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Introduction of Rickettsia

Howard Taylor Ricketts (1909): Isolated Rickettsia from a patient suffering from Rocky Mountain spotted fever, late on he died while working with this organism. Obligate intracellular parasite and Gram-negative pleomorphic rods are poorly stained with Gram stain but properly stained with Giemsa or Gimenez stains.

Normal habitat

Parasite of arthropods e.g. fleas, lice, ticks, mites

Scientific classification of Rickettsia

Domain: Bacteria

Phylum: Proteobacteria

Class: Alphaproteobacteria

Order: Rickettsiales

Family: Rickettsiaceae

Genus: Rickettsia

Species group: Spotted fever group

Species: R. rickettsii

Binomial name: Rickettsia rickettsii

Note: Rickettsiae include the genera Rickettsia, Ehrlichia, Orientia, and Coxiella. which are zoonotic pathogens that cause infections that disseminate in the blood to many organs.

Morphology of Rickettsia

Rickettsia is a short rod, pleomorphic, Gram-negative, measuring 600 nm x 300. They may appear singly, in pairs, in short-chain, filamentous form, or in coccus, from containing both DNA and RNA. The cell wall contains peptidoglycan that contains muramic acid and diaminopimelic acid. They divide like bacteria. They are obligate intracellular organisms and unable to grow in cell-free media.  In tissues ( HEP-2 cell, HeLa cell, and also in the yolk sac of chick embryo) culture, the generation time is 8- 10 hours at 34°C.

Other properties:

1. Rickettsial growth is enhanced when the metabolic activity of the cell is lowered.
2.  The growth of rickettsiae is enhanced in the presence of Sulphonamide.
3.  Rickettsiae are sensitive to tetracycline and chloramphenicol.
4. Most important species of Rickettsia

Typhus group (Louse borne)

• R. prowazenki
• R. typhi

Scrub typhus (Mite borne)

• R. tsutsugamushi

Spotted fever (Ticket borne)

• R. conori
• R. siberica
• R. rickettsii

Pathogenesis of Rickettsia

Mode of transmission: by the bite of infected arthropod.

When the organism reaches the blood, disseminate and get localized chiefly in endothelial cells of small blood vessels of the skin and different organs and causing vasculitis. There is thrombosis of blood vessels leading to rupture and necrosis, also leading to develop disseminated intravascular coagulation (DIC)and homeostasis disturbances.

Clinical Features

Early signs and symptoms are nonspecific and may be similar to viral illnesses, making the diagnosis more difficult but later may show the following cardinal features-

• fever
• headache
•  rash
• confusion
• myalgia

Complications of rickettsial diseases

If treatment is not proper at right time, the patient may show the following complications-

• Bronchopneumonia
• Congestive heart failure
• Multi-organ failure
• Deafness
• Disseminated intravascular coagulation (DIC)
• Myocarditis
• Endocarditis
•  Glomerulonephritis

Diseases   Causative agent  Reservoir

Epidemic typhus, Brill-Zinser disease:   R. prowazekii  Human body louse
Endemic typhus: R. typhi      Rat flea
Rocky- Mountain Spotted fever:  R. rickettsii  Ticks
Australian tick fever:  R. australis   Ticks
Siberian tick typhus:  R. siberica   Ticks
Boutonneuse fever:   R. coroni  Ticks
Rickettsial pox:    R. akari    Mites

Laboratory diagnosis of Rickettsia

Specimen

Skin biopsies, blood, and serum

Staining

Giemsa stain: blue

Macchiavello’s stain : red

Gimenez stains: Organism: red/magenta/pink

Background: blue-green (malachite green counterstain)

Culture and  isolation

Isolation of Rickettsia from blood and tissue is technically difficult. Whole blood or emulsified blood clot is inoculated in guinea pigs/mice or yolk sac of chick embryo and after 3 rd to 4th week. This laboratory animal response to different rickettsial species can vary.  It is difficult and dangerous because of the highly infectious nature of rickettsiae having symptoms like rising in temperature in all species, scrotal inflammation, swelling, necrosis by R. typhi, R. conori,  except R. prowazekii.

Serology test

• Weil – Felix reaction: Heterophile agglutination test and uses to find rickettsial antibodies in patient’s serum.
• Indirect immunofluorescence test for IgM: More sensitive and specific which allows discrimination between IgM and  IgG antibodies which helps in early diagnosis.  Direct immunofluorescence is the only serologic test that is useful for clinical diagnosis having  100% specific and 70% sensitive allowing diagnosis in 3-4 days into the illness.
• CFT: Not very sensitive  and  time consuming
• Latex particle agglutination
• ELISA

Weil- Felix Reaction in Rickettsial disease

• Non motile Proteus vulgaris
• OX 19,  OX 2,  OXK
• (As a source of antigen)
• Typhus fever +++      +/-       –
• Scrub typhus group –    –       +++/-
• Spotted fever group  +/++   +/++   –

 Procedure

Dilute serum in three separate series of tubes followed by the addition of equal amounts of OX 19, OX 2, OX K in 3 separate series of tubes. Incubate at 37°C for overnight and then after incubation observes for agglutination.

Result Interpretation

1. Strong agglutination with OX 19: Epidemic and endemic typhus.
2. Strong agglutination with OX 19 and OX 2: Spotted fever
3.  Strong agglutination with OX K:  Scrub typhus ( causative agent- Orientia tsutsugamushi )

Molecular Test

Rickettsial DNA or Omp genes detection by PCR assay is more rapid than isolation and allows specific identification.

Other tests

Hematology: Thrombocytopenia

Biochemistry: Slightly elevation of alanine​ aminotransferase (ALT) and aspartate aminotransferase (AST) may be detected. Neutropenia may be seen in more severe diseases.

CSF: Mild cerebrospinal fluid pleocytosis (presence of an abnormally large number of lymphocytes ) with monocyte predominance.

Treatment 

Antimicrobial therapy early in the first week of illness is highly effective and is found with the best outcome. Fever usually subsides within 1-3 days after starting antibiotic therapy. If fever fails to subside with the use of a suitable antibiotic, the diagnosis of rickettsial disease should be reconsidered. Doxycycline is the drug of choice; it is preferred over other tetracyclines ( earlier drug of choice) for the treatment of rickettsial infections. Chloramphenicol may be used as an alternative. Recent reports from Europe suggest that fluoroquinolones, such as ciprofloxacin and ofloxacin, may be effective in the treatment of certain rickettsial diseases.

Prevention and control of Rickettsia

• Following preventive measures against rickettsial infections are as follows-

1.  use of protective clothing
2.  insect repellents
3. avoidance of tick-infested areas,
4. personal hygiene

• The vaccine is not available against rickettsial diseases and thus in case of infections, we have doxycycline and chloramphenicol to treat them.

Key Notes on Rickettsia

• Despite the similar name, Rickettsia bacteria do not cause rickets, which is a result of vitamin D deficiency.
• No Human to human transmission
• Ricketts was devoted to his research and, on several occasions, injected himself with pathogens to study their effects.  In 1910, Ricketts became interested in a strain of typhus known as tabardillo, due to a major outbreak in Mexico City. Days after isolating the organism that he believed caused typhus, he himself died of the disease.
• The antibodies present in the serum 2–3 weeks after the onset of the disease remain in the serum after a very long period of time.
• Scrub typhus is a mite-borne disease caused by Orientia tsutsugamushi (previously called Rickettsia tsutsugamushi).

References

1. https://www.ncbi.nlm.nih.gov/books/NBK7624/
2. https://reference.medscape.com/article/
3. https://www.slideshare.net/NnirJhor/rickettsia
4. https://en.wikipedia.org/wiki/Rickettsia_rickettsii
5. http://textbookofbacteriology.net/Rickettsia_2.html
6. https://encyclopedia2.thefreedictionary.com/Macchiavello
7. https://www.sciencedirect.com/topics/medicine-and-dentistry/rickettsiaceae
8. https://www.cdc.gov/typhus/scrub/index.html
9. https://www.uptodate.com/contents/scrub-typhus-clinical-features-and-diagnosis