Rickettsial diseases
Rickettsial diseases are infections caused by bacteria primarily belonging to the genera Rickettsia, Orientia, and Coxiella, which lead to illnesses such as spotted fevers, scrub typhus, and Q fever. They are typically transmitted through the bites of arthropod vectors like ticks, lice, fleas, or mites, with wild animal populations often serving as reservoirs. The most notable rickettsial illness in the United States is Rocky Mountain spotted fever (RMSF), which can result in severe complications if untreated. Symptoms of rickettsial diseases often include high fever, headache, muscle pain, and characteristic rashes, although specific symptoms may vary by disease. Epidemic typhus, in particular, has historically influenced significant events, with its spread attributed to unsanitary conditions, especially during wartime. Diagnosis can be challenging due to the nonspecific nature of early symptoms, making timely treatment with doxycycline crucial. Prevention strategies focus on vector control, including the use of insect repellents to mitigate the risk of transmission. Understanding these diseases is essential for effective management and prevention, especially in areas where vectors are prevalent.
Rickettsial diseases
Definition
Rickettsial diseases include infections caused by bacteria of the genera Rickettsia (which causes spotted fevers and epidemic and endemic typhus), Orientia (which causes scrub typhus), and Coxiella (which causes Q fever). Some authorities also include as rickettsial diseases members of the more distantly related genera Ehrlichia and Anaplasma.
Causes
When an arthopod vector (such as a tick, louse, flea, or mite) obtains a blood meal from an infected animal, bacteria in the blood are inoculated directly into the arthropod, where they subsequently multiply within its gastrointestinal tract and appear later in its feces. When the arthropod next feeds on an uninfected individual (animal or human), the Rickettsia are transmitted to the new host either directly or by contamination of the bite with fecal material from the arthropod.
Risk Factors
With the exception of Q fever (which can be transmitted as an aerosol), all rickettsial diseases are spread through the bite of an arthropod vector. Wild animal populations serve as the natural reservoir for most species of Rickettsia, and humans are often incidental hosts. Epidemic typhus is unique in that it is the only rickettsial disease for which there is no wild animal reservoir. The disease is spread only by the human body or by head lice. Epidemic typhus has played perhaps a more important role than any other disease in shaping world history. It has been said, for example, that French emperor Napoleon I’s retreat from Russia in the early nineteenth century was started by a louse, and that lice have defeated the most powerful armies of Europe and Asia. The typhus epidemic during World War I spread throughout Eastern Europe and led to almost three million deaths. Because of the crowded and often unsanitary conditions that characterize land-based military operations during wartime, the spread of lice between soldiers has always been a problem. In all wars involving the United States before World War II, more soldiers died from typhus than from combat-related injuries.
Symptoms
After an incubation period of one to three weeks, symptoms of epidemic typhus include an abrupt onset of high fever, chills, headache, and myalgia. Several days later, a characteristic rash will appear, beginning on the trunk and spreading to the extremities, except for the face, palms, and soles of the feet. Mortality rates in untreated cases are 10 to 30 percent.
Endemic typhus, which is maintained in the wild rat population and transmitted by the bite of the rat flea, is like epidemic typhus, although its onset is less abrupt and the symptoms are generally less severe. The disease can be mild in children, and the illness rarely lasts more than two weeks.
Rocky Mountain spotted fever (RMSF) was first reported in the Western United States but is now most prevalent in the southeastern United States. The rickettsial pathogens are maintained in populations of many dogs and small wild animals, and the disease is transmitted by the bite of wood or dog ticks.
RMSF is a systemic infection triggered by rickettsial growth in cells lining small blood vessels throughout the body. Symptoms include high fever, severe headache, myalgia, nausea, and vomiting. The rash will develop within three to five days, appearing first on the wrists and ankles and then spreading within hours to the trunk, covering the entire body. The illness can last up to three weeks, and in severe cases significant complications are not uncommon (hearing loss, neuropathy, incontinence, motor dysfunction, and occasionally shock and death).
The bacteria responsible for Q fever is widespread in domestic animal populations (such as cattle, sheep, and goats), and the disease can be spread either from the bite of an infected tick or, more commonly, from inhalation of dust containing bacteria from dried animal feces or urine; Q fever also can be spread through the consumption of unpasteurized milk. The symptoms and course of infection are similar to RMSF, although a rash usually fails to develop.
Screening and Diagnosis
Diagnosis of the various rickettsial diseases may be difficult because their early signs and symptoms are often nonspecific or may resemble benign viral infections.
Treatment and Therapy
The drug of choice for treating all rickettsial diseases is doxycycline, administered as early in the infection cycle as possible. In persons who are intolerant of tetracycline antibiotics, intravenous chloramphenicol or fluoroquinolones have been effective.
Prevention and Outcomes
For disease prevention, vector control is of utmost importance. The use of insect repellents containing NN-diethyl metatoluamide (DEET) generally prevents tick attachment.
Bibliography
Brachman, Philip S., and Elias Abrutyn, eds. Bacterial Infections of Humans: Epidemiology and Control. 4th ed. New York: Springer, 2009.
Didier, Raoult, and Phillipe Parola, eds. Rickettsial Diseases. New York: Informa Health Care, 2007.
Hechemy, Karim E., et al., eds. Rickettsiology and Rickettsial Diseases: Fifth International Conference. Boston: Wiley-Blackwell, 2009.
Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010.
Petri, William A., Jr. "Overview of Rickettsial Infections." Merck Manual, Jan. 2024, www.merckmanuals.com/home/infections/rickettsial-and-related-infections/overview-of-rickettsial-infections. Accessed 7 Feb. 2025.
Shakespeare, Martin. Zoonoses. 2d ed. London: Pharmaceutical Press, 2009.