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Four genera: Rickettsia, orientia, coxeilla, ehrlichia

A. Rickettsia & Orientia
  1. Gram negative bacilli (0.3 x 1 µm)
  2. Obligate intracellular parasites
  3. Require arthropod vector for transmission to man
  4. Parasitize vascular endothelial cells
  5. Unable to grow on cell free media. Embryonated hens egg (yolk sac), mouse fibroblast, HeLa,
  6. HEp-2
Group Disease Agent Vector Mode of transmission
Typhus fever Epidemic typus
Recrudescent typhus
Murine typhus
R. prowazekii
R. prowazekii
R. typhi
rat flea
Louse feces
Flea feces
Spotted fevers RMSF
Boutonneuse fever
Australian tick typhus
Siberian tick typhus
Rickettsial pox
R. rickettsii
R. conorii
R. australis
R. siberica
Ixodid ticks
Scrub typhus Scrub typhus O. tsutsugamushi Trombiculid mite Bite
f. Weil-felix reaction: heterophile agglutination test. OX19, OX2 (P. vulgaris), OXK (P. mirabilis)
Disease OX19 OX2 OXK
Epidemic typhus ++++ + -
Brill-Zinsser +/- - -
Endemic typhus ++++ + -
Spotted fever ++++ to + + to ++++ -
Scrub typhus - - +++

Neil-Mooser/ tunica reaction: Intraperitoneal inoculation with blood from a case of endemic typhus or culture of R. typhi in male guinea pig causes fever and scrotal inflammation. The scrotum becomes enlarged and the testes cannot be pushed back into the abdomen because of the inflammatory adhesions between the layers of tunica vaginalis.

B. Coxiella burnetii
  1. Obligate intracellular pathogen, filterable through bacterial filters
  2. Resistant to heat and drying, survives the Holder method of pasteurisation
  3. Causes Q fever
  4. Two cycles of infection: wild animals and ticks; domestic animals (ticks not required) maintained by airborne transmission]
  5. Human infection: infected milk, handling infected wool, hide, soil contaminated with feces of infected animals, tick do not play significant role
  6. Fever, chills, pneumonia, hepatitis, endocarditis
C. Ehrlichia
  1. Gram negative bacilli, obligate intracellular, multiply within membrane bound vacuoles in phagocytes (morula), primary veterinary pathogens
  2. E. sennetsu: lymphoid hyperplasia & atypical lymphocytosis, Japan & Malaysia, ? ingestion of raw fish
  3. E. chaffeensis (Human monocytic ehrichiosis): infects monocytic cells, febrile illness with leucopenia, thrombocytopenia, transmitted by ticks. USA, Europe, Africa
  4. E. phagocytophila (Human granulocytic ehrlichiosis) : infects granulocytic cells. febrile illness with leucopenia, thrombocytopenia, tick borne. USA, Europe
Ehrlichia Disease Vector
E. chaffeensis Human Mono cytotropic Ehrlichiosis (HME) Tick borne
E. phagocytophila
(Anaplasma phagocytophila)*
Human Granulocytic Ehrlichiosis(HGE) Tickborne
E. sennetsu (Neorickettsia sennetsu)* Sennetsu fever Unknown
* New Names
  1. Bartonella
    1. B. bacilliformis: Oroya fever, transmitted by sand flies; verruga peruana/Peruvian wart
    2. B. Quintana: trench fever, louse borne
    3. B. henselae: cat-scratch disease in immunocompetent, bacillary angiomatosis in HIV

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