Human Ehrlichiosis : Taxonomy

by CDC

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The genus Ehrlichia is currently classified as a member of the tribe Ehrlichieae, of the family Rickettsiaceae, in the order Rickettsiales. The genus includes seven recognized species: E. canis, E. chaffeensis, E. equi, E. phagocytophila, E. risticii, E. ewingii, and E. sennetsu. A number of other named ehrlichiae, such as "E. platys," "E. bovis," E. ovina," and "E. ondiri," also cause disease in animals. The names of the latter organisms are enclosed in quotation marks because they have not been formally proposed and accepted according to the rules of the International Code of Nomenclature of Bacteria, Bacteriological Code.

The ehrlichiae were initially grouped according to the type of blood cell most commonly infected (granulocyte, lymphocyte, monocyte, platelet), and disease classes have been termed "granulocytic (or granulocytotropic) ehrlichiosis" or "monocytic (or monocytotropic) ehrlichiosis." However, this type of classification may be misleading because some of the Ehrlichia species have been found in cells other than their chief target cell type. In addition, more than one species may be responsible for the broad category of "monocytic" or "granulocytic" ehrlichiosis (e.g., compare the HGE agent and E. ewingii).

Ehrlichia chaffeensis primarily infects mononuclear leukocytes (predominantly monocytes and macrophages), but may also be seen occasionally in the granulocytes of some patients with severe disease.

The pathogen that causes human granulocytic ehrlichiosis (HGE) primarily infects granulocytes (neutrophils and rarely eosinophils). The pathogen is often referred to as the agent of HGE or the HGE agent. This species is very similar, or likely identical, to E. phagocytophila and E. equi.

Ehrlichia ewingii primarily infects neutrophils and occasionally eosinophils and produces a disease clinically similar to HME and HGE. Most patients with this form of ehrlichiosis have also had other medical conditions causing immunosuppression (e.g., HIV infection, splenectomy, transplantation, immunosuppressive drugs).

Modern Classification

Using modern molecular biology techniques, we now know that Ehrlichia species form three distinct groups. The species contained within these "genogroups" are also related to organisms not previously considered to be members of this genus. The classification of the genus Ehrlichia requires revision, and future studies may provide the additional data needed.

Natural History

Studies throughout the world have shown that many cases of human illness are caused by zoonotic pathogens that are maintained by animal hosts in their natural cycles. Many zoonoses require a vector (e.g., mosquito, tick, mite) to be transmitted from the animal host to the human host. The ehrlichial disease cycle includes four components: the pathogen, the animal reservoir species, the vectors, and the human host. For the types of human ehrlichiosis in the United States, the arthropod vectors are ticks.

Only one of the three tick families, Ixodidae (hard ticks), is associated with ehrlichiae. These ticks have four stages in their life cycle: egg, larva, nymph, and adult. After the eggs hatch, each stage must feed once to develop into the next stage. Larvae are uninfected with ehrlichiae when they begin to look for a bloodmeal. Ticks become infected with ehrlichiae while feeding on blood from the host in either the larval or nymphal stage. After the tick develops into the next stage, the ehrlichiae may be transmitted to the following host during the feeding process. Both male and female ticks may bite humans but it is the females that are responsible for most transmission. In the United States, it appears that both the nymphal and adult stages are responsible for transmission of ehrlichiae, but one stage may be more important for each Ehrlichia species.

Ehrlichia chaffeensis is principally transmitted by the lone star tick (Amblyomma americanum). White-tailed deer are a major host of lone star ticks and appear to represent one natural reservoir for E. chaffeensis. Antibody to E. chaffeensis has been found throughout deer populations in the southeastern and midwestern United States, and the organism has been cultured from deer blood.

The HGE agent has been associated with the blacklegged tick (Ixodes scapularis) in the northeastern and upper midwestern United States. The western blacklegged tick (Ixodes pacificus) is a vector in northern California. Ixodes ricinus has been shown to be a vector of E. phagocytophila in Europe. Deer, elk, and wild rodents are likely reservoirs.

The natural history of Ehrlichia ewingii is not completely known. However, dogs may be a reservoir host and the lone star tick (Amblyomma americanum) may be an important vector. Canine granulocytic ehrlichiosis caused by E. ewingii has been described in south central and southeastern states, including Arkansas, Georgia, Mississippi, Missouri, North Carolina, Oklahoma, Tennessee, and Virginia. To date, human cases have been observed in Missouri, Oklahoma, and Tennessee. It has been demonstrated experimentally that the lone star tick is able to transmit the disease among dogs. Other potential reservoirs and vectors remain to be identified.

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