Previously known as Cowdria ruminantium, this obligate intracellular bacterium has been reclassified along with several other organisms in the order Rickettsiales and it is now classified as Ehrlichia ruminantium.
Under natural conditions, E. ruminantium is transmitted by Amblyomma ticks. These 3-host ticks become infected during either the larval or nymphal stages and transmit the infection during one of the subsequent stages (transstadial transmission). The progeny of an infected female tick are most probably not infective (ie, there is no epidemiologically significant transovarial transmission). Therefore, the infection rate in tick populations tends to be low. Intrastadial transmission by male ticks may also occur, as well as some degree of vertical transmission from cow to calf (eg, via colostrum), in areas where the disease is endemic.
E ruminantium can be propagated experimentally by serial passage, either by inoculating infective blood into, or by feeding infected nymphal or adult stages of a vector tick on susceptible animals. The organism can also be propagated in tissue culture, most reliably in endothelial cells, but also in primary neutrophil cultures and macrophage cell lines. At room temperature, infective material loses its infectivity within a few hours, but the organism, together with suitable cryoprotectants, may be viably preserved in liquid nitrogen for years.
Immunity to heartwater appears to be chiefly, if not exclusively, cell mediated. There is no, or only partial, cross-protection between different strains (stocks) of E ruminantium. Most of these stocks are infective for, but cannot be serially passaged in, mice; however, a few are pathogenic to mice infected by the IV route. One of these, the Kümm stock, can even be passaged by the intraperitoneal route. Molecular analysis has established that the traditional Kümm stock was made up of organisms of 2 distinct genotypes.
E ruminantium seems to initially reproduce in macrophages; it then invades and multiplies in the vascular endothelium. During the febrile stage, and for a short while thereafter, the blood of infected animals is infective to susceptible animals if subinoculated.
- Pruneau L et al (2012) Global gene expression profiling of Ehrlichia ruminantium at different stages of development. FEMS Immunol Med Microbiol 64(1):66-73