Image Courtesy of Jere McBride.

Ehrlichia chaffeensis, a member of the order Rickettsiales, is a tick-borne small obligate intracellular gram-negative, dimorphic bacterium that resides as a microcolony in a membrane-lined intracellular vacuole (morula), primarily in monocytes and macrophages of mammalian hosts. Each morula may contain 100 or more ehrlichiae. Ultrastructurally, E. chaffeensis has two forms, termed reticulate and dense-cored cells, resembling morphologically similar reticulate and elementary body forms of chlamydiae. Ehrlichiae have relatively small genomes (~1.2 Mbp) and do not possess extrachromosomal plasmids. Members of the genus Ehrlichia include E. chaffeensis, E. canis, E ewingii, E. ruminantium, and E. muris, and phylogenetic analysis of 16s rDNA and groEL genes support the close genetic relatedness of E. chaffeensis and E. canis. The genus Ehrlichia is closely related to the genera of Anaplasma and Neorickettsia, whose members have similar intracellular existence.

Historically, ehrlichioses have been known as tick-transmitted diseases of veterinary importance, until 1987 when the first case of human ehrlichiosis was described, followed by cultivation and identification of the etiologic agent, E. chaffeensis, in 1991. From 1986 through 1997, human ehrlichiosis cases (1,223) were reported in 30 states, with the highest incidence in Missouri and Arkansas. However, ehrlichioses are globally distributed and have been reported in Europe, Asia, Africa, and South America, although case reports have originated predominantly from the United States. Two forms of human ehrlichiosis are recognized; human monocytotropic ehrlichiosis (HME), caused by E. chaffeensis, and human granuocytotropic ehrlichiosis (HGE), caused by two distinct organisms, Anaplasma phagocytophilia and E. ewingii. The currently recognized distribution of ehrlichial diseases is more a reflection of the organized effort to define the disease epidemiology, particularly in the United States. The number of cases reported, thus far, is likely to significantly underestimate the true incidence since many cases are not reported, or are not diagnosed by physicians. In the United States, the majority of the tabulated cases of human ehrlichiosis are HME (60.7%), the remaining cases (36.7%) were diagnosed as HGE, while 2.6% cases were not ascribed to a specific ehrlichial agent. HME is now recognized as a serious, sometimes fatal, emerging infectious disease, with a case-fatality of 2.7%. The pathogenesis of disease in humans is poorly understood, but fatal cases of HME involve multisystem failure probably mediated in part by cytokines induced during the infection.

Ehrlichiae have a complex life cycle involving an arthropod and a mammalian host, including strategies to establish persistent infection in their natural hosts to insure its survival and transmission in nature. E. chaffeensis is transmitted primarily by Amblyomma americanum (lone star) ticks and is maintained in nature by persistent infection of the white-tailed deer. Ticks acquire E. chaffeensis infection by feeding on infected deer, maintain the infection transtadially as the tick molts from stage to stage, before infecting other susceptible mammalian hosts. The mechanism of persistent ehrlichial infection has not yet been determined, but recent studies have resulted in the identification of some important immunoreactive proteins and potential pathogenic mechanisms of ehrlichiae. Members of the genus Ehrlichia have a multigene family encoding homologous but distinct 28-kDa surface proteins. A recently characterized 24 kb DNA locus of E. chaffeensis has been identified that contains 22 paralogs encoding homologous, but nonidentical 28-kDa proteins. There are four hypervariable regions found in each p28 amino acid sequence that may be involved in antigenic variation. In addition, differential gene expression and/or gene conversion of this gene family may also play a role in persistence of infection by altering the expression of important surface antigens. Two large molecular weight glycoproteins have been identified in E. chaffeensis (gp120 and gp200) along with the corresponding orthologs in E. canis. These glycoproteins are among the first such proteins described in pathogenic bacteria, and appear to be important targets of the host immune response and elicit a strong antibody response. Although some important genes, including those encoding immunoreactive proteins (p28, p120, gp200 and dsb) and the 16S rRNA, nadA, and groE, have been identified and sequenced, there is a very limited knowledge of the genes involved in pathogenesis. Considering the inherent difficulties related to in vitro cultivation and genetic manipulation of ehrlichiae, the complete genome sequence will dramatically advance research towards understanding pathogenesis and immunity, and promote vaccine development.

The significance of the genome sequences of E. chaffeensis

1. Pathogenesis. Ehrlichiae attach to a host cell receptor through an adhesin, enter the host cell through phagocytosis and prevent phagosomal fusion with the lysosome, multiply in the vacuole, and finally are released from the host cell to begin a new life cycle. None of virulence factors involved in these processes has been identified other than evidence that gp120 is as adhesin.
2. Vaccine development and immunity. Defining the virulence factors and immunoprotective antigens is necessary for vaccine development.
3. Prophylaxis. Design of improved therapeutic and preventive pharmaceuticals would be desirable.
4. Genetic manipulation. Genetic manipulation of the intracellular pathogens is not currently possible. Sequence data will aid in the identification of bacteriophages and transposons that may be used for genetic manipulation of E. chaffeensis.