Rickettsiae are small, Gram-negative coccobacilli that structurally resemble bacteria but are one tenth to one half as large. Polychromatic stains (Giemsa stain) are better than simple stains or the Gram stain for demonstrating rickettsiae in cells.
Rickettsiosis is the general name given to any disease caused by rickettsiae (Table 7.3). These organisms are considered to be obligate intracellular parasites; that is, they cannot exist anywhere except inside the bodies of living organisms. Diseases caused by rickettsiae are transmitted by arthropod vectors, such as lice, fleas, ticks, or mites. Rickettsial diseases are divided into the following general groups:
Typhus-like fevers
Rocky Mountain spotted fever (caused by Rickettsia rickettsia; transmitted by ixodid [hard] ticks)
Scrub typhus (caused by Orientia tsutsugamushi; transmitted by mites)
Q fever (caused by rickettsial-like bacillus Coxiella burnetii; chronic or acute)
Other rickettsial diseases
Q fever, caused by C. burnetii, is characterized by an acute febrile illness, severe headache, rigors, and possibly pneumonia or hepatitis. It can cause encephalitis in children and has been isolated in breast milk and in the placenta of infected mothers, making it possible for a fetus to be infected in utero. Both complement fixation and fluorescent antibody tests can detect antibodies to the organism. C. burnetii displays an antigenic variation during an infection. Phase I antibodies are preponderant during the chronic phase, whereas phase II antibodies predominate during the acute phase. A diagnosis is made when the phase I titer in a convalescent serum specimen is four times greater than that in an acute serum specimen.
Early diagnosis of rickettsial infection is usually based on observation of clinical symptoms such as fever, rash, and exposure to ticks. Biopsy specimens of skin tissue from a patient with suspected Rocky Mountain spotted fever can be tested with an immunofluorescent stain and diagnosed 3–4 days after symptoms appear. Signs and symptoms include the following:
Fever
Skin rashes
Parasitism of blood vessels
Extreme exhaustion
Stupor and coma
Headache
Ringing in the ears
Dizziness
Rickettsial diseases are often characterized by an incubation period of 10–14 days, followed by an abrupt onset of the signs and symptoms listed, in a patient with a history of arthropod bites. Cultures of rickettsia are performed only in reference laboratories. Rickettsial infections usually are diagnosed by serologic methods, using acute and convalescent serum specimens. A fourfold rise in serum antibody titer is preferable, but a single titer greater than 1:64 is highly suggestive of infection (seeChapter 8).