What is rickettsia?

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 also have 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 all 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 production of most amino acids and
nucleotides. They do possess the enzymes for aerobic metabolism, but 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 and include 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. Mortality can be up to 70 percent of those infected, 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 much like epidemic typhus, but the disease is much milder, and 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 transovarian 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, are able to transmit this disease. Ticks must remain attached for
some time for disease transmission because the bacteria are in a dormant state 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. Other spotted fevers are transmitted by ticks or mites and show similar infection patterns and symptoms, although the symptoms may be milder.

Drug Susceptibility

Doxycycline, a tetracycline-type antibiotic, is the drug of choice for treating
rickettsial diseases. Tetracycline and chloramphinicol also are used. They are
taken orally for one week or more, although fever usually disappears in two to
three days. The antibiotics can be administered intravenously in severe
cases.

Bibliography

Didier, Raoult, and Phillipe Parola, eds. Rickettsial Diseases. New York: Informa Health Care, 2007. After a brief introduction to the organisms, this book explains many rickettsial diseases in detail.

Hechemy, Karim E., et al., eds. Rickettsiology and Rickettsial Diseases. Boston: Wiley-Blackwell, 2009. This brings together current research on all phases of Rickettsia and rickettsial diseases.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010. A comprehensive college textbook that provides broad coverage of microbiology and bacterial diseases.