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Species of Rickettsia are small (0.3-0.5 mcm x 1-2 mcm) obligately intracellular bacteria (Proteobacteria). They have a gram-negative cell wall structure. Rickettsia are found in arthropod hosts for at least part of their life cycle.
Rickettsial infections in the United States are caused by 2 major groups within the genus Rickettsia: spotted fever group and typhus fever group. The spotted fever group includes Rickettsia rickettsii (Rocky Mountain spotted fever), Rickettsia akari, Rickettsia conorii (Boutonneuse fever), Rickettsia australis (Queensland tick typhus), and Rickettsia sibirica (North Asian tick typhus). The typhus fever group includes Rickettsia typhi (murine typhus; endemic typhus), and Rickettsia prowazekii (epidemic typhus).
Rickettsia rickettsiae is the most common rickettsial species encountered in the United States and is transmitted through a tick vector (Dermacentor species or, less commonly, Rhipicephalus sanguineus). Following a 2- to 14-day incubation period, patients most commonly present with fever, chills, and myalgia. A maculopapular rash typically appears 2 to 5 days after fever onset, though approximately 10% of patients will not develop a rash. Antibodies to the spotted fever group agents are detectable within 7 to 10 days after illness onset. Demonstration of either 1) seroconversion or 2) a 4-fold change in IgG specific antibody titers in acute and convalescent serum samples is consistent with acute or ongoing disease.
An aid in the diagnosis of spotted fever group rickettsial infections
This test detects reactivity to the group-specific rickettsia. For example, antibody reactivity to the Rickettsia rickettsii will also react with other organisms within the spotted fever group.
> or =1:256:
-Serum end point titers of > or =1:256 are considered presumptive evidence of recent or current infection by organisms of appropriate rickettsial antigen group.
<1:256 and > or =1:64:
-Single serum end point titers > or =1:64 and <1:256 are suggestive of infection at an undetermined time and may indicate either past infection or early response to a recent rickettsial infection.
-A 4-fold or greater increase in IgG titer between 2 serum specimens drawn 1 to 2 weeks apart and tested in parallel is considered presumptive evidence of a recent or current infection.
-In patients infected with organisms within the rickettsial groups, IgG antibody is generally detectable within 1 to 2 weeks of onset of symptoms, peaking within 1 to 2 months, and declining thereafter. Following prompt antimicrobial treatment, titers generally decline below detectable levels within 8 to 11 months. With relapse, prior immunization, or delayed antibiotic treatment, IgG levels may remain elevated for more than a year post-onset.
> or =1:64:
-Titers of > or =1:64 are considered presumptive evidence of recent or current infection by organisms of appropriate rickettsial antigen group.
-Titers <1:64 suggest that the patient does not have an acute rickettsial infection.
-IgM class antibody is transiently detected within 1 to 2 weeks of onset of symptoms, usually declining rapidly within 3 months following prompt antibiotic treatment. These levels will also be elevated for an extended period with relapse, prior immunization, or delayed antibiotic treatment.
Cross-reactivity within the spotted fever group precludes the speciation of the infecting rickettsia by this procedure. Sera reactive with Rickettsia rickettsii must be termed "spotted fever group-positive." Spotted fever and typhus fever intragroup cross-reactivity is weak: cross-reactive titers are typically at least 16-fold lower than group-specific titers.
Antibody is variably absent for 1 to 2 weeks after onset of symptoms and an initial negative titer should not be used to exclude the diagnosis of rickettsial disease. A second serum specimen should be drawn 1 to 2 weeks later to establish the diagnosis in such patients.
IgM titers must be interpreted with caution, especially in the absence of IgG. Cases should be further evaluated clinically or serologically, by testing acute and convalescent serum in parallel to demonstrate a 4-fold or greater change in IgG or IgM titer.
Diagnosis of recent infection based on a single elevated IgG titer is complicated by the slow decline of antibody titer from past infection in many individuals. Titers may remain elevated for longer than 12 months, especially where antibiotic treatment was delayed or prior immunization was involved.
Some patients may maintain a long-term IgM titer, with or without IgG. It is important to check the IgM titer 1 to 2 weeks following testing of an acute specimen.
1. Walker DH, Bouyer DH: Rickettsia. In Manual of Clinical Microbiology. Eighth edition. Edited by PR Murray, EJ Baron, JH Jorgenson et al. Washington, DC, ASM Press, 2003, pp 1005-1014
2. Helmick C, Bernard K, D'Angelo L: Rocky Mountain spotted fever. Clinical laboratory and epidemiological features of 262 cases. J Clin Microbiol 1984;12:343-350
3. Centers for Disease Control and Prevention. Tickborne diseases of the United States. A Reference Manual for HealthCare Providers. First Edition, 2013. Available from URL: http://www.cdc.gov/lyme/resources/TickborneDiseases.pdf