Pathogen Characteristics
Top of page
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.
References
Top of pageAdakal H, Gavotte L, Stachurski F, Konkobo M, Henri H, Zoungrana S, Huber K, Vachiery N, Martinez D, Morand S, Frutos R, 2010. Clonal origin of emerging populations of Ehrlichia ruminantium in Burkina Faso. Infection, Genetics and Evolution, 10(7):903-912. http://www.sciencedirect.com/science/journal/15671348
Allsopp MTEP, Hattingh CM, Vogel SW, Allsopp BA, 1999. Evaluation of 16S, map1 and pCS20 probes for detection of Cowdria and Ehrlichia species. Epidemiology and Infection, 122(2):323-328; 23 ref.
Allsopp MTEP, Visser ES, Plessis JLdu, Vogel SW, Allsopp BA, 1997. Different organisms associated with heartwater as shown by analysis of 16S ribosomal RNA gene sequences. Veterinary Parasitology, 71(4):283-300; 50 ref.
Cowdry EV, 1925a. Studies on the etiology of heartwater I. Observation of a rickettsia, Rickettsia ruminantium (n. sp.), in the tissues of infected animals. Journal of Experimental Medicine, 42(2):231-252.
Dumler JS, Barbet AF, Bekker CP, Dasch GA, Palmer GH, Ray SC, Rikihisa Y, Rurangirwa FR, 2001. Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and 'HGE agent' as subjective synonyms of Ehrlichia phagocytophila. International Journal of Systematic and Evolutionary Microbiology, 51:2145-2165.
Faburay B, Geysen D, Munstermann S, Taoufik A, Postigo M, Jongejan F, 2007. Molecular detection of Ehrlichia ruminantium infection in Amblyomma variegatum ticks in The Gambia. Experimental and Applied Acarology, 42(1):61-74. http://springerlink.metapress.com/content/k0053661734l2107/?p=68fefacc431a47b7aabc4be0118f04e5&pi=6
Faburay B, Munstermann S, Geysen D, Bell-Sakyi L, Ceesay A, Bodaan C, Jongejan F, 2005. Point seroprevalence survey of Ehrlichia ruminantium infection in small ruminants in the Gambia. Clinical and Diagnostic Laboratory Immunology, 12(4):508-512. http://cdli.asm.org/cgi/content/abstract/12/4/508
Hart A, Prozesky L, Jacobs PD, Brett S, 1992. A transmission electron microscopical study of the life cycle of Cowdria ruminantium in vitro. Journal of the South African Veterinary Association, 63(2):90.
Jongejan F, Zandbergen TA, Wiel PAvan de, Groot Mde, Uilenberg G, 1991. The tick-borne rickettsia Cowdria ruminantium has a Chlamydia-like developmental cycle. Onderstepoort Journal of Veterinary Research, 58(4):227-237; 46 ref.
Kocan KM, Bezuidenhout JD, 1987. Morphology and development of Cowdria ruminantium in Amblyomma ticks. Onderstepoort Journal of Veterinary Research, 54(3):177-182; 18 ref.
Kocan KM, Bezuidenhout JD, Hart A, 1987. Ultrastructural features of Cowdria ruminantium in midgut epithelial cells and salivary glands of nymphal Amblyomma hebraeum. Onderstepoort Journal of Veterinary Research, 54(1):87-92; 17 ref.
Molia S, Frebling M, Vachiéry N, Pinarello V, Petitclerc M, Rousteau A, Martinez D, Lefrançois T, 2008. Amblyomma variegatum in cattle in Marie Galante, French Antilles: prevalence, control measures, and infection by Ehrlichia ruminantium. Veterinary Parasitology, 153(3/4):338-346. http://www.sciencedirect.com/science/journal/03044017
OIE Handistatus, 2002. World Animal Health Publication and Handistatus II (dataset for 2001). Paris, France: Office International des Epizooties.
OIE Handistatus, 2003. World Animal Health Publication and Handistatus II (dataset for 2002). Paris, France: Office International des Epizooties.
OIE Handistatus, 2004. World Animal Health Publication and Handistatus II (data set for 2003). Paris, France: Office International des Epizooties.
OIE Handistatus, 2005. World Animal Health Publication and Handistatus II (data set for 2004). Paris, France: Office International des Epizooties.
Okoh AEJ, Oyetunde IL, Ibu JO, 1987. Heartwater infection (cowdriosis) in a Sitatunga (Tragelaphus spekei) in Nigeria. Journal of Wildlife Diseases, 23(2):211-214.
Pienaar JG, 1970. Electron microscopy of Cowdria (Rickettsia) ruminantium (Cowdry, 1926) in the endothelial cells of the vertebrate host. Onderstepoort Journal of Veterinary Research, 37(1):67-78.
Plessis JLdu, Camus E, Oberem PT, Malan L, 1987. Heartwater serology: some problems with the interpretation of results. Onderstepoort Journal of Veterinary Research, 54(3):327-329; 19 ref.
Plessis JLdu, Gas Lvan, Olivier JA, Bezuidenhout JD, 1989. The heterogenicity of Cowdria ruminantium stocks: cross-immunity and serology in sheep and pathogenicity to mice. Onderstepoort Journal of Veterinary Research, 56(3):195-201; 23 ref.
Prozesky L, Bezuidenhout JD, Paterson CL, 1986. Heartwater: an in vitro study of the ultrastructure of Cowdria ruminatium. Onderstepoort Journal of Veterinary Research, 53(3):153-159.
Sulsona CR, Mahan SM, Barbet AF, 1999. The map1 gene of Cowdria ruminantium is a member of a multigene family containing both conserved and variable genes. Biochemical and Biophysiological Research Communications, 257(2):300-305.
Villiers EP de, Brayton KA, Zweygarth E et al., 2000. Genome size and genetic map of Cowdria ruminantium. Microbiology, 146(10):2627-2634.
Waghela SD, Rurangirwa FR, Mahan SM, Yunker CE, Crawford TB, Barbet AF, Burridge MJ, 1991. A cloned DNA probe identifies Cowdria ruminantium in Amblyomma variegatum ticks. Journal of Clinical Microbiology, 29(11):2571-2577; [;Abstract number refers to VB volume 63 (1993)]; 43 ref.