Development of improved vaccines for heartwater

Abstract

Heartwater is a tick-borne disease of ruminants which causes major economic losses for domestic livestock owners throughout sub-Saharan Africa and the Caribbean. It is caused by the intracellular rickettsia Ehrlichia (formerly Cowdria) ruminantium and the only commercially available vaccination procedure is over 50 years old. It involves infecting animals with cryopreserved sheep blood containing virulent E. ruminantium organisms, followed by antibiotic treatment when fever develops. Experimental attenuated, inactivated, and nucleic acid vaccine procedures have been investigated over the last half-century, but none of them has yet been particularly successful. We have developed two new experimental vaccines, a live attenuated vaccine and a nucleic acid vaccine. The attenuated vaccine was developed by continuous passage of E. ruminantium organisms of the virulent Welgevonden isolate in a continuous canine macrophage-monocyte cell line. After more than 50 passages the cultures produced no disease when inoculated into mice or sheep, and the inoculated animals were 100% immune to a subsequent lethal homologous needle challenge. The nucleic acid vaccine is based on four E. ruminantium genes from a genetic locus involved in nutrient transport. A cocktail of all four genes, cloned in a DNA vaccine vector and used to immunize sheep, engendered 100% protection against a subsequent lethal needle challenge with the homologous isolate and with each of five different virulent heterologous isolates. Sheep immunized with this cocktail were also exposed to a field challenge in a heartwater-endemic area and few animals survived. This suggests that the local E. ruminantium genotypes were different from any which were administered by needle challenge, or that needle challenge is not a good model for tick challenge in the field.