RICKETTSIAL, EHRLICHIAL, AND Anaplasma INFECTIONS AT A GLANCE
- Rickettsiae primarily target vascular endothelial cells, causing febrile illness and rash in the mammalian host.
- Transmission is predominantly via tick bites, with certain pathogens transmitted by human body lice and mites.
- Fever, headache, myalgia, and malaise are common to rickettsial, ehrlichial, and Anaplasma infections; rash is common in rickettsial disease, occasional in ehrlichial infection, and rare in anaplasmosis.
- The early signs and symptoms of infection are often nonspecific and can mimic self-limited viral illnesses or other life-threatening illnesses.
- Early empiric treatment with doxycycline should be considered in highly suspicious cases until rickettsial infection is definitively ruled out, as delayed treatment can lead to severe sequelae and high mortality rates.
Rickettsial diseases are curable infections that, if unrecognized, can be readily lethal. Early nonspecific symptoms can mimick benign viral illnesses and should be considered in any patient who presents with constitutional symptoms, fever, headache, and a characteristic petechial rash. The advent of modern molecular technologies, including genetic analysis, has allowed for significant taxonomic reclassification of the rickettsiae including moving the family Bartonella (see Chapter 192) and the genus Coxiella out of the order Rickettsiales to the orders Rhizobiales and Legionellales, respectively. “Rickettsiae” now includes a polyphyletic group of microorganisms in the class Proteobacteria, comprising species belonging to the genera Rickettsia, Orientia, Ehrlichia, Anaplasma, and Neorickettsia. Several non-rickettsial agents that were historically included in the group of infections loosely termed the rickettsioses remain incorporated in the discussion of rickettsial disease herein to reflect that precedent.1
Historically rickettsiae and Rickettsia-like organisms (e.g., Coxiella burnetii) were endemic pathogens, however the rise of international travel has allowed for the spread of rickettsial infections to nonendemic areas. Rickettsia prowazekii and C. burnetii, agents of epidemic typhus and Q fever, respectively have emerged as potential agents of bioterrorism.2
The rickettsioses have had profound effects on civilization, particularly during times of war or famine. The first contemporary account of rickettsial disease appeared during the civil wars of Granada in 1489–1490, where a typhus-like disease killed 17,000 Spanish soldiers. The eighteenth century physician James Lind astutely recognized that the vector of epidemic typhus was carried on the bodies of men and clothes. In 1909, Charles Nicolle identified the body louse as the vector for epidemic typhus. During World War I and the Russian Revolution, epidemic typhus caused 3 million deaths. Howard Tyler Ricketts, for whom the genera was named, is credited with first demonstrating the organism (now known to be R. prowazekii) in typhus patients and infected lice. Ironically, Ricketts ultimately died of typhus in Mexico in 1910. Before his death, however, Ricketts also confirmed that the wood tick was a vector of Rocky Mountain spotted fever (RMSF). S. Burt Wolback at Harvard University Medical School is credited with identifying the organism that causes RMSF within the endothelial cells of vasculitic lesions.