Rickettsia
Rickettsia
Rickettsial disease transmission
Rickettsia is 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). Rickettsia was named after American pathologist Howard Taylor Ricketts (1871–1910), who found that the Rocky Mountain wood tick carried bacteria that later would cause a disease called Rocky Mountain spotted fever.
Specific species of rickettsia include Rickettsia rick-ettsii, which causes the dangerous Rocky Mountain spotted fever; R. akari, which causes the relatively mild rickettsial pox; R. prowazekii, which causes the serious diseaseepidemic typhus; R. typhi, the cause of the more benign endemic or rat typhus; and R. tsutsugamushi, the cause of scrub typhus.
Rickettsial disease transmission
Rickettsia are transmitted to humans by insects such as ticks, mites, and chiggers. Usually the insect has acquired the bacteria from larger animals that 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. Fever and a rash characterize most rickettsial diseases. 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.
The spotted fevers
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 U.S. Centers for Disease Control and Prevention (CDC) report about 600 to 1,000 cases occurring annually in the United States, but this number may be underestimated due to under-reporting. 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 fatal if not treated.
A similar but milder disease is rickettsial pox, caused by R. akari. Mites that live preferentially on the common house mouse, only occasionally biting humans, transmit these bacteria. A rash that does not affect the palms or soles of the feet characterizes rickettsial pox. 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
KEY TERMS
Pathogenic— Able to cause disease.
Rocky Mountain spotted fever— A disease caused by Rickettsia ricketsii transmitted by the hard-shelled tick. The disease is characterized by a fever and a rash that starts on the extremities, including the soles of the feet and palms of the hands.
Typhus— A disease caused by various species of Rickettsia, characterized by a fever, rash, and delirium. Typhus is transmitted by insects such as lice and chiggers. Two forms of typhus, epidemic disease and scrub typhus, are fatal if untreated.
of the rash has sometimes led to its misdiagnosis as chicken pox, but the eschar clearly distinguishes rick-ettsial 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.
Rickettsial typhus diseases
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 rainforests, 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.
Nonpathogenic rickettsia
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 nonpathogenic (they do not cause disease) to humans as well.
Prevention
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.
It is important to know how to remove a tick if one is found on the skin. It takes several hours from the time a rickettsia-infected tick attaches to the skin for the rickettsia to be transmitted to the human bloodstream, so removing a tick promptly is crucial. When removing a tick, be careful not to crush it, as crushing may release rickettsia that can contaminate the hands and fingers. Use tweezers to grasp the tick as close to the skin as possible, and then pull slowly away from the skin. Make sure the mouthparts are removed from the skin (sometimes the body of a tick will separate from the head as it is being pulled). Do not try to remove a tick with gasoline or try to burn a tick off the skin with a match. After the tick is removed, wash hands immediately. If the tick cannot be removed, seek immediate medical help.
Resources
BOOKS
Cormican, M.G., and M.A. Pfaller. “Molecular Pathology of Infectious Diseases,” Clinical Diagnosis and Management by Laboratory Methods. 20th ed. Philadelphia, PA: W.B. Saunders, 2001.
Harden, Victoria Angela. Rocky Mountain Spotted Fever: History of a Twentieth-Century Disease. Baltimore, MD: Johns Hopkins University Press, 1990.
Prescott, L., J. Harley, and D. Klein. Microbiology 5th ed. New York: McGraw-Hill, 2002.
OTHER
Todar, Kenneth, Department of Bacteriology, University of Wisconsin at Madison. “The Rickettsiae.” 2005. <http://textbookofbacteriology.net/Rickettsia.html> (accessed accessed November 19, 2006).
National Institute of Allergy and Infectious Diseases. Rocky Mountain Spotted Fever. Bethesda, MD: U.S. Department of Health, Education, and Welfare, Public Health Service, National Institute of Health, National Institute of Allergy and Infectious Diseases, Office of Reporting and Public Response, 1975.
Kathleen Scogna
Rickettsia
Rickettsia
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.
Rickettsial disease transmission
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 which 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.
The spotted fevers
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 600-1,000 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 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 has sometimes led 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.
Rickettsial typhus diseases
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 rainforests, 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.
Nonpathogenic rickettsia
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 nonpathogenic (they do not cause disease) to humans as well.
Prevention
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.
It is important to know how to remove a tick if one is found on the skin. It takes several hours from the time a rickettsia-infected tick attaches to the skin for the rickettsia to be transmitted to the human bloodstream, so removing a tick promptly is crucial. When removing a tick, be careful not to crush it, as crushing may release rickettsia that can contaminate the hands and fingers. Use tweezers to grasp the tick as close to the skin as possible, and then pull slowly away from the skin. Make sure the mouthparts are removed from the skin (sometimes the body of a tick will separate from the head as it is being pulled). Do not try to remove a tick with gasoline or try to burn a tick off the skin with a match. After the tick is removed, wash your hands immediately. If you cannot remove the tick yourself, seek medical help.
Resources
books
Cormican, M.G., and M.A. Pfaller. "Molecular Pathology of Infectious Diseases," Clinical Diagnosis and Management by Laboratory Methods. 20th ed. Philadelphia: W. B. Saunders, 2001.
Harden, Victoria Angela. Rocky Mountain Spotted Fever: History of a Twentieth-Century Disease. Baltimore: Johns Hopkins University Press, 1990.
Prescott, L., J. Harley, and D. Klein. Microbiology. 5th ed. New York: McGraw-Hill, 2002.
periodicals
Miksanek, Tony. "An Independent Diagnosis." Discover 14 (February 1993): 26.
Petri, William Jr. "Tick-borne Diseases." American FamilyPhysician 37 (June 1988): 95-105.
Salgo, Miklos P., et al. "A Focus of Rocky Mountain Spotted Fever within New York City." The New England Journal of Medicine 318 (May 26, 1988): 1345-48.
other
National Institute of Allergy and Infectious Diseases. RockyMountain Spotted Fever. Bethesda, MD: U.S. Department of Health, Education, and Welfare, Public Health Service, National Institute of Health, National Institute of Allergy and Infectious Diseases, Office of Reporting and Public Response, 1975.
Kathleen Scogna
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Pathogenic
—Able to cause disease.
- Rocky Mountain spotted fever
—A disease caused by Rickettsia ricketsii transmitted by the hardshelled tick. The disease is characterized by a fever and a rash that starts on the extremities, including the soles of the feet and palms of the hands.
- Typhus
—A disease caused by various species of Rickettsia, characterized by a fever, rash, and delirium. Typhus is transmitted by insects such as lice and chiggers. Two forms of typhus, epidemic disease and scrub typhus, are fatal if untreated.