Rickettsiae

Aka: Rickettsiae, Rickettsiaceae, Rickettsia, Rickettsioses, Ehrlichia, Coxiella, Rochalimaea, Rickettsial Disease, Weil-Felix Reaction

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II. Pathophysiology

  1. Rickettsiaceae are primarily Vector-Borne Infections
    1. Transmitted by arthropods (ticks, fleas, lice)
    2. Coxiella Burnetii (Q Fever) does not require a vector-borne transmission
      1. However, Coxiella is now classified in the Coxiellaceae family instead
  2. Rickettsiaceae target vascular endothelial cells
  3. Rickettsiaceae are more akin to viruses than Bacteria (similar to Chlamydia)
    1. Tiny Gram Negative Cocci
      1. Rickettsiaceae are only 350 nm (nearly the diameter of a large virus)
      2. Like other Bacteria, they have both DNA and RNA (while viruses have one or the other)
    2. Obligate intracellular Parasites
      1. Similar to viruses (as well as a few other Bacteria families including Chlamydia)
      2. Rickettsia uses cellular ATP, but can also generate its own energy (unlike Chlamydia)
      3. Rickettsia replicates freely in the host cytoplasm
  4. Rickettsiaceae is a family of 6 genera (3 of which are important in human disease)
    1. Rickettsia (most prominent in human disease)
      1. Spotted Fever Group
        1. Rocky Mountain Spotted Fever (Rickettsia rickettsii)
        2. Other species in the spotted fever group are found worldwide
      2. Typhus Group
        1. Rickettsia prowazekii (Louse-Borne Typhus)
        2. Rickettsia typhi (Murine Typhus)
    2. Orientia
      1. Orientia tsutsugamushi (Scrub Typhus)
    3. Ehrlichia
      1. Human Monocytic Ehrlichiosis (Ehrlichia chaffeensis, Ehrlichia ewingii)
      2. Human Granulocytic Anaplasmosis (Ehrlichia equi, Ehrlichia phagocytophila)
    4. Coxiella (previously in Rickettsiae family, now in Coxiellaceae)
      1. Coxiella Burnetii (Q Fever)

III. Types: Genus Rickettsia

  1. Spotted Fever Group (transmitted by fleas, mites and ticks)
    1. Rocky Mountain Spotted Fever (Rickettsia rickettsii)
    2. Rickettsial Pox (R. akari in North America)
    3. American Boutonneuse fever (R. parkeri in southeast U.S.)
      1. First identified in Southwestern coastal U.S. in 2002
      2. Patient presented with fever, Headache, echars and Regional Lymphadenopathy
    4. African Tick BiteFever or ATBF (R. africae in africa)
      1. Important cause of Fever in the Returning Traveler from Sub-Sahara Africa
    5. Finders Island Spotted Fever (R. honei in northwest U.S. as well as Australia and southeast Asia)
    6. Mediterranean Spotted Fever or Boutonneuse Fever (R. connori in the Mediterranean)
    7. Queensland Tick Typhus (R. australis in australia)
    8. Siberian Tick Typhus (R. sibirica in China)
  2. Typhus Group
    1. See below for Orentia tsutsugamushi (Scrub Typhus)
    2. Rickettsia prowazekii (Louse-Borne Typhus, Epidemic Typhus)
    3. Rickettsia typhi (Murine Typhus)
    4. Rickettsia mooseri

IV. Types: Other Genera

  1. Genus Orentia
    1. Orentia tsutsugamushi (Scrub Typhus)
      1. Previously categorized as Rickettsia tsutsugamushi
  2. Genus Ehrlichia
    1. Human Monocytic Ehrlichiosis (Ehrlichia chaffeensis, Ehrlichia ewingii)
    2. Human Granulocytic Anaplasmosis (Ehrlichia equi, Ehrlichia phagocytophila)
  3. Genus Coxiella
    1. Previously in Rickettsiae family (now in Coxiellaceae)
    2. Coxiella Burnetii (Q Fever)

V. Labs: Modern Testing

  1. Complete Blood Count
    1. Thrombocytopenia
  2. Comprehensive Metabolic Panel
    1. Hyponatremia
    2. Hepatic Enzyme Transaminitis (AST, ALT)
  3. Serology
    1. Indirect Immunofluorescence Test (IFA)
    2. Complement Fixation Test (CF)
    3. Enzyme Linked Immunosorbent Assay (ELISA)
  4. DNA
    1. Polymerase chain reaction (PCR)

VI. Labs: Weil-Felix Reaction (Historical)

  1. Rickettsiaceae share similar Antigens to Proteus Vulgaris
    1. Rickettsiaceae shares the Antigens OX-19, OX-2 and OX-K with Proteus Vulgaris
    2. Proteus Vulgaris is an otherwise completely unrelated organism
  2. Efficacy
    1. Weil-Felix Reaction was historically used to identify Rickettsiaceae organisms
    2. However, the test has low efficacy and has been replaced by other methods
  3. Technique
    1. Latex Agglutination test using each of the Proteus Vulgaris Antigens (OX-19, OX-2, OX-K)
    2. Serum of the test patient is mixed with these test Antigens
    3. Agglutination occurs if patient has serum Antibody to the Antigen tested
  4. Interpretation
    1. Rocky Mountain Spotted Fever is OX-19 and OX-2 positive
    2. Epidemic Typhus and Endemic Typhus are both OX-19 positive
    3. Scrub Typhus is OX-K positive

VII. Management

  1. See Rocky Mountain Spotted Fever
  2. See Typhus
  3. See Anaplasmosis
  4. See Ehrlichiosis
  5. See Q Fever
  6. Antibiotics
    1. Doxycycline
    2. Chloramphenicol

VIII. References

  1. Gladwin, Trattler and Mahan (2014) Clinical Microbiology, Medmaster, Fl, p. 115-21
  2. Abdad (2018) J Clin Microbiol 56(8):e01728-17 +PMID: 29769278 [PubMed]
  3. Blanton (2019) Infect Dis Clin North Am 33(1):213-29 +PMID: 30712763 [PubMed]

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