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 (centripetal)
Rash of typhus (both murine and epidemic typhus) begins centrally (on 4th day of fever; spares face) and moves toward the extremities, i.e. centrifugal (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

Rickettsial Disease, Agents and Vectors

	Disease	Agent	Vectors
Typhus group	Epidemic typhus or Brill Zinsser disease	R. prowazekii	Louse
Typhus group	Murine/Endemic typhus	R.typhi (R.mooseri)	Flea
Spotted fever group	Rocky mountain spotted fever	R.rickettsii	Tick (Dermacentor)
	R. pox	R.akari	Mite
	Fever boutonneuse or Mediterranean spotted fever or Indian tick typhus	R.conorii	Tick
Other	Q. fever	Coxiella burnettii	Nil (Air borne)
	Trench fever / Five day fever / Quintan fever	Rochalimaea Quintana (Bartonella Quintana)	Louse
	Scrub typhus (Chigger borne typhus)	Rickettsia or Orientia tsutsugambushi	Mite
	Ehrlichiosis	Ehrlichiae – human granulocytic ehrlichiosis (Anaplasma phagocytophilum); human monocytic ehrlichiosis (Ehrlichia chaffeensis)	Tick

Mnemonics to remember vectors and agents of rickettsial diseases

LET

Louse-borne

Epidemic typhus
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

Ehrlichiosis
Rocky mountain spotted fever
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

Pox (R.pox) – R.akari
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.

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:

OK! Scrub now (Scrub typhus OX-K)

Two typhus (Endemic and epidemic): OX-2

1920 m Rocky Mountain (RMSF OX-19 and OX-2)