Rickettsia << rih KEHT see uh >> is any of a group of microorganisms that cause certain infectious diseases in human beings. Rickettsias were once classified as viruses, but they are now usually regarded as a special kind of bacteria. They differ from most bacteria in two major ways. They are smaller, and they cannot reproduce outside of living cells.
Rickettsias live primarily in the cells of certain insects and other arthropods, such as mites and ticks. A few species can infect human cells and cause such diseases as epidemic typhus, Rocky Mountain spotted fever, scrub typhus, and Q fever.
Epidemic typhus is spread among people by the bite of infected body lice. This disease occurs in the colder regions of the world, particularly those where people have poor sanitary facilities. The rickettsia that causes Rocky Mountain spotted fever lives in ticks and wild rodents and is spread to people by tick bites. This disease occurs most frequently in the Southeastern United States. Scrub typhus is common in southern and Southeast Asia, northern Australia, and Japan. Rodents and chigger mites carry the rickettsia that causes this disease, and it is spread to people by mites.
Unlike other rickettsias, the Q-fever organism infects people by being inhaled. Q fever is common in many livestock-raising regions. Cattle and sheep bitten by infected ticks pass Q-fever rickettsias in their urine and in the fluids lost while giving birth. The rickettsias may infect people who inhale dust from livestock pens.
The symptoms of rickettsial diseases include chills, fever, headaches, and rashes. Untreated rickettsial diseases–particularly epidemic typhus and Rocky Mountain spotted fever–are often fatal. However, these diseases can be treated effectively with antibiotics. Rickettsias were named for Howard Taylor Ricketts, an American pathologist who first identified the organisms in 1909.
