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Often severe infectious diseases caused by several diverse and specialized bacteria, the rickettsiae and rickettsia-like organisms. The best-known rickettsial diseases infect humans and are usually transmitted by parasitic arthropod vectors.
Rickettsiae and rickettsia-like organisms are some of the smallest microorganisms visible under a light microscope. Although originally confused with viruses, in part because of their small size and requirements for intracellular replication, rickettsiae and rickettsia-like organisms are characterized by basic bacterial (gram-negative) morphologic features. Their key metabolic enzymes are variations of typical bacterial enzymes. The genetic material of rickettsiae and rickettsia-like organisms likewise seems to conform to basic bacterial patterns. The genome of all rickettsia-like organisms consists of double-stranded deoxyribonucleic acid (DNA).
Rickettsiae enter host cells by phagocytosis and reproduce by simple binary fission. The site of growth and reproduction varies among the various genera.
Clinically, the rickettsial diseases of humans are most commonly characterized by fever, headache, and some form of cutaneous eruption, often including diffuse rash, as in epidemic and murine typhus and Rocky Mountain spotted fever, or a primary ulcer or eschar at the site of vector attachment, as in Mediterranean spotted fever and scrub typhus. Signs of disease may vary significantly between individual cases of rickettsial disease. Q fever is clinically exceptional in several respects, including the frequent absence of skin lesions.
All of the human rickettsial diseases, if diagnosed early enough in the infection, can usually be effectively treated with the appropriate antibiotics. Tetracycline and chloramphenicol are among the most effective antibiotics used; they halt the progression of the disease activity, but do so without actually killing the rickettsial organisms. Presumably, the immune system is ultimately responsible for ridding the body of infectious organisms. Penicillin and related compounds are not considered effective. See Antibiotic
Most rickettsial diseases are maintained in nature as diseases of nonhuman vertebrate animals and their parasites. Human infection may usually be regarded as peripheral to the normal natural infection cycles, and human-to-human transmission is not the rule. However, the organism responsible for epidemic typhus (Rickettsia prowazekii) and the agent responsible for trench fever (Rochalimaea quintana) have the potential to spread rapidly within louse-ridden human populations. See Zoonoses
All known spotted fever group organisms are transmitted by ticks. Despite a global distribution in the form of various diseases, nearly all spotted fever group organisms share close genetic, antigenic, and certain pathologic features. Examples of human diseases include Rocky Mountain spotted fever (in North and South America), fièvre boutonneuse or Mediterranean spotted fever (southern Europe), South African tick-bite fever (Africa), Indian tick typhus (Indian subcontinent), and Siberian tick typhus (northeastern Europe and northern Asia). If appropriate antibiotics are not administered, Rocky Mountain spotted fever, for example, is a life-threatening disease. See Infectious disease