Epidemiology
Ehrlichiosis and anaplasmosis have a seasonal distribution that coincides with higher tick activity in warmer months (e.g., April through October in the US), although infections can be seen in other months with less frequency.
Human monocytotropic/monocytic ehrlichiosis (HME)
Passive surveillance from 2008 to 2012 indicates an average annual incidence of 3.2 cases per million persons.[15] However, incidence and prevalence are thought to be grossly underestimated, as studies have revealed an incidence of 330 to 414 cases per 100,000 population per year, with seroprevalence studies showing the presence of anti-ehrlichial antibodies in about 12.5% of the population.[16][17][18][19] Cases of Ehrlichia chaffeensis infection are increasing in the US, with 2093 reported in 2019, but decreased in 2020 and 2021 due to the COVID-19 pandemic (1337 cases in 2021).[20]
In the US, HME occurs mostly across the South Central and Southeastern states, although cases have been reported in almost all states following the distribution of the Ehrlichia chaffeensis vector, the Lone Star tick (Amblyomma americanum).[21] The American dog tick (Dermacentor variabilis) is also a possible vector. HME has also been documented outside the US (e.g., Cameroon, South Korea).[22]
The main zoonotic reservoir in the US is the white-tailed deer.
Ehrlichia muriseauclairensis (formerly known as Ehrlichia muris-like agent, EMLA) causes a presentation similar to HME.
Ehrlichia muris eauclairensis
Formerly known as E. muris-like agent or EMLA.
This pathogen was discovered in 2009 in the upper Midwest. More than 115 cases have been reported since its discovery.[21]
Human granulocytotropic/granulocytic anaplasmosis (HGA)
Passive surveillance from 2008 to 2012 indicates an average annual incidence of 6.3 cases per million persons.[23] However, underreporting is also likely, since studies have revealed an incidence of 24 to 58 cases per 100,000 population per year, and seroprevalence studies have shown that 14.9% of the population have anti-Anaplasma antibodies.[24] In the US, reported anaplasmosis infections peaked in 2017 with 5762 cases. In 2018, reported cases were substantially lower although in 2019, cases rose to 5655. Cases decreased in 2020 due to the COVID-19 pandemic, but rose again in 2021 (6729 cases).[25]
In the US, HGA occurs mostly across northeastern states, the Upper Midwest, and the Pacific Northwest following the distribution of the Anaplasma phagocytophilum vector, the deer tick, also known as the black-legged tick or bear tick (Ixodes scapularis or I pacificus).[21] HGA has been reported in Europe and Asia, where the main vectors are I ricinus and I persulcatus, respectively.[26]
The main zoonotic reservoir in the US is the white-footed mouse.
These tick species also harbor Borrelia burgdorferi, Babesia microti, and the tick-borne encephalitis virus, 3 infectious agents that can coexist with A phagocytophilum within the vector and cause dual infections in humans.[21]
Human ewingii ehrlichiosis (HEE)
Passive surveillance from 2008 to 2012 indicates an average annual incidence of 0.04 cases per million persons.[15] A total of 302 cases have been reported from 2008-2021.[20]
E ewingii has not been isolated in culture to date. It appears to be less virulent than E chaffeensis, but the tick vector and zoonotic host are the same as for E chaffeensis; therefore, distribution is similar.
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