Laboratory diagnosis, treatment and prevention of Rickettsia rickettsii

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Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever (RMSF), a potentially fatal tick-borne disease. Laboratory diagnosis of R. rickettsii infection is important for early and appropriate treatment, as well as for epidemiological surveillance and prevention. However, laboratory diagnosis of R. rickettsii can be challenging due to the following reasons:

  • The clinical manifestations of RMSF are nonspecific and can mimic other febrile illnesses.
  • The rash, which is a characteristic sign of RMSF, may be absent or delayed in some patients.
  • The organism is difficult to culture and requires biosafety level 3 facilities and precautions.
  • The serological tests may have low sensitivity in the early phase of infection or cross-reactivity with other rickettsial species.

Therefore, a combination of clinical, epidemiological, and laboratory criteria is recommended for the diagnosis of RMSF. The laboratory methods that can be used for the detection of R. rickettsii include:

  • Direct detection methods: These methods aim to identify the presence of R. rickettsii antigen or DNA in clinical specimens, such as skin biopsies or serum. The advantages of these methods are that they can provide rapid and specific results, and that they do not require prior exposure or antibody production by the host. The disadvantages are that they may have low sensitivity due to the low number of organisms in the specimens, and that they may require specialized equipment and expertise. The direct detection methods include:

    • Giemsa or Gimenez stains: These are microscopic techniques that use specific dyes to stain the rickettsiae in tissue sections or smears. The stained organisms appear as small purple dots (coccobacilli) within or outside the host cells. However, these stains are not specific for R. rickettsii and may also stain other bacteria or cellular debris.
    • Direct fluorescent antibody (DFA) test: This is an immunofluorescence technique that uses fluorescent-labeled antibodies that bind to R. rickettsii antigen in tissue sections or smears. The bound antibodies emit light when viewed under a fluorescence microscope, allowing the visualization of the rickettsiae as bright spots. This test is more specific than the stains, but it requires fresh or frozen specimens and a fluorescence microscope.
  • Culture: This method involves the isolation and growth of R. rickettsii in artificial media, such as embryonated egg or tissue cultures. The advantages of this method are that it can provide definitive identification and characterization of the isolate, and that it can be used for further studies, such as typing or susceptibility testing. The disadvantages are that it is time-consuming, labor-intensive, and hazardous, as it requires biosafety level 3 facilities and precautions. Moreover, culture is often unsuccessful due to the fastidious nature of R. rickettsii and the presence of inhibitory substances in the specimens.
  • Serology: This method detects the presence of R. rickettsii-specific antibodies in the serum of infected patients. The advantages of this method are that it is widely available, relatively inexpensive, and easy to perform. The disadvantages are that it may have low sensitivity in the early phase of infection, as antibodies usually appear after 7 to 10 days of illness onset, and that it may have low specificity due to cross-reactivity with other rickettsial species or non-rickettsial antigens. The serological tests include:

    • Weil-Felix test: This is an agglutination test that uses certain strains of Proteus vulgaris bacteria as antigens. These bacteria share some cross-reactive antigens with R. rickettsii and other rickettsiae, and thus can agglutinate with the serum antibodies of infected patients. However, this test is nonspecific and outdated, and is rarely used in modern laboratories.
    • Microimmunofluorescence (MIF) test: This is an immunofluorescence technique that uses R. rickettsii-specific antigens (outer membrane proteins or lipopolysaccharide) coated on slides. The serum antibodies bind to these antigens and are detected by fluorescent-labeled secondary antibodies under a fluorescence microscope. This test is considered the reference method for serology, as it has high sensitivity and specificity, and can differentiate between different rickettsial species and antibody classes (IgM or IgG).
    • Indirect immunofluorescent antibody (IFA) test: This is a similar technique to MIF, but uses whole-cell antigens instead of specific antigens. This test has lower specificity than MIF, but may have higher sensitivity in some cases.
    • Enzyme-linked immunosorbent assay (ELISA): This is an enzyme-based technique that uses R. rickettsii-specific antigens coated on plates. The serum antibodies bind to these antigens and are detected by enzyme-labeled secondary antibodies that produce a color change when exposed to a substrate. This test has similar sensitivity and specificity to MIF, but may be more convenient and automated.

The serological diagnosis of RMSF is usually based on the demonstration of a fourfold rise in antibody titers between acute and convalescent sera (collected at least two weeks apart), or a single serum titer of 1:64 or higher by MIF or IFA.

  • Molecular diagnosis: This method detects the presence of R. rickettsii DNA or genes in clinical specimens by polymerase chain reaction (PCR). The advantages of this method are that it can provide rapid and specific results, and that it can detect low levels of organisms in the specimens. The disadvantages are that it may be affected by inhibitors or contaminants in the specimens, and that it requires specialized equipment and expertise. The molecular targets that are used for PCR include gene sequences for outer membrane proteins (OmpA, OmpB), the 17-kDa lipoprotein, citrate synthase, or other genes.

PCR has been shown to be useful for detecting R. rickettsii DNA in skin biopsy specimens from patients with rash, with good sensitivity and high specificity.