Rickettsiae are microorganisms which have characteristics common to both bacteria and viruses. They are small, Gram-negative, aerobic coccobacilli that are obligate intracellular parasites. Like bacteria, they possess metabolic enzymes and cell membranes, utilize oxygen, and are susceptible to broad-spectrum antibiotics. They resemble viruses in that they grow only within living cells- in the cytoplasm or nucleus of the host cell.
Several species of Rickettsia can cause illnesses in humans. The term "rickettsiae" conventionally embraces a polyphyletic group of microorganisms in the class Proteobacteria, comprising of species belonging to the genera Rickettsia, Orientia, Ehrlichia, Anaplasma, Neorickettsia, Coxiella, and Bartonella. Rickettsia species cause Rocky Mountain Spotted Fever, rickettsialpox, other spotted fevers, epidemic typhus, and murine typhus. Orientia (formerly Rickettsia) tsutsugamushi causes scrub typhus. Many rickettsial illnesses occur in specific geographical regions.
These agents are usually not transmissible directly from person to person except by blood transfusion or organ transplantation, although sexual and placental transmission has been hypothesized for Coxiella. Transmission generally occurs via an infected arthropod vector or through exposure to an infected animal reservoir host. However, research has suggested that sennetsu fever (a rare disease also known Human Ehrlichioses) is acquired following consumption of raw fish products; Q fever is spread by inhalation of the airborne agent. At the site of arthropod infection, the rickettsiae either multiply in the endothelial cells of small blood vessels or in white blood cells. The diseases that follow generally produces a fever, headache, and a rash, often followed with general weakness and a cough. Sometimes, rickettsial diseases cause vomiting and diarrhea, and progress towards kidney failures, dangerously low blood pressure, and swelling of the liver and spleen.
Rickettsial agents that cause human disease are typically categorized not by disease manifestation but according to antigenic similarity. The clinical severity and duration of illnesses associated with different rickettsial infections vary considerably, even within a given antigenic group. Rickettsioses range in severity from diseases that are usually relatively mild (rickettsialpox, cat scratch disease, and African tick-bite fever) to those that can be life-threatening (epidemic typhus, Rocky Mountain spotted fever, and Oroya fever), and they vary in duration from those that can be self-limiting to chronic (Q fever and bartonelloses) or recrudescent (Brill-Zinsser disease). Most patients with rickettsial infections recover with timely use of appropriate antibiotic therapy.
History of Rickettsiae
Carried by its arthropod vector from person-to-person, rickettsial diseases have tormented large, densely-quartered human populations throughout history. Historians calculated that louse-borne typhus caused more deaths than wars combined, especially in cramped army encampments without adequate sanitation facilities. Rickettsiae were first discovered by Howard Taylor Ricketts, an American pathologist who noticed the similarities between Rocky Mountain Spotted Fever and typhus fever. Dr. Ricketts died in Mexico City in 1910 when he contracted typhus fever, the disease he was studying.
At the turn of the 21st Century, squalid conditions in heavily populated areas continued to foster outbreaks of rickettsial disease. In 1998, over 30,000 cases of human rickettsial diseases were reported in Burundi refugee camps.
Rickettsiae as Biological Agents
As potential biological agents, rickettsiae are generally stable organisms of a small sized that can be spread by aerosol. Infection spreads with low doses and causes morbidity and mortality. However, large quantities of weapons quality rickettsiae require expertise, elaborate procedures, and high-tech facilities. Additionally, rickettsiae diseases do not transmit person-to-person, and effective treatments of the diseases are readily available.
|Join the GlobalSecurity.org mailing list|