Rickettsial Diseases Made Easy

Important features of Rickettsia and Rickettsial Diseases

  • Obligate intracellular parasites like Chlamydia – can survive only in host cells
  • Cannot produce their own ATP, so they utilize the ATP of a host cell
  • Gram negative coccobacillia, and short bacilli that grow strictly in eukaryotic cells (unable to grow in cell media)
  • Contain DNA, RNA; divide by binary fission
  • All rickettsial infections are transmitted by an insect vector except Q fever (respiratory spread)
  • Humans are only incidental hosts except in louse born epidemic typhus
  • All are associated with a rash, except Q fever and ehrlichiosis
  • Rocky mountain spotted fever (RMSF) rash (on 2nd-6th day of fever) begins on the extremities (involvement of palms and soles is characteristic) and moves centrally
  • Rash of typhus (both murine and epidemic typhus) begins centrally (on 4th day of fever; spares face) and moves toward the extremities (spares palms and soles)
  • Rickettsial pox is the only rickettsial disease characterized by vesicular rash (with eschar at the site of bite)
  • No rash is observed in Q fever
  • All the members of the spotted fever group, except Rocky Mountain spotted fever, cause an eschar, as does scrub typhus.
  • Rochalimaea or Bartoneall quintana (Trench fever) can be cultivated in blood agar under 10% CO2
  • Doxycycline (drug of choice), Chloramphenicol¬†and others (macrolides, rifampicin) are effective for the treatment of rickettsial diseases

rickettsia

 

Rickettsial Disease, Agents and Vectors

DiseaseAgentVectors
Typhus groupEpidemic typhus or Brill Zinsser diseaseR. prowazekiiLouse
Murine/Endemic typhusR.typhi (R.mooseri)Flea
Spotted fever groupRocky mountain spotted feverR.rickettsiiTick (Dermacentor)
R. poxR.akariMite
Fever boutonneuse or Mediterranean spotted fever or Indian tick typhusR.conoriiTick
OtherQ. feverCoxiella burnettiiNil (Air borne)
Trench fever / Five day fever / Quintan feverRochalimaea Quintana (Bartonella Quintana)Louse
Scrub typhus (Chigger borne typhus)Rickettsia or Orientia tsutsugambushiMite
EhrlichiosisEhrlichiae – human granulocytic ehrlichiosis (Anaplasma phagocytophilum); human monocytic ehrlichiosis (Ehrlichia chaffeensis)Tick

Mnemonics to remember vectors and agents of rickettsial diseases

LET

Louse-borne

  1. Epidemic typhus
  2. Trench fever

Take the 2nd letter from these. The name of the agent starts with the same letter.

  • ePidemic typhus: Prowazekii (R.prowazekii)
  • tRench fever: Rochalimaea quintana

TERM

Tick-borne

  1. Ehrlichiosis
  2. Rocky mountain spotted fever
  3. Mediterranean spotted fever

Take the 1st letter from these. The name of the agent starts with the same letter.

  • Ehrlichiosis: Ehrlichia chaffeensis
  • Rocky mountain spotted fever: R. Rickettsii
  • Mediterranean spotted fever (exception): R.conorii

MPS

Mite-borne

  1. Pox (R.pox) – R.akari
  2. Scrub typhus – Orientia tsutsugambushi

At last, Endemic typhus is flea borne and caused by R.typhi.

Weil-Felix reaction

It is a heterophile agglutination test based on sharing of a common antigen between Typhus rickettsia and some strain of Proteus bacilli (OX 19, OX – 2, OX K). It doesn’t differentiate between epidemic and endemic typhus and is of no value in rickettsial pox, trench fever and Q fever.

weil felix reaction
OX-19 (+++): In Epidemic and Endemic typhus

OX 19, OX-2 (++): In Rocky Mountain Spotted fever

OX-K (+++): In Scrub typhus

OX-19, OX-2 and OX-L (all negative): In Rickettsial pox, Q fever, Trench fever (Quintan fever)

Mnemonic for Weil-felix reaction: PQ-RST

  1. Pox, Q fever and Quintan (trench) fever: Negative for all
  2. Rocky mountain spotted fever (RMSF): 19 and 2
  3. Scrub typhus: K
  4. Typhus (epidemic and endemic): 2

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