Abstract:
Ehrlichia ruminantium is an obligately intra-cellular "-proteobacterium which causes a
disease known as heartwater or cowdriosis in some wild, and all domestic, ruminants.
The organism is transmitted by ticks of the genus Amblyomma, and it is of serious
economic importance wherever the natural vectors occur, an area which includes all of
sub-Saharan Africa, and several islands in the Caribbean. The disease was first
recognized in South Africa in the 19th century, where its tick borne nature was
determined in 1900, but the organism itself was not demonstrated until 1925, when it
was recognized to be a rickettsia, initially named Rickettsia ruminantium. It was thus the
first species of what are now known as Ehrlichia to be discovered, and most of the early
work to elucidate the nature of the organisms, and its reservoirs and vectors, was
performed in South Africa. The next milestone was the development, in 1945, of an
infection and treatment regimen to immunize livestock, and this is still the only
commercially available “vaccine” against the disease. Then in 1985, after fruitless
attempts over many years, the organism was propagated reliably in tissue culture,
opening the way for the first application of the newly developed techniques of molecular
genetics. From 1990 onwards the pace of heartwater research accelerated rapidly, with
notable advances in phylogeny, diagnosis, epidemiology, immunology, and vaccine
development. The complete genome sequence was published in 2005, and during the
last two years a new understanding has arisen of the remarkable genetic variability of
the organism and new experimental vaccines have been developed. Despite all this the
goal of producing an effective vaccine against the disease in the field still remains
frustratingly just beyond reach. This article summarises our current understanding of
the nature of E. ruminantium, at a time when the prospects for the development of an
effective vaccine against the organism seem better than at any time since its discovery
83 years ago.
