Rickettsioses

Species	Disease	Vector	Clinical Clues
Rickettsiae: Spotted fever group
Rickettsia rickettsii	Rocky Mountain spotted fever	Multiple Dermacentor, Amblyomma, and Rhipicephalus ticks	More severe disease. Centripetal rash without eschar in the Americas
Rickettsia conorii	Boutonneuse fever, Mediterranean spotted fever	Rhipicephalus sanguineus ticks	More severe disease. Rash with eschar, predominantly Mediterranean and Black Sea
Rickettsia japonica	Oriental spotter fever	Dermacentor, Haemaphysalis, or Ixodes ticks	Rash with eschar, acquired in Asia
Rickettsia sibirica mongolitimonae		Rhipicephalus sanguineus	Rash with eschar, acquired in Europe, Africa, or Asia
Rickettsia sibirica sibirica		Dermacentor ticks	Rash with eschar, acquired in Asia or southern Russia
Rickettsia australis		Ixodes ticks	Vesicular rash with eschar, acquired in Australia
Rickettsia slovaca	Tick-borne lymphadenopathy	Dermacentor marginatus	Eschar (often scalp) with regional lymphadenopathy, acquired in Europe
Rickettsia africae	African tick-bite fever	Amblyomma hebraeum and Amblyomma variegatum	Vesicular rash with eschar, acquired in Africa or the Caribbean
Rickettsia honei		Bothriocroton, Haemaphysalis, Ixodes, or Rhipicephalus ticks	Rash with eschar acquired in Flinders Islands, eastern Australia, Thailand, or Nepal
Rickettsia aeschlimanii		Hyalomma or Rhipicephalus ticks	Rash with eschar, acquired in Africa or Europe
Rickettsia helvetica		Tick
Rickettsia parkeri		Amblyomma maculatum ticks	Rash with eschar, acquired in the Americas
Rickettsia heilongjianghensis		Dermacentor or Haemaphysalis ticks	Rash with eschar, acquired in Asia
Rickettsia raoultii		Dermacentor ticks	Eschar (often scalp) with regional lymphadenopathy, acquired in Europe
Rickettsia massiliae		Rhipicephalus sanguineus	Rash with eschar, acquired in Europe, Africa, and the Americas
Rickettsia amblyommii		Tick
Rickettsia monacensis		Tick
Rickettsia philipii strain 364D		Dermcentor occidentalis	Eschar (often scalp) with regional lymphadenopathy, acquired in California
Rickettsia felis		Flea	Rash with eschar and flea exposure
Rickettsia akari	Rickettsialpox	Liponyssoides sanguineus house mouse mites	Vesicular rash with eschar, worldwide
Rickettiae: Typhus group
Rickettsia typhi	Endemic (murine) typhus	Xenopsylla cheopis (rat flea) and Ctenocephalides felis (cat flea)	May have centrifugal rash without eschar in the tropics or subtropics, with flea exposure
Rickettsia prowazekii	Epidemic (louse-borne) typhus	Pediculus humanus humanus (human body louse)	More common in homeless shelters. May have centrifugal rash without eschar, with exposure to flying squirrel or other
Rickettsiae: Scrub typhus group
Orientia tsutsugamushi	Scrub typhus	Leptotrombidium mites (chiggers, trombiculid mites)	Rash with eschar, acquired in Asian Pacific, Chile, or Dubai
Anaplasma
Amblyomma phagocytophilum	Human granulocytotropic anaplasmosis	Tick
Ehrlichia
Ehrlichia chaffeensis	Human granulocytotropic anaplasmosis	Amblyomma americanum ticks (the Lone Star tick)
Ehrlichia ewingii	Human granulocytotropic anaplasmosis	Amblyomma americanum and Dermacentor variabilis ticks
Ehrlichia canis		Tick
Other
Neoehrlichia mikurensis		Tick
Neorickettsia sennetsu		Raw fish
Wolbachia species		Helminths

Diagnosis

• The mainstay of diagnosis is serology
  • Indirect immunofluorescence antibody assays of IgG antibodies
    • IgM do not appear any earlier in the course of disease and are less specific
  • Usually as acute and convalescent titres drawn 2 to 4 weeks apart showing a 4-fold increase in titres
    • Rickettsia africae may take up to 4 weeks after onset of illness to seroconvert
  • Cross-reactivity is common within each group
    • Serologic tests for Rickettsia conorii or Rickettsia rickettsii may be used to diagnosis other spotted fever group diseases such as Rickettsia africae, Rickettsia akari, Rickettsia australis, and Rickettsia sibirica
    • Serologic tests for Rickettsia typhi may be used to diagnose other typhus group diseases such as Rickettsia prowazekii
    • Cross-reactivity also common between the two groups, but the titres will usually be highest in the group containing the causative organism
• PCR is becoming more widely available
  • Blood and tissue (e.g. eschar biopsy)
  • Done as PCR for the group followed by species-specific sequencing

Management

• First-line for essentially all infections is doxycycline 100 mg po BID, continue for 3 days following resolution of symptoms
• Alternatives include azithromycin and chloramphenicol

Further Reading

• Syndromic classification of rickettsioses: an approach for clinical practice. Int J Infect Dis. 2014;28:126-39. doi: 10.1016/j.ijid.2014.05.025