Rickettsia and Rickettsial Pox (World of Microbiology and Immunology)
Rickettsia are a group of bacteria that cause a number of serious human diseases, including the spotted fevers and typhus. Rod- or sphere-shaped, rickettsia lack both flagella (whip-like organs that allow bacteria to move) and pili (short, flagella-like projections that help bacteria adhere to host cells). Specific species of rickettsia include Rickettsia rickettsii, which causes the dangerous Rocky Mountain spotted fever; R. akari, which causes the relatively mild rickettsial pox; R. prowazekii, which causes the serious disease epidemic typhus; R. typhi, the cause of the more benign endemic or rat typhus; and R. tsutsugamushi, the cause of scrub typhus.
Rickettsia are transmitted to humans by insects such as ticks, mites, and chiggers. Usually the insect has acquired the bacteria from larger animals which they parasitize, such as rats, mice, and even humans. When an insect infected with rickettsia bites a human, the bacteria enter the bloodstream. From there, unlike most other bacteria that cause infection by adhering to cells, rickettsia enter specific human cells, where they reproduce. Eventually these host cells lyse (burst open), releasing more rickettsia into the bloodstream. Most rickettsial diseases are characterized by fever and a rash. Although all can be effectively cured with antibiotics, some of the rickettsial diseases, such as epidemic typhus and Rocky Mountain spotted fever, can be fatal if not treated promptly.
Rocky Mountain spotted fever is one of the most severe rickettsial diseases. First recognized in the Rocky Mountains, it has since been found to occur throughout the United States. The Centers for Disease Control report about 600,000 cases occurring annually, but this number may be underestimated due to underreporting. Rickettsia rickettsii are carried and transmitted by four species of the hard-shelled tick, all of which feed on humans, wild and domestic animals, and small rodents. When a tick feeds on an infected animal, the bacteria are transmitted to the tick, which can in turn infect other animals with its bite. Human-to-human transmission of R. rickettsii does not occur. Once inside the human bloodstream, the bacteria invade cells that line the small blood vessels.
The symptoms of Rocky Mountain spotted fever reflect the presence of bacteria inside blood vessel cells. Within two to 12 days of being bitten by an infected tick, the infected person experiences a severe headache, fever, and malaise. After about two to four days, a rash develops, first on the extremities, then the trunk. A characteristic sign of this disease is that the rash involves the soles of the feet and palms of the hands. If the disease is not treated with antibiotics, the infected blood vessel cells lyse, causing internal hemorrhage, blockage of the blood vessels, and eventual death of the cells. Shock, kidney failure, heart failure, and stroke may then occur. Rocky Mountain spotted fever is often fatal if not treated.
A similar but milder disease is rickettsial pox, caused by R. akari. These bacteria are transmitted by mites which live preferentially on the common house mouse, only occasionally biting humans. Rickettsial pox is characterized by a rash that does not affect the palms or soles of the feet. The rash includes a lesion called an eschar-a sore that marks the spot of the infected mite bite. The mild course of this disease and the presence of the rash sometimes leads to its misdiagnosis as chicken pox, but the eschar clearly distinguishes rickettsial pox from chicken pox.
Outside of the United States, spotted fevers such as North Asian tick typhus, Queensland tick typhus, and boutonneuse fever are caused by other rickettsia species. As their names suggest, these diseases are found in Asia, Mongolia, and the Siberian region of Russia; in Australia; and in the Mediterranean region, Africa, and India, respectively. Symptoms of these spotted fevers resemble those of rickettsial pox. Although these spotted fevers share some of the symptoms of Rocky Mountain spotted fever, they are milder diseases and are usually not fatal.
Three forms of typhus are also caused by rickettsia. Epidemic typhus is caused by R. prowazekii, a bacterium that is transmitted by the human body louse. Consequently, episodes of this disease occur when humans are brought into close contact with each other under unsanitary conditions. Endemic typhus and scrub typhus are caused by R. typhi and R. tsutsugamushi, respectively. Transmitted by rat fleas, endemic typhus is a mild disease of fever, headache, and rash. Scrub typhus, named for its predilection for scrub habitats (although it has since been found to occur in rain forests, savannas, beaches, and deserts as well) is transmitted by chiggers. Unlike endemic typhus, scrub typhus is a serious disease that is fatal if not treated.
Not all rickettsia cause disease. Some species, such as R. parkeri and R. montana, normally live inside certain species of ticks and are harmless to the insect. These rickettsia are non-pathogenic (they do not cause disease) to humans as well.
With the exception of epidemic typhus, no vaccine exists to prevent rickettsial infection. Prevention of these diseases should focus on the elimination of insect carriers with insecticides and wearing heavy clothing when going into areas in which rickettsial carriers dwell. For instance, appropriate clothing for a forest expedition should include boots, long-sleeved shirts, and long pants. Treating the skin with insect repellents is also recommended to prevent insect bites.