This blog post is the outline of my lecture class for bachelor level (MBBS). It will be useful for the students for a quick review of the most important facts regarding typhus fever and other related diseases of the Rickettsiaceae family.
Important characteristics of the 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 staining, etc.
- Morphology: Very short rods (coccobacilli) barely visible in a 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 the 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 bloodstream
- Endothelial cells in the bloodstream engulf these organisms (induced by the organism itself) and are carried to the cell cytoplasm within a vacuole.
- Organisms escape from the vacuole or phagosome.
- Multiplication of the organisms- which causes cell injury and cell deaths manifested as vascular lesions which disseminated throughout the body
- Skin, heart, brain, lungs, and muscles are primarily affected
- Organisms transmit inside the body via cell-to-cell spread, lymphatic drainage, and 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 is 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 the 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.
- The rash becomes petechial and spreads over the entire body but spares the face palms and soles
Brill Zinsser disease (Recrudescent typhus)
The 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 of incubation, the 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 the untreated mortality rate is 2%.
- Was a prominent problem during World War II and the 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 is available.
- Prevention by controlling mite populations
Laboratory diagnosis of Typhus fever
Sample: Blood is drawn soon after the onset of illness.
Culture: Limited usefulness; technically difficult and also hazardous
Culture: In guinea pigs, mice, or embryonated eggs. Culture facilities are 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 immunofluorescence antibody test: used to detect rickettsiae in ticks and sections 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 evidence of infection occurs after the second week of illness.
- A four-fold or greater rise in titer between the acute and convalescent serum (obtained 2 weeks after the acute sample was taken) samples is diagnostically significant.
- A single acute titer of 1: 128 or greater is accepted as presumptive evidence.
Commonly employed tests are:
- Indirect immunofluorescence assay (IFA):
- Enzyme Immunoassay e.g. ELISA
- Weil-Felix test
- Least specific but widely used test
- Test based on immunological cross-reaction: antigenic cross-reactivity.
- Proteus vulgaris OX-19 and OX-2 and Proteus mirabilis OX-K strain agglutination
- 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. The sensitivity of PCR to detect Rocky Mountain Spotted Fever is about 70% compared to that of skin biopsy with immunocytology.
- Line blot
- Western immunoblotting
After reading this blog, test your knowledge with these questions: Top Ten most important Multiple Choice Questions (MCQs) about rickettsial disease.