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

Add Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.