Microbiology of Rickettsia

General features:

• Small gram negative, aerobic, obligate intracellular (need CoA and NAD+) that infect mammals and arthropods.
• Non-motile, non-capsulated, non-sporing
• Contains both DNA and RNA
• Pleomorphic
• Microcapsule and slimelayer

Main Species:

1. R.prowazekii
2. R.typhi
3. R.ricketsii
4. Orientia tsutsugamushi

Pathogenesis:

Virulence Factors: Endotoxin, Phospholipase A, Slime Layer
Sites: Vascular system producing vasculitis
Characteristic triad of symptoms: Fever, Headache, Rash (Vasculitis)

1. Vector: a)bites   b)rubs faeces   c)is crushed into abraded skin
2. Entry of rickettsia into human body
3. Infection of endothelial cells (phospholipase A may help penetration)
4. Multiply in endothelial cells of microvessels
5. Eschar formation and Lymphadenitis
6. Reaches lymphatic circulation and are engulfed by macrophages
7. Able to survive and multiply intracellularly
8. Released by budding or lysis into the bloodstream
9. Infects endothelial cells
10. Vasulitis

Diseases, Vectors, Reservoirs and Incubation period:

• Rickettsial rash starts on hands and feet
• Typhus rash starts centrally and spreads outward without involving palms or soles
• “Rickettsia on the wRists, Typhus on the Trunk.”

Weil-Felix Reaction:

Patients with rickettsial infection have antibodies against Rickettsia. When patient serumis mixed with Proteus antigens, antirickettsial antibodies cross-react and agglutinate (Weil-Felix is negative in Coxiella infection).

Lab Diagnosis:

A. Specimen: Blood or emulsified blood clot

B. Culture and Isolation:
Yolk sac inoculation and tissue culture (HeLa, HEp2, Mouse fibroblast):

• Typhus group grow in cell cytoplasm
• Spotted fever group grow in nucleus

Grown over 4-7 days

Orientia tsutsugambushi: intraperitoneal inoculation of mice

C. Direct antigen detection:

• Direct immunofluorescent staining with fluorescence-conjugated polyvalent antigen
• PCR

D. Serological tests:

1. Weil-Felix reaction: The Weil-Felix test looks for the production of serum antibody that is reactive against Proteus OX19, OX2 or OXK antigens but it is not always reliable.
2. Rickettsial agglutination
3. Complement fixation
4. Indirect fluorescence
5. Direct fluorescence

Control:

Sanitary: Arthropod and rodent control; Milk pasteurization

Vaccine:

• a formaldehyde-inactivated suspension of Rickettsia prowazekii grown in embryonated eggs
• effective against epidemic typhus
• primary immunization consists of 2 subcutaneous injections 4 or more weeks apart
• booster doses are required every 6–12 months, as long as the possibility of exposure exists.
• A vaccine containing living rickettsiae of an attenuated strain of R. prowazekii has also been used.

Chemotherapeutic: Tetracycline and doxycycline