1. RICKETTSIACEAE FAMILY:
General characteristics:
Consists of 3 genera:
Rickettsia,
Ehrlichia,
Coxiella
Obligate intracellular parasites.
Small Gram (-) coccobacilli ( um)
Cell membrane similar to Gram (-) bacteria with LPS & peptidoglycan.

2. General characteristics:
The organisms will not show up on Gram stain, but can be seen when either Gimenez or Giemsa stains are used.
Require growth co-factors (acetyl-CoA, NAD, ATP).
Will not grow on artificial media.
Grown in embryonated eggs or tissue culture.
Cultivation is costly and hazardous because aerosol transmission can easily occur.
All, except Coxiella, are transmitted by arthropod vectors as lice, ticks, fleas and/or mites.

3. Gimenez stain of tissue culture cells infected with Rickettsia rickettsii

4. Rickettsia: Transmission:
Rickettsia are usually introduced into human skin by the bite of an insect (flea or louse) or an arachnid (tick or
mite).
Rickettsiae multiply in endothelial cells of microvessels.
Incubation period: ~1 week
Virulence factors of Rickettsial species:
Induced phagocytosis
Recruitment of actin for intracellular spread

5. Arthropod Vector Rickettsia rickettsii

6. Pathogenesis:
- During the first few days of incubation period, local reaction caused by hypersensitivity to tick or vector products
- Bacteria multiply at the site & later disseminate via lymphatic drainage
- Bacteria is phagocytosed by macrophages (1st barrier to rickettsial multiplication)
- After 7-10 days, organisms disseminate. Replicate in the nucleus or cytoplasm of endothelial cells causing vasculitis

7. Pathogenesis:
- Infected cells show intracytoplasmic inclusions & intranuclear inclusions
- Endothelial damage & vasculitis progress causing the development of maculopapular skin rashes, perivascular tissue necrosis, thrombosis & ischemia
- Disseminated endothelial lesion lead to increased capillary permeability, edema, hemorrhage & hypotensive shock
- Endothelial damage can lead to activation of clotting system ---> DIC

8. Pathogenesis: Rickettsia cell-to-cell spread1

9. Rocky Mountain Spotted Fever:
Etiologic agent: Rickettsia rickettsiae
Most common rickettsial disease
At risk: individuals younger than 19 years old.
Males affected twice as often as females
Disease common during summer months
Serious disease with 35% mortality rate
Transmitted by ticks that must remain attached for hours in order to transmit the disease.
Incubation of 2-6 days
Followed by a severe headache, chills, fever, aching, and nausea.

10. After 2-6 days, a maculopapular rash develops, first on the extremities, including
palms and soles, and spreading
to the chest and abdomen.
If left untreated, the rash
will become petechial with
hemorrhages in the skin and
mucous membranes due to
vascular damage as the
organism invades the blood
vessels.
Death may occur during the end of the second week due to kidney or heart failure.

11. Rocky Mountain Spotted Fever1

12. Endemic Typhus: Etiologic agent: Rickettsia typhi
Incubation period: 5-18 days.
Transmitted to man by rat fleas.
The disease occurs sporadically.
Symptoms: severe headache, chills, fever, and after a fourth day, a maculopapular rash caused by subcutaneous hemorrhaging as Rickettsia invade the blood vessels.
The rash begins on the upper trunk and spread to involve the whole body except the face, palms of the hands, and the soles of the feet.
The disease lasts about 2 weeks and the patient may have a prolonged convalescence.

13. Ehrlichia: Disease: Ehrlichiosis Transmitted via tick vectors
Etiologic agent: E. chaffeensis
Parsitize leukocytes and grow in cytoplasmic vacuoles making characteristic inclusions known as morulae.
Symptoms resemble Rocky Mountain spotted fever
Clinically manifests as acute fever with leucopenia, thrombocytopenia & elevations of aminotransferase levels
Rash is infrequent
Vasculitis is rare

14. Coxiella burnetii: The only species of Coxiella genus.
Causal agent of “Q fever”.
Found in infected animals, arthropods or humans and highly infectious.
Transmission via:
Inhalation of airborne organisms (infected dusts in barnyards and slaughterhouses).
Ingestion of milk from infected animals.
It has spore-like form that resists heat and dryness allowing it to survive extracellulary.

15. Q fever Q for “query” or mysterious febrile illness
Occurs in veterinarians, ranchers, and animal researchers who are in contact with infected placenta from sheep, cattle, or goats (no arthropod vector for C. burnetii)
• Incubation period: days
• Disease characterized by fever, influenza-like syndromes; but no skin rash
Some patients present with bronchopneumonia with patchy interstitial infiltrates
Rare complications: hepatitis, endocarditis, and meningoencephalitis

16. Doughnut shaped noncaseating granuloma of Q fever

17. Laboratory Diagnosis:
1. Culture & isolation: difficult & dangerous because of the highly infectious nature of rickettsiae
2. Serologic test: widely used
Weil-Felix test: based on cross-reactivity between some strains of Proteus & Rickettsia
B. Complement fixation: not very sensitive & time consuming
C. Indirect fluorescence (EIA): more sensitive & specific; allows discrimination between IgM & IgG antibodies which helps in early diagnosis
D. Direct immunofluorescence: the only serologic test that is useful for clinical diagnosis, 100% specific & 70% sensitive allowing diagnosis in 3-4 days into the illness