An attenuated Ehrlichia ruminantium (Welgevonden stock) vaccine protects small ruminants against virulent heartwater challenge.

Heartwater is a tick-borne disease of ruminants caused by the intracellular rickettsia Ehrlichia ruminantium. The only commercially available immunization procedure involves infecting animals with cryopreserved sheep blood containing virulent E. ruminantium organisms, followed by treatment with tetracyclines when fever develops. The virulent Welgevonden stock of E. ruminantium was attenuated by continuous propagation of the organisms in a canine macrophage-monocyte cell line (DH82), followed by re-adaptation to grow in a bovine endothelial cell line (BA 886). The material used for the present experiments consisted of the attenuated stock between passages 43 and 64 after re-adaptation. When inoculated into sheep or goats the attenuated organisms did not produce disease, and the only symptom observed was a rise in body temperature in most, but not all, animals. All sheep injected with 2 ml of culture suspension were subsequently found to be fully protected against a lethal needle challenge with the virulent homologous stock or with one of four different heterologous stocks (Ball 3, Gardel, Mara 87/7, Blaauwkrans). Titrations of elementary body suspensions showed that 2ml of a 1:10,000 dilution of culture suspension injected into sheep or goats was still sufficient to trigger an immune response which resisted a lethal needle challenge with the virulent Welgevonden stock. Adult Amblyomma hebraeum ticks, fed as nymphs on sheep immunized with DH82-derived organisms of passage 111, were able to transmit the attenuated stock to a naive sheep, which was found to be protected against a subsequent lethal homologous needle challenge.

The genome of the heartwater agent Ehrlichia ruminantium contains multiple tandem repeats of actively variable copy number.

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.