The group of ehrlichoisis diseases in humans include:
Human Monocytic Ehrlichiosis (HME)
HME is caused by E. chaffeensis, a bacteria whose primary reservoir is deer [1,8]. HME can be sometimes life-threatening, especially in immunocompromised people. Most of the human ehrlichiosis cases reported in the US have been caused by E. chaffeensis . HME is usually reported in the south-eastern and south-central regions of the country where there is a high density of lone star ticks . These include Arkansas, Maryland, Mississippi, Oklahoma, and Tennessee .
HME was first described in 1986 and since then the cases kept gradually increasing . For instance, in 2000, only 200 cases of HME were reported. By 2019, the number of cases reported was 2,093. Though the number of cases increased, the mortality rate declined over the course of time to about 1% in recent years . The median age of people diagnosed with HME was 53 years.
Human Granulocytic Anaplasmosis (HGA)
HGA was formerly known as Human Granulocytic Ehrlichiosis or HGE. HGA is caused by Anaplasma phagocytophilum (formerly referred to as Ehrlichia phagocytophila), primarily spread by mice, rats, and voles (small rodents that live near rivers or in fields) . HGA has a fatality rate of less than 1% . Together, HME and HGA are responsible for the majority of ehrlichiosis infections in humans so far .
HGA was first identified in 1990 . Between 1994 and 2005, around 2,900 HGA cases in the US were reported to the CDC. The annual incidence of HGA cases during that time was around 1.6 cases per million . The highest number of cases were reported in 2017 (5,762 cases) and the second-highest number of cases were reported in 2019 (5,655 cases) .
The disease is prevalent in the US, Europe, and Australia. In the Asian continent, HGA is found in China, Korea, Russia, and Siberia . In the US, the states of Connecticut, Wisconsin, and New York had the highest incidence rates. HGA cases peak during the summer season, as black-legged ticks are usually active during this season. The median age of people diagnosed with HGA is 51 years .
Human Ewingii Ehrlichiosis (HEE)
E. ewingii is the species of bacteria that causes Human Ewingii Ehrlichiosis. E.ewingii’s primary reservoirs are dogs and deers [1,8]. It was first discovered in 1992 but it was only in 1998 that it was recognized as one of the causes of human ehrlichiosis .
In the United States, HEE cases are mostly reported from the south-eastern, south-central, and mid-Atlantic regions . They include the states of Tennessee, Missouri, and Oklahoma. According to the CDC, the number of HEE cases reported from 2008 to 2019 was 261. None of them were found to be fatal . A majority of people infected with HEE are either HIV patients or those who had an organ transplant (immunosuppressed individuals) .
Other types of Human Ehrlichiosis
Other species that cause ehrlichiosis in humans include E. canis, E. muris eauclairensis, E. Wisconsin-Minnesota, and N. sennetsu.
- E.canis causes severe illness in dogs and can be fatal . Though dogs are its primary reservoir, human E.canis infections have been reported in South America, Mexico, and Venezuela. Individuals affected by this infection can experience a wide range of illnesses – from asymptomatic to severe [5, 8].
- E. muris eauclairensis is a relatively new species of Ehrlichia . It was identified in patients in the upper mid-western regions of the United States (mainly Wisconsin and Minnesota) in 2009 [5,7]. As of today, over 115 cases of human ehrlichiosis caused by E. muris eauclairensis were reported with no fatalities .
- Human Sennetsu Ehrlichiosis (HSE) or Sennetsu Fever is a rare disease with no fatalities reported so far . It was first discovered in Japan in 1953. HSE is mainly reported in southeast Asia in countries like Japan, Laos, Thailand, the Philippines, and Malaysia. These are countries where eating raw fish is common — one of the ways in which an HSE infection spreads to humans [11,12].
How is it Spread?
Ehrlichiosis can be transmitted to humans through:
Tick bites: People get infected when they are bitten by ticks that carry the bacteria. The lone star tick is the primary vector that transmits different types of ehrlichiosis, including HME, HSE, and HEE. Other types of ehrlichiosis, namely, HGA and HWME, are transmitted by black-legged or deer ticks [1,7]. The American dog tick is another vector that spreads ehrlichiosis infections .
Blood transfusion or organ transplant: Ehrlichiosis can also be transmitted through blood transfusion or organ transplant .
Eating Raw fish: In some cases, eating an infected raw fish can cause Human Sennetsu Ehrlichiosis or Sennetsu Fever .
Biology of the Disease
The bacteria that cause ehrlichiosis in humans infect white blood cells (leukocytes). The type of white blood cells targeted depends on the species of ehrlichiosis. For instance, E. chaffeensis mainly affects monocytes and, in severe cases, the granulocytes. A. phagocytophilum and E. ewingii, however, primarily infect granulocytes (neutrophils and occasionally eosinophils).
When ehrlichiosis pathogens enter the human body, they make their way into white blood cells through a cellular process called endocytosis. Once inside, they start to multiply and then rupture the cell walls or host cell membranes to get released into the bloodstream. Now, the larger numbers of bacteria can infect more white blood cells. This vicious cycle continues to repeat, which increases the severity of the infection .