Rickettsia
Rickettsia are obligate, intracellular, parasitic bacteria that are typically gram-negative and coccobacilli in shape. These microorganisms are primarily energy parasites, relying on their hosts for adenosine triphosphate (ATP) due to their incomplete genome, which lacks certain metabolic enzymes. While humans are often accidental hosts, various mammals and arthropods, such as ticks and fleas, serve as reservoirs for Rickettsia species. The two main groups of Rickettsia include the typhus group and the spotted fever group, each associated with distinct transmission routes and clinical symptoms.
Typhus group infections, like epidemic typhus caused by R. prowazekii, are typically transmitted through body lice, while spotted fever group infections, including Rocky Mountain spotted fever from R. rickettsii, are spread via tick bites. Symptoms can range from fever and headache to severe complications in untreated cases, with mortality rates varying based on the specific infection. Treatment generally involves antibiotics such as doxycycline, which are effective if administered promptly. In recent years, the emergence of new rickettsial pathogens has raised concerns about the increasing prevalence of these infections, highlighting the need for continued research and public health vigilance.
Rickettsia
- TRANSMISSION ROUTE: Blood
Definition
Rickettsia are obligate, intracellular, parasitic, gram-negative coccobacilli. Their ATP transport system allows them to be energy parasites. Humans are usually accidental hosts, while other mammals and arthropods serve as reservoirs. Rickettsial-type organisms have also been linked to plant diseases.
Natural Habitat and Features
Because they are small, obligate, intracellular parasites, Rickettsia spp. were originally thought to be viruses. Further studies showed them to be true bacteria. All have a gram-negative-type cell wall and are normally visualized by Giemsa staining. Their genomes are made of deoxyribonucleic acid (DNA) and are incomplete, lacking genes for enzymes of anaerobic metabolism and for the production of most amino acids and nucleotides. They do possess the enzymes for aerobic metabolism, but they normally use a unique ATP transport system to absorb ATP from their hosts instead of making it themselves. This allows them to be energy parasites.
The genome of one of these bacteria, prowazekii, is the most closely related bacterial genome to the genome of mitochondria. No Rickettsia spp. can be grown on artificial media; instead, they must be cultured in living tissue, usually a chick embryo. In infected humans, Rickettsia spp. usually induce their own phagocytosis by the endothelial cells lining blood vessels. Inside the cells, they escape from the phagosome into the cytoplasm, where they replicate. Many species escape the cell by causing lysis, which destroys the host cell. Other species exit by extrusion through filipodia, finger-like projections on the cell surface.
Serology and DNA studies have separated these bacteria into two main groups: the typhus group (prowazekii and typhus) and the spotted fever group (all others). Another group, formerly called the Rickettsia scrub typhus group, has been separated into the related genus Orientia. The genera Ehrlichia, Anaplasma, and Coxiella are similar but only distantly related small intracellular parasites.
The most common reservoirs for Rickettsia are ticks, fleas, and mites. Rodents and other mammals also serve as reservoirs. Prowazekii, the causative agent of epidemic typhus, has a human reservoir and is transmitted from human to human through body lice.
Pathogenicity and Clinical Significance
Transmission and the course of the disease are slightly different between the typhus and spotted fever groups. In the typhus group, prowazekii, the causative agent of epidemic typhus, are deposited on the host’s skin in the feces of human body lice. Irritation caused by the louse’s saliva causes humans to scratch; the louse feces, with the bacteria, can then enter through the scratch-abraded skin.
Symptoms appear suddenly after about eight days of incubation, including fever, chills, headache, and muscle and joint pain. One week later, a rash appears in some infected persons. This rash starts on the trunk and spreads toward the extremities. Stupor and delirium may follow. Up to 70 percent of those infected can die, and full recovery can take several months.
Humans are the main reservoir of the disease; however, other mammals can serve as reservoirs. In the Eastern United States, flying squirrels are important reservoirs. The lice themselves are not reservoirs because they die soon after becoming infected; thus, crowded conditions are needed for epidemic spread.
Typhi, the causative agent of endemic typhus, is deposited on humans in the feces of rat or cat fleas. The course of the disease is similar to epidemic typhus, but it is much milder. Humans recover in less than three weeks, even when not treated.
In the spotted fever group, the bacteria are released into the arthropod’s saliva and then enter the mammalian host. The arthropods may emerge from the egg already infected because there is a transovarial transfer of bacteria from the female to her eggs. Uninfected arthropods also may become infected when they take a blood meal from an infected mammal. R. rickettsia causes Rocky Mountain spotted fever, the most common rickettsial disease in the United States. Several species of tick, including the dog tick, can transmit this disease. Ticks must remain attached for some time for disease transmission because the bacteria are dormant and must become active before they can enter the saliva and then the mammal, a process that may take up to forty-eight hours. The ticks themselves are the main reservoirs, while wild rodents serve as secondary reservoirs.
The onset of symptoms is sudden, two to twelve days after the tick bite, and includes fever, chills, headache, and muscle pain. A rash appears in almost all infected persons two or three days later. This rash begins on the hands and feet, often includes the palms and soles, and spreads toward the trunk. Complications include respiratory and renal failure, seizures, and coma. Mortality is about 20 percent in untreated persons. Ticks or mites transmit other spotted fevers and show similar infection patterns and symptoms, although the symptoms may be milder.
In the twenty-first century, discoveries are providing further insight into Rickettsia. A newly discovered rickettsial pathogen was reported as causing severe cases of an illness much like Rocky Mountain Spotted Fever in California. This new pathogen represented a novel genotype. Data has also shown that Rickettsia infections are on the rise. In 2022, an outbreak of rickettsioses occurred in Mexico, affecting many children, causing hospitalizations and deaths. The increasing prevalence of Rickettsia suggests the need for further research and vigilance.
Drug Susceptibility
Doxycycline, a tetracycline-type antibiotic, is the drug of choice for treating rickettsial diseases. Tetracycline and chloramphenicol are also used. They are taken orally for one week or more, although fever usually disappears in two to three days. Fluoroquinolones, such as ciprofloxacin, may be recommended for adults, but they are not approved for use in children. Azithromycin may be used as well, although its effectiveness is not as high. The antibiotics can be administered intravenously in severe cases.
Bibliography
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