, accounting for 10 (9.2%) of 109 ehrlichiosis cases during the study period. 
E. ewingii–positive samples were from 9 men and 1 woman (median age 58 years, range 24–74 years). The samples were from Missouri (n = 4), Indiana (n = 3), Arkansas (n = 1), New Jersey (n = 1), and New York (n = 1) and were collected in June (n = 1), July (n = 3), August (n = 4), and September (n = 2). Case histories and laboratory results were obtained for 5 patients (Table 3).

This report strengthens the decision of the CDC to have a separate surveillance program for E. ewingii, and will provide us a better, more granular, species level understanding of the epidemiology of the disease, especially with respect to whether there are any differences in the clinical course of the illness or the therapeutic plan adopted to combat it.


ResearchBlogging.org1. Harris, R., Couturier, B., Sample, S., Coulter, K., Casey, K., & Schlaberg, R. (2016). Expanded Geographic Distribution and Clinical Characteristics of Infections, United States Emerging Infectious Diseases, 22 (5), 862-865 DOI: 10.3201/eid2205.152009

2. Adams D, Fullerton K, Jajosky R, Sharp P, Onweh D, Schley A, Summary of notifiable infectious diseases and conditions—United States, 2013. MMWR Morb Mortal Wkly Rep. 2015;62:1–122.


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