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
Spectrum of Rickettsial diseases
Rickettsia encompasses following three groups of bacteria:
- The spotted fever group: Rickettsia akari , R.rickettsii
- The typhus group: R. prowazekii, R. typhi
- 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
- Arthropod vectors deposit the organism in the blood stream
- Endothelial cells in the blood stream engulf these organisms (induced by the organism itself) and carried to the cell cytoplasm within a vacuole.
- Organism escape from the vacuole or phagosome.
- Multiplication of the organisms- which causes cell injury and cell deaths manifested as vascular lesions which disseminated through out the body
- Skin, Heart, Brain, Lungs and Muscles are primarily affected
- Organisms transmits inside the body via cell to cell spread, lymphatic drainage, hematogenous route and can also be latent (e.g. R. prowazekii)
- Formation of disseminated endothelial lesions
- Activation of clotting systems
- disseminated intravascular coagulopathy (DIC)
- Death usually by cardiac failure
Epidemic typhus (Louse borne typhus)
- The most severe of the rickettsial disease
- Caused by Rickettsia prowazekii
- Not zoonoses
- Transmitted by human body lice (Pediculus humanus corporis) and humans are only natural vertebrate host-
- Sudden onset of chills, fever, headache and other influenza like symptoms approximately 1-3 weeks after the louse bite
- Development of maculopapular rash on the trunk and spread peripherally.
- Rash becomes petechial and spreads over the entire body but spares the face palms and soles
Brill Zinsser disease (Recrudescent typhus)
Recurrent form of epidemic typhus.
Signs and symptoms: similar to those of epidemic typhus but are
- less severe
- shorter duration
- and rarely fatal
- Does not cause skin rash
- Flea borne endemic typhus
- Causative agent: Rickettsia typhi
- Vector: Fleas from infected rats
- Autoinoculation: via itching.
- After 10 to 14 days’ incubation, onset of fever, chills and a crushing headache is abrupt followed by a rash in 3 to 5 days.
- The disease is self limiting and lasts about 2 weeks
- If untreated mortality rate is 2%.
- Was a prominent problem during World War II and Vietnam war
- Causative agent Rickettsia tsutsugamushi
- Vector: Chigger mite.
- After 10- 12 days of incubation, scrub typhus begins abruptly with fever, chills and headache.
- Many patients develop sloughing lesions at the bite sites and later a generalized spotty rash.
- Fatality rate up to 50%, rare with prompt antibiotic treatment
- No vaccine available.
- 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.
- Obligate Intracellular bacteria: Cant be grown in routine bacteriological culture media
- Rickettsiae can be grown in cell culture or embryonated egg,
- Risk of laboratory acquired infections is very high
- Should be handled only by very experienced technicians and
- only in specially equipped isolation laboratories.
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:
- Indirect immunofluorescence assay (IFA):
- Widely used
- used to detect IgG and IgM
- Enzyme Immunoassay e.g. ELISA
Other tests are
Proteus vulgaris OX-19 and OX-2 and Proteus mirabilis OX-K strain agglutination
- Least specific but widely used test
- Test based on immunological cross-reaction: Antigenic cross reactivity.
- detects antirickettsial antibodies in a patient’s serum by agglutination of the Proteus organisms
- Principle: Several rickettsiae, such as Rickettsia prowazekii, Rickettsia tsutsugamushi, and R. rickettsii shares antigens with the OX strains of Proteus vulgaris.
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.