Ehrlichia ruminantium

Other Scientific Names

• Cowdria ruminantium (Cowdry, 1925) Moshkovski, 1947
• Rickettsia ruminantium

• Domain: Bacteria
•     Phylum: Proteobacteria
•         Class: Alphaproteobacteria
•             Order: Rickettsiales
•                 Family: Ehrlichiaceae
•                     Genus: Ehrlichia
•                         Species: Ehrlichia ruminantium

E. ruminantium is best demonstrated in fixed sections stained with Giemsa. It resembles a Gram-negative coccus, the colour varying from lilac purple to blue, varying between 0.2 and 2.5 µm in diameter. They most often occur in clumps (from 1 or 2 to several thousand) in a vacuole in the cytoplasm of endothelial cells. Generally the small and medium-sized organisms occur in large colonies, while the larger ones occur in small colonies. Pleomorphic forms; horseshoe, ring and bacillary shaped, are frequently seen in those colonies that contain very large organisms (Pienaar, 1970).

The ultrastructural morphology of Ehrlichia reveals that they occur within a bilaminar membrane-bound cytoplasmic vacuole, each organism consisting of a wall, made up of 2 membranes, and an internal structure consisting of electron dense and electron-pale areas. Some authors differentiate elementary, intermediate and reticulated bodies (Prozesky et al., 1986; Jongejan et al., 1991).

Taxonomy

Traditional rickettsial taxonomy assigned Cowdria ruminantium as the sole member of the genus Cowdria in the tribe Ehrlichieae. This was one of three tribes within the family Rickettsiaceae in the order Rickettsiales which initially encompassed all intracellular bacteria but from which the Chlamydiae were later removed (Moulders, 1984). The obligate intracellular nature of E. ruminantium, coupled with morphological features suggestive of a Chlamydia-like life cycle led to confusion as to its position in the ehrlichial hierarchy (Uilenberg, 1983). In 2001, Dumler et al. defined after 16S ribosomal DNA and groESL gene comparisons that all members of the tribes Ehrlichieae and Wolbachieae had to be transferred to the family Anaplasmataceae and that the family Rickettsiaceae had to be eliminated. In Anaplasmataceae, four genera are present Anaplasma, Ehrlichia, Wolbachia and Neorickettsia. The genus Ehrlichia now includes E. ruminantium (formerly Cowdria ruminantium) but no longer Anaplasma bovis, phagocytophila, and HGE (belonging now to genus Anaplasma) and the genus neorickettsia (including Ehrlichia sennetsu and Ehrlichia risticii). In this reorganization, E. ruminantium is closer to E. chaffeensis, ewingii, muris, ovis and canis.  

Disease(s) associated with this pathogen is/are on the list of diseases notifiable to the World Organisation for Animal Health (OIE). The distribution section contains data from OIE's WAHID database on disease occurrence. Please see the AHPC library for further information from OIE. Also see the website: www.oie.int.