| Rickettsia rickettsii | |
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| Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Prokaryote |
| Phylum: | Proteobacteria |
| Class: | Alphaproteobacteria |
| Order: | Rickettsiales |
| Family: | Rickettsiaceae |
| Genus: | Rickettsia |
| Species: | R. rickettsii |
| Binomial name | |
| Rickettsia rickettsii Brumpt, 1922 |
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Rickettsia rickettsii (abbreviated as R. rickettsii) is a unicellular, gram-negative coccobacillus (plural coccobacilli) that is native to the New World. It belongs to the spotted fever group (SFG) of Rickettsia and is most commonly known as the causative agent of Rocky Mountain spotted fever (RMSF). By nature, R. rickettsii is an obligate intracellular parasite that survive by an endosymbiotic relationship with other cells.
R. rickettsii is a non-motile, non-spore forming aerobic organism. Cells are typically 0.3-0.5 X 0.8-2.0 um in size. They lack a distinct nucleus and membrane bound organelles. Their outer membrane is composed mostly lipopolysaccharides.
RMSF is transmitted by the bite of an infected tick while feeding on warm-blooded animals, including humans. Humans are considered to be accidental hosts in the rickettsia–tick life cycle and are not required to maintain the rickettsiae in nature.
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Rocky Mountain spotted fever first emerged in the Idaho Valley in 1896. At that time, not much information was known about the disease; it was originally called Black Measles because patients had a characteristic spotted rash appearance throughout their body.
Howard Ricketts (1871-1910) was an American pathologist and infectious disease researcher who was the first to identify and study the organism that causes Rocky Mountain spotted fever. He received his undergraduate degree in zoology from the University of Nebraska and his medical degree from Northwestern University School of Medicine. Rickets completed his internship at Cook County Hospital in Chicago, IL, followed by a fellowship in pathology and cutaneous diseases at Rush Medical College.
In 1902, Ricketts became the associate professor of pathology at the University of Chicago. The trademark rash, which first appeared in the Idaho Valley, now began to slowly emerge in the Bitterroot Valley region, a highly influential area in western Montana and had an 80-90% mortality rate. During his tenure as associate professor, Ricketts was funded and recruited by the University of Chicago, the State of Montana, and the American Medical Association to conduct research on Rocky Mountain spotted fever.
His research entailed interviewing victims of the disease and collecting and studying infected animals. He was also known to inject himself with pathogens to measure its effects.
Ricketts was also known for his research on typhus, which was very similar to spotted fever. Ironically, days after isolating the organism that he believed to cause typhus, he died. It was speculated that his death was likely caused from an insect bite.
In 1922, French pathologist, Dr. Emile Brumpt, proposed the name Rickettsia rickettsia, in honor of Howard Ricketts.
S. Burt Wolbach is credited for the first detailed description of the etiologic agent in 1919. He clearly recognized it as an intracellular bacterium which was seen most frequently in endothelial cells. He was struck by the fact that in the tick, and also in mammalian cells, the microorganism was intranuclear. The nucleus was often completely filled with minute particles and often was distended. Although Wolbach recognized its similarity to the agent of typhus and tsutsugamushi fever (scrub typhus), he did not regard the designation 'rickettsia' as appropriate. He proposed the name Dermacentroxenus rickettsi. Emile Brumpt felt that the etiologic agent of RMSF, despite some uncertainty about its properties, belonged in the genus Rickettsia and in 1922 proposed the name Rickettsia rickettsii.
The life cycle of Rickettsia rickettsii is considered to be a complex one. Survival is dependent on both an invertebrate vector, (the hard tick- Family Ixodidae) and a vertebrate host (including mice, dogs, rabbits). Humans are considered to be accidental vectors and are not essential in the rickettsial cycle. In addition, a sequence of events occur between both hosts in the successful transmission of rickettsial disease. Rickettsia rickettsii mostly affects canines and humans.
Three arthropod vectors in the United States have been identified in transmitting R. rickettsii to humans, causing the potentially fatal disease Rocky Mountain Spotted Fever. The American dog tick (Dermacentor variabilis), the Rocky Mountain Wood Tick (Dermacentor andersoni), and the Brown dog tick (Rhipicephalus sanguineus) can acquire Rickettsia rickettsii in a number of different ways.
First, an uninfected tick can become infected when feeding on the blood of an infected mammalian host in the larval or nymph stages, a mode of transmission called "transstadial transmission". Once a tick becomes infected with this pathogen, they are infected for life. Both the American dog tick and the Rocky Mountain wood tick serve as long-term reservoirs for Rickettsia rickettsii, in which the organism resides in the tick posterior diverticulae of the midgut, the small intestine and the ovaries.
Due to its confinement in the midgut and small intestine, it is possible for mammals, including humans, to contract rickettsial disease from open skin/wound contact with the feces of the organism. In addition, an infected male tick can transmit the organism to an uninfected female during mating. Once infected, the female tick can transmit the infection to her offspring, in a process known as "transovarial transmission".
An uninfected mammal can become infected with Rickettsia rickettsii when eating food that contains the feces of the infected tick. They can also succumb to infection through the bite of an infected tick.
Humans acquire Rickettsia rickettsii infection from infected vectors. After getting bitten by an infected tick, rickettsiae are transmitted to the bloodstream by tick salivary secretions or, as mentioned previously, through contamination of broken skin by infected vector feces.
All these modes of transmission ensures the survival of Rickettsia in nature.
R. rickettsii have evolved a number of strategical mechanisms or virulence factors that allow them to evade the host immune system and successfully infect the host.
R. rickettsii invades the endothelial cells that line the blood vessels. Endothelial cells are not phagocytic in nature; however, after attachment to the host cell surface, the pathogen causes changes in the host cell cytoskeleton that induces phagocytosis. They are able to avoid lysosomal fusion and oxidative burst by escaping from the phagosome into the cytoplasm where they multiply and spread.
Over the years, different virulence factors have been identified in R. rickettsii.
OmpA (rOmp) and Omp B (rOmp) have been identified as rickettsial outer surface proteins and are implicated in adherence of the bacterium to the host cell. The genes that encode these two surface proteins are designated as ompA and ompB, respectively.
rOmp B is the predominant surface membrane protein in R. rickettsii; Policastro et al, identified the rOmpA to rOmpB ratio to be 1:9 (1994). While the surface proteins of the bacterium have been identified, the host cell protein receptor(s) have not.
Entry into the host cell is mediated by a Type 4 secretion system' (T4SS) which is found in all rickettsiae. The organization of the T4SS apparatus is a rather elaborate one; it is a tunnel-shaped structure that is embedded in the bacterial inner membrane and extends to the outer membrane. At least 12 or more proteins help form the tunnel-like apparatus. Once adherence to the host cell is established, the T4SS of rickettsiae recruits substrates to the bottom of the apparatus, activating the complex via an ATP-dependent process that results in the direct transfer of the bacterium's DNA and other proteins into the host cell.
Invasion of the host endothelial cell immediately triggers phagocytosis, where the rickettsiae escape from the phagosome and into the cytosol where replication takes place. Although the escape from the phagosome is not well understood, it is thought to be mediated by phospholipase A2 activity.
In the cytosol, another virulence factor, ActA, allows survival of the bacterium. ActA is a bacterial surface protein that is associated with actin nucleation and tail formation in R. rickettsii. After interacting with the host cytoskeletal proteins, the bacterium develops an actin tail. The actin-based motility of R. rickettsii allows swift, unidirectional movement across the cytoplasm into adjacent cells, promoting cell to cell spread.
Endothelial cell damage caused by R. rickettsii can lead to end organ failure, DIC, and even death.
In vivo studies reveal that Rickettsia rickettsii invade endothelial lining of small to medium vessels in the human host, causing vascular permeability. When tested in vitro, it is shown that the bacterium infects every kind of cell of the mammalian host.
As the name may imply, Rocky Mountain spotted fever is limited to regions of the United States in proximity to the Rocky Mountains (the mountainous regions encompassing the northern most part of British Columbia to southwestern United States); however, the name RMSF is somewhat of a misnomer. Cases of Rocky Mountain spotted fever have been reported in every continent except Antarctica, and in every state in the U.S. except for Maine, Alaska, and Hawaii.
Approximately 90% of all infections occur within the months of April to September, the time period in which adult and nymphal ticks are the highest. The areas of the U.S. with the greatest reported cases of RMSF are the mid to south Atlantic states, including DE, MD, DC, VA, WV, NC, SC.
It is estimated that approximately 1200 or more new cases of RMSF will present on a yearly basis.
The Centers for Disease Control and Prevention states that the diagnosis of RMSF must be made based on the clinical signs and symptoms of the patient and later confirmed using specialized laboratory tests. However, the diagnosis of RMSF is often missed due to its non-specific onset. The clinical signs and symptoms that a patient may experience could appear and may be misdiagnosed as other diseases even by the most experienced physician.
During the initial stages of the disease, the patient will experience fever, nausea, vomiting, and loss of appetite.
The classic RMSF rash occurs in about 90% of patients and develops 2 to 5 days after the onset of fever. The characteristic rash appear as small, flat pink macules that develop peripherally on the patient's body, such as the wrists, forearms, ankles, and feet. During the course of the disease, the rash will take on a more darkened red to purple spotted appearance and a more generalized distribution.
Diarrhea, abdominal and joint pain, and pinpoint reddish lesions (petechiae) are observed during the late stages of the disease.
Patients with severe infections may require hospitalization. They may become thrombocytopenic, hyponatremic, experience elevated liver enzymes, and other more pronounced symptoms. It is not uncommon for severe cases to involve the respiratory system, central nervous system, gastrointestinal system or the renal system. This disease is worst for elderly patients, males, African-Americans, alcoholics, and patients with G6PD deficiency.
A proper physician's diagnosis is crucial during the early stages of RMSF. However, due to the fact that the signs and symptoms are very non-specific at onset, RMSF can often be misdiagnosed. For this reason, it is vital for a physician to treat the patient based on suspicion alone.
Rocky Mountain Spotted Fever is often diagnosed using an indirect immunofluorescence assay (IFA), which is considered the reference standard by the Centers for Disease Control and Prevention (CDC). The IFA will detect an increase in IgG or IgM antibodies.
A more specific lab test used in diagnosing RMSF is polymerase chain reaction or PCR which can detect the presence of rickettiae DNA.
Immunohistochemical (IHC) staining is another diagnostic approach where a skin biopsy is taken of the spotted rash; however, sensitivity is only 70%.
Doxycycline and Chloramphenicol are the most common drugs of choice for reducing the symptoms associated with RMSF. When it is suspected that a patient may have RMSF, it is crucial that antibiotic therapy be administered promptly. Failure to receive antibiotic therapy, especially during the initial stages of the disease, may lead to end-organ failure (heart, kidney, lungs, meningitis, brain damage, shock, and even death.
Brenner, D., Krieg, N., Garrity, G., & Staley, J. (2005). Bergey’s Manual of Systematic Bacteriology, Volume Two: The Proteobacteria (Part C). USA: Williams & Wilkins.
Weiss K., The Role of Rickettsioses in History. 2 - 14. In: Walker, D.H., Biology of Rickettsial Diseases, Volume I., 1988. CRC Press Inc, Boca Raton, Florida.
Weiss, E., History of Rickettsiology. 15 - 32. In: Walker, D.H., Biology of Rickettsial Diseases, Volume I., 1988. CRC Press Inc, Boca Raton, Florida.
Wilson, B., Salyers, A., Whitt, D., & Winkler, M. (2011) Bacterial Pathogenesis: A Molecular Approach, Third Edition. USA: ASM Press.
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