Rickettsia

Rickettsia, genus of bacteria which are intermediate in size between viruses and most bacteria. The bacteria were named after the American pathologist Howard Taylor Ricketts, who first discovered them. Like viruses, they can survive and multiply only inside cells. The microorganisms, which are either coccus-shaped (round) or bacilliform (elongate), are found in a great variety of mammals and cause several diseases in humans; they are generally carried by arthropods, such as fleas, lice, mites, and ticks. One exception infects domestic animals such as cattle and sheep, and can be transmitted to humans through ingestion of infected raw milk, or through inhalation of airborne particles of the milk, urine, feces, or tissues of infected animals. Numerous insect-infesting species of rickettsia cause no mammalian animal diseases. Rickettsia can be grown in the laboratory only in cultures of living cells.

The rickettsial diseases are commonly characterized by sudden onset and cause lethargy, high fever, headache, muscle aches, skin rashes in most cases, and damage to the lining of the blood vessels; damage to tissues of the central nervous system often follows. Rickettsial diseases include epidemic typhus; endemic typhus; Rocky Mountain spotted fever; tsutsugamushi fever, or scrub typhus; and trench fever. Q fever and rickettsialpox are also rickettsial diseases. Diagnosis of most of the rickettsial diseases is facilitated by the development in the blood of infected patients of specific antibodies that can be detected by serologic tests.

Prevention of rickettsial diseases involves eradication of the arthropod carriers. Until 1948, no specific treatment was available for the rickettsial diseases; now broad-spectrum antibiotics, such as the tetracycline drugs and chloramphenicol, which act specifically on rickettsia, provide an effective cure.

Scientific classification: Epidemic typhus is caused by Rickettsia prowazeki, endemic typhus by Rickettsia thyphi, and Rocky Mountain spotted fever by Rickettsia rickettsii. Tsutsugamushi fever, or scrub typhus, is caused by Rickettsia orientalis or Rickettsia tsutsugamushi. Trench fever is caused by Rickettsia quintana. Q fever is caused by Coxiella burnetii, which can be transmitted by airborne particles or food. Rickettsialpox is caused by Rickettsia akari.