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Describe etiology, pathogenesis and laboratory diagnosis of Typhus fever

This blog post is the outline of my lecture class in bachelor level (MBBS).  It will be useful for the students for quick review of the most important facts regarding Typhus fever and other related disease of Rickettsiaceae family.

Important characteristics of Rickettsiaceae  family

—  Obligate intracellular parasite

—  Multiply in the cytoplasm of host cells by binary fissions

—  Most rickettsiae survive only for short times outside a host (reservoir/vector.

—  Transmission by hematogenous route by blood sucking arthropod vectors: Lice, fleas, ticks and mites.

—  Gram staining reaction: Stain poorly with gram stain, cell wall resembles with that of gram negative rods

—  Alternative staining: Giemsa stain, acridine orange etc.

—  Morphology: Very short rods (coccobacilli) barely visible in light microscope

—  Rickettsiae grow readily in yolk sacs of embryonated eggs, but isolation is only done in reference laboratories because of bio-safety issues.

Rickettsiaceae family includes five genera

    1. Rickettsia
    2. Orientia
    3. Ehrlichia
    4. Anaplasma
    5. Neorickettsia

—  Spectrum of Rickettsial diseases

Rickettsia encompasses following three groups of bacteria:

    1. The spotted fever group: Rickettsia akari , R.rickettsii
    2. The typhus group: R. prowazekii, R. typhi
    3. The scrub/Chigger-borne typhus group: Orientia tsustsugamushi

Epidemiology and Pathogenesis

—  Human is an accidental host in many cases and disease is prevalent in wild animals.

—  Passed between animals and from animals to humans by insect vectors

—  In most cases Humans become infected by the bite of infected arthropod vectors

Pathogenesis

  1. —  Arthropod vectors deposit the organism in the blood stream
  2. —  Endothelial cells in the blood stream engulf these organisms (induced by the organism itself) and carried to the cell cytoplasm within a vacuole.
  3. —  Organism escape from the vacuole or phagosome.
  4. —  Multiplication of the organisms- which causes cell injury and cell deaths manifested as vascular lesions which disseminated through out the body
  5. —  Skin, Heart, Brain, Lungs and Muscles are primarily affected
  6. —  Organisms transmits inside the body via cell to cell spread, lymphatic drainage, hematogenous route and can also be latent (e.g. R. prowazekii)
  7. —  Formation of disseminated endothelial lesions
  8. —  Activation of clotting systems
  9. —  disseminated intravascular coagulopathy (DIC)
  10. —  Death usually by cardiac failure

Epidemic typhus  (Louse borne typhus)

Brill Zinsser disease (Recrudescent typhus)

—  Recurrent form of epidemic typhus.

—  Signs and symptoms: similar to those of epidemic typhus but are

  1. less severe
  2. shorter duration
  3. and rarely fatal
  4. Does not cause skin rash

Murine typhus

Scrub Typhus

  1. —  Was a prominent problem during World War II and Vietnam war
  2. —  Causative agent Rickettsia tsutsugamushi
  3. —  Vector: Chigger mite.
  4. —  After 10- 12 days of incubation, scrub typhus  begins abruptly with fever, chills and headache.
  5. —  Many patients develop sloughing lesions at the bite sites and later a generalized spotty rash.
  6. —  Fatality rate up to 50%, rare with prompt antibiotic treatment
  7. —  No vaccine available.
  8. —  Prevention by controlling mite populations

—  Laboratory diagnosis of Typhus fever

—  Culture: Limited usefulness; technically difficult and also hazardous

Sample: Blood drawn soon after onset of illness.

Culture: In Guinea pigs, mice or embryonated eggs.  Culture facility not available in Nepal.

Staining

—  Direct immunofluoresecent antibody test: used to detect rickettsiae in ticks and section of tissues. Most useful to detect R. rickettsii in skin biopsy specimens to aid in the diagnosis of Rocky Mountain Spotted Fever.

Serological tests: Laboratory diagnosis of rickettsial diseases is based on serologic analysis.

—  Serologic evidences of infection occurs after a second week of illness.

—  Four fold or greater rise in titer between the acute and convalescent serum (obtained 2 weeks after the acute sample taken) samples is diagnostically significant.

—  Single acute titer of 1: 128 or greater is accepted as presumptive evidence.

—  Commonly employed tests are:

  1. —  Indirect immunofluorescence assay (IFA):
  2. —  Widely used
  3. —  used to detect IgG and IgM
  4. —  Enzyme Immunoassay e.g. ELISA

—  Other tests are

—  Proteus vulgaris OX-19 and OX-2  and Proteus mirabilis OX-K strain agglutination

—  Line blot

—  Western immunoblotting

Weil-Felix test

Other tests: 

Polymerase Chain Reactions (PCR): Has been used to diagnose Rocky Mountain Spotted Fever, Scrub typhus, murine typhus and Q Fever.

Sensitivity of PCR to detect Rocky Mountain Spotted Fever is about 70% comparable to that of Skin Biopsy with immunocytology.

After reading this blog, test your knowledge with these questions: Top Ten most important Multiple Choice Questions (MCQs) about rickettsial disease.