There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
During most of the 20th century, the epidemiology of tick-borne rickettsioses could be summarized as the occurrence of a single pathogenic rickettsia on each continent. An element of this paradigm suggested that the many other characterized and noncharacterized rickettsiae isolated from ticks were not pathogenic to humans. In this context, it was considered that relatively few tick-borne rickettsiae caused human disease. This concept was modified extensively from 1984 through 2005 by the identification of at least 11 additional rickettsial species or subspecies that cause tick-borne rickettsioses around the world. Of these agents, seven were initially isolated from ticks, often years or decades before a definitive association with human disease was established. We present here the tick-borne rickettsioses described through 2005 and focus on the epidemiological circumstances that have played a role in the emergence of the newly recognized diseases.
Background Rickettsia are intracellular symbionts of eukaryotes that are best known for infecting and causing serious diseases in humans and other mammals. All known vertebrate-associated Rickettsia are vectored by arthropods as part of their life-cycle, and many other Rickettsia are found exclusively in arthropods with no known secondary host. However, little is known about the biology of these latter strains. Here, we have identified 20 new strains of Rickettsia from arthropods, and constructed a multi-gene phylogeny of the entire genus which includes these new strains. Results We show that Rickettsia are primarily arthropod-associated bacteria, and identify several novel groups within the genus. Rickettsia do not co-speciate with their hosts but host shifts most often occur between related arthropods. Rickettsia have evolved adaptations including transmission through vertebrates and killing males in some arthropod hosts. We uncovered one case of horizontal gene transfer among Rickettsia, where a strain is a chimera from two distantly related groups, but multi-gene analysis indicates that different parts of the genome tend to share the same phylogeny. Conclusion Approximately 150 million years ago, Rickettsia split into two main clades, one of which primarily infects arthropods, and the other infects a diverse range of protists, other eukaryotes and arthropods. There was then a rapid radiation about 50 million years ago, which coincided with the evolution of life history adaptations in a few branches of the phylogeny. Even though Rickettsia are thought to be primarily transmitted vertically, host associations are short lived with frequent switching to new host lineages. Recombination throughout the genus is generally uncommon, although there is evidence of horizontal gene transfer. A better understanding of the evolution of Rickettsia will help in the future to elucidate the mechanisms of pathogenicity, transmission and virulence.
In serum samples obtained from all the healthy humans, horses, dogs, and donkeys present on three farms in the Pedreira Municipality, an endemic area for Brazilian spotted fever, an indirect immunofluorescence assay (IFA) detected antibodies against Rickettsia rickettsii in 17 (77.3%) horses, 5 (31.3%) dogs (titers ranging from 64 to 4,048), and none of 4 donkeys or 50 humans. Five canine and eight equine sera with high antibody titers to R. rickettsii were also tested by IFA against R. bellii, R. akari, and R. africae antigens. Sera from two horses and two dogs that showed similar high antibody titers against two rickettsial antigens were evaluated after cross-absorption. Sera from seven horses and two dogs contained antibodies specific for R. rickettsii, and one dog serum had antibodies against a Rickettsia species very closely related to R. africae. The latter may have been caused by infection with the recently identified COOPERI strain.