Heartwater, a tick-borne disease of domestic and wild ruminants, is caused by the intracellular rickettsia Ehrlichia ruminantium (previously known as Cowdria ruminantium). It is a major constraint to livestock production throughout subSaharan Africa, and it threatens to invade the Americas, yet there is no immediate prospect of an effective vaccine. A shotgun genome sequencing project was undertaken in the expectation that access to the complete protein coding repertoire of the organism will facilitate the search for vaccine candidate genes. We report here the complete 1,516,355-bp sequence of the type strain, the stock derived from the South African Welgevonden isolate. Only 62% of the genome is predicted to be coding sequence, encoding 888 proteins and 41 stable RNA species. The most striking feature is the large number of tandemly repeated and duplicated sequences, some of continuously variable copy number, which contributes to the low proportion of coding sequence. These repeats have mediated numerous translocation and inversion events that have resulted in the duplication and truncation of some genes and have also given rise to new genes. There are 32 predicted pseudogenes, most of which are truncated fragments of genes associated with repeats. Rather then being the result of the reductive evolution seen in other intracellular bacteria, these pseudogenes appear to be the product of ongoing sequence duplication events.gene duplication ͉ bacterial genome ͉ molecular sequence data ͉ intracellular adaptation E hrlichia ruminantium (previously known as Cowdria ruminantium) is an obligate intracellular bacterium in the order Rickettsiales. Species in this order cause serious diseases in man and domestic animals throughout the world. E. ruminantium is transmitted by ticks of the genus Amblyomma and causes heartwater, a fatal and economically important disease of wild and domestic ruminants. The disease occurs throughout subSaharan Africa and on several Caribbean islands, from which it threatens to invade the Americas (1), but the existing immunization procedures are rudimentary and relatively ineffective (2). E. ruminantium is a fragile bacterium with exacting culture requirements in eukaryotic cell lines; genetic manipulation has not been attempted, and little is known about its mechanisms of virulence or pathogenesis. Heartwater affects all domestic ruminants, and 80-95% of naïve animals die within 3 weeks, but those that recover have a T cell-mediated immunity to subsequent homologous challenge (3). In the absence of any directed strategy to identify T cell-stimulatory proteins we sequenced the E. ruminantium genome in the expectation that access to the complete protein-coding repertoire of the organism would facilitate the search for vaccine candidate genes.

No abstract

Ehrlichia ruminantium is an obligate intracellular bacterial pathogen which causes heartwater, a serious tick-borne disease of ruminants throughout sub-Saharan Africa. The development of promising recombinant vaccines has been reported previously, but none has been as effective as immunisation with live organisms. In this study we have used reverse vaccinology to identify proteins that elicit an in vitro cellular immune response similar to that induced by intact E.

In 1994 a batch of apparently healthy goats was selected for intended export to the USA from a heartwater-free and vector tick-free region of South Africa. The animals were tested serologically for heartwater, using either or both an IFA and an ELISA test, and 52% were found to be serologically positive. A PCR assay based on Ehrlichia ruminantium 16S gene sequences gave positive results for 54% of the animals, suggesting that apparently non-pathogenic E. ruminantium variants existed in this heartwater-free area. To identify and characterise the agents responsible for the positive serological and PCR results, ticks and animal blood samples were collected from two of the three farms involved in the original survey during two successive seasons of expected peak tick activity. Ticks were kept alive for a minimum of 3 weeks to allow digestion of any blood meal before being processed.Over the two seasons, 28% of the livestock and 15% of the ticks sampled were found to be carrying E. ruminantium. E. ruminantium 16S and pCS20 sequences were detected in all of the four tick species collected from the livestock (Rhipicephalus evertsi evertsi, Rhipicephalus evertsi mimeticus, Hyalomma truncatum, Hyalomma marginatum rufipes),suggesting that some of the species may act as vectors. Animals generally carried multiple E. ruminantium 16S genotypes, whereas ticks rarely carried more than one.Infection levels in both animals and ticks were too low to generate a marked response openUP when a blood stabilate was sub-passaged in a clean sheep, preventing the subsequent establishment of any of the organisms in culture.

No abstract

No abstract

No abstract

No abstract