Rickettsioses

Rickettsioses are arthropod-borne zoonoses caused by obligate intracellular bacteria from Rickettsia or Orientia genera.

Rickettsioses are acute febrile, zoonotic diseases caused by rickettsiae, which are obligate intracellular Gram-negative bacteria that invade endothelial cells and induce the formation of vasculitis.

Members of the genus Rickettsia are obligately intracellular Gram-negative organisms that are transmitted to humans via hemophagous arthropod vectors. The rickettsiae have traditionally been classified as belonging to either the spotted fever or typhus group. Rickettsia typhi and Rickettsia felis are flea-borne pathogens, which both cause acute undifferentiated febrile illness throughout the world.

Various arthropods, including lice, fleas, ticks, and mites, can act as vectors, whereas mammals and sometimes the arthropods themselves constitute the principal reservoirs.

Rickettsioses are difficult to distinguish from other conditions causing acute fever in endemic areas, especially during the early phase. Common presentations include fever, abdominal discomfort, headache, myalgia, and rashes. Lung, liver, and kidney involvement may complicate the disease.
Rickettsioses

Rickettsial infections

Rickettsial infections are caused by bacterial organisms (Rickettsiae), which are found throughout the world. The genus Rickettsia is included in the bacterial tribe Rickettsiae, family Rickettsiaceae, and order Rickettsiales.

Family Rickettsiaceae comprise a group of microorganisms that phylogenetically occupy a position between bacteria and viruses. Rickettsiae are small, non-flagellate, gram negative pleomorphic cocco-bacilli adapted to obligate intracellular parasitism and transmitted by arthropod vectors.

The diseases caused by Rickettsia species are often collectively referred to as rickettsioses. Rickettsiae live in small insects such as ticks, fleas, mites, and lice and can infect mammals (including humans), but cannot survive for any significant period outside their host.

Among returned travelers, rickettsial diseases have been estimated to be the fourth most common cause of fever, with symptoms such as rash, abdominal pain, and a dry, black/dark scab at the site of the infecting bite.

The incubation period ranges from 2-14 days. The disease begins with fever, myalgia and headache. Headache is usually quite severe, young children may not complain of pain at all. Onset of nausea, vomiting, pain abdomen, diarrhea and abdominal tenderness occur in substantial number of patients suggesting gastroenteritis or an acute surgical abdomen.

Tick-borne rickettsioses are often acquired in rural settings in risk areas; however, urban transmission also occurs. Most rickettsial infections are transmitted to humans by the bite or feces of fleas, ticks, mites, or lice. Even the smallest forms of ticks (such as larvae) can transmit the infection to humans, meaning that many infected individuals do not recall a tick bite before symptoms appear.

Rickettsiae microorganisms appear to exert their pathologic effects by adhering to and then invading the endothelial lining of the vasculature within the various organs affected.

Infected ticks and mites transmit rickettsiae primarily through their infected salivary glands directly into the bite. In lice and fleas, rickettsiae do not infect the salivary glands but multiply in the cells of the gut wall and are excreted in the faeces in large amounts, contaminating the skin area around the bite.

The most important pathophysiologic effect is increased vascular permeability with consequent edema, loss of blood volume, hypoalbuminemia, decreased osmotic pressure, and hypotension.
Rickettsial infections

Evolutionary Intricacies Of Rickettsia Pathogens Revealed By Scientists

Scientists from the Virginia Bioinformatics Institute (VBI) at Virginia Tech and the University of Maryland School of Medicine have unveiled some of the evolutionary intricacies of rickettsial pathogens by analyzing over a decade's worth of genomic data.

Some species of Rickettsia are known to cause harmful diseases in humans, such as epidemic typhus (R. prowazekii) and Rocky Mountain spotted fever (R. rickettsii), while others have been identified as emerging pathogens and organisms that might possibly be used for the development of biological weapons. The new data, which are publicly available via the PATRIC project web site (patric.vbi.vt.edu), open up exciting new possibilities for future research.

Dr. Joseph Gillespie, a bioinformatician at the Virginia Bioinformatics Institute and leader of the study, remarked: "Over the past ten years, an average of one genome per year has been sequenced for the Rickettsia, which represents a considerable genomic treasure trove for evolutionary studies. We have systematically probed the genomic data available for Rickettsia to reveal how rickettsial genomes have given rise to the great diversity of organisms that we know today. This approach sheds light on the evolutionary intricacies of Rickettsia and suggests how some members of the group have developed into potent pathogens responsible for significant diseases in humans."

In the study, the researchers defined a core Rickettsia genome by looking at a large number of genes that could potentially encode for proteins in the ten genomes under investigation. This information was used to generate over 700 groups of orthologous proteins that theoretically could have originated from a common ancestor. A similar exercise yielded over 1,300 orthologous groups of proteins that define the accessory genome. Digging further into the accessory genome yielded signature proteins that define the four major rickettsial groups, as well as species infecting common arthropod hosts and species harboring plasmids. Surprisingly, and contrary to previous dogma regarding rickettsial genome evolution, the accessory genome contained many likely elements of the bacterial mobile gene pool.

VBI Director and PATRIC Principal Investigator Bruno Sobral remarked: "Virulent species of Rickettsia are of great interest both as emerging agents of infectious disease and potential bioterror agents. We believe the current work provides a robust evolutionary framework that allows for the interpretation of the genomic characteristics of the four main lineages of Rickettsia. As such it provides an ideal resource for research directed at developing vaccines, diagnostics and therapeutics for the diverse group of pathogens that constitute the Rickettsia."

Dr. Gillespie concluded: "The results obtained in this study are consistent with the recent explosion in the number of identified plasmids in Rickettsia. By making these data available we hope to enable future research into these intriguing organisms."
Evolutionary Intricacies Of Rickettsia Pathogens Revealed By Scientists
Article by: Medical News Today