What is the prevalence of ehrlichiosis in the US?

Updated: Jun 22, 2021
  • Author: Chinelo N Animalu, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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Answer

The distribution of ehrlichiosis in the United States mirrors the tick distribution and appropriate mammalian vectors (eg, white-footed mouse, white-tailed deer). Ehrlichiosis occurs where mammalian hosts are in contact with the appropriate tick vector (ie, A americanum, D variabilis, Ixodes ticks). 

Map of the United States showing the distribution Map of the United States showing the distribution of the Lone Star Tick, which is the principle vector for ehrlichiosis.
Established and reported distribution of anaplasmo Established and reported distribution of anaplasmosis vectors Ixodes scapularis and Ixodes pacificus, by county, in the United States from 1907-1996. Courtesy of the Division of Vector-Borne Infectious Diseases at the Centers for Disease Control and Prevention.

Most cases of ehrlichiosis in the United States occur in California and Texas and in the southeast and northeast regions of the country, with some cases occurring in the north-central states west of the Great Lakes.

In 2016, 4 states (Missouri, Arkansas, New York, Virginia) accounted for 50% of all reported cases of ehrlichiosis in the United States. [5]

Ehrlichiosis is a seasonal disease observed mainly from April to September. In 1999, ehrlichiosis became reportable to the US Centers for Disease Control and Prevention (CDC). In 2005, 786 cases of human granulocytic anaplasmosis (HGA) were reported. The 3 states that reported the most cases were New York (221 cases), Minnesota (186 cases), and Wisconsin (155 cases). [6, 7] In 2006, 646 cases of HGA were reported. The 3 states that reported the most cases were New York (235 cases), Minnesota (177 cases), and Wisconsin (49 cases). [8]

In the year 2000, only 200 cases of ehrlichiosis were reported, whereas more than 1,377 cases were reported in 2016. [5]

This graph displays the number of human cases of e This graph displays the number of human cases of ehrlichiosis caused by Ehrlichia chaffeensis reported to the Centers for Disease Control and Prevention (CDC) annually from 2000 through 2016. *From 2000 to 2008, ehrlichiosis was included in the reporting category “human monocytic ehrlichiosis” in reports to the National Notifiable Diseases Surveillance System (NNDSS). **Since 2008, ehrlichiosis has been reported to the NNDSS under the categories “Ehrlichia chaffeensis infections,” “Ehrlichia ewingii infections,” and “Undetermined ehrlichiosis/anaplasmosis infections”, which include infections caused by Ehrlichia muris eauclairensis. Only E chaffeensis infections are shown above. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/index.html).

 

This graph shows the number of ehrlichiosis cases This graph shows the number of ehrlichiosis cases caused by Ehrlichia chaffeensis reported from 2000 through 2016 by month of onset to illustrate the seasonal trends. Cases are reported in each month of the year, although most are reported in June and July. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/index.html).

A 2011 study confirmed that B burgdorferi and A phagocytophilum share the same enzootic life cycle suggesting that it is important to monitor areas endemic for Lyme disease for HGA. In this study, La Crosse, WI is centrally located in a well-documented Lyme disease focus. HGA was identified by PCR in the blood of 53 patients with clinical findings consistent with HGA confirming that this region endemic for Lyme should now also be considered part of the upper Midwestern focus of endemicity for HGA. [9]

In 2005, 506 cases of human monocytic ehrlichiosis (HME) were reported. The 3 states that reported the most cases were New York (85 cases), Oklahoma (79 cases), and New Jersey (64 cases). In 2006, 578 cases of HME were reported. The 3 states that reported the most cases were New York (141 cases), Missouri (73 cases), and New Jersey (67 cases).

A 2011 report identified a new ehrlichia species in 4 patients in the Minnesota and Wisconsin areas. All patients had the traditional clinical syndrome and responded to treatment. On testing, 17 of 697 Ixodes scapularis ticks collected in Minnesota or Wisconsin were positive for the same ehrlichia species by polymerase chain-reaction testing and genetic analyses revealed that this new ehrlichia species was closely related to E muris. [10]

Notably, while cases and incidence rose, the case fatality rate (ie, the proportion of patients with ehrlichiosis who died as a result of infection) has declined since 2000, although the case fatality rate in recent publications is still roughly 1% of cases.


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