Overview
Rickettsiae are small, pleomorphic, Gram-negative bacilli that are obligate intracellular parasites, requiring an arthropod vector as part of their natural transmission cycle. Named after Howard Taylor Rickett, who died of typhus contracted while studying this organism, they possess both DNA and RNA and a peptidoglycan cell wall, yet cannot be seen by ordinary light microscopy.
Family Rickettsiaceae contains genera Rickettsia, Orientia and Ehrlichia. Related but reclassified organisms — Coxiella burnetii (Q fever) and Bartonella species — are covered together in this lesson due to their overlapping clinical relevance as fastidious, vector-associated pathogens.
Learning Objectives
After this lesson you will be able to…- Enumerate the members of family Rickettsiaceae and describe their general characteristics.
- Describe the cultivation requirements and antigenic structure of rickettsiae.
- Differentiate the typhus fever group from the spotted fever group of diseases.
- Explain the Weil-Felix reaction and its diagnostic significance.
- Describe Ehrlichia, Coxiella burnetii (Q fever) and Bartonella species and the diseases they cause.
Clinical Story
Why This MattersA trekker returns from a forested hill region with high fever, severe headache, and a dark scab-like lesion (eschar) at the site of an insect bite, followed by a maculopapular rash spreading from the trunk. The physician suspects scrub typhus. The lab performs a Weil-Felix reaction, testing the patient's serum against non-motile Proteus strains — strong agglutination with OXK antigen supports the clinical suspicion of Orientia tsutsugamushi infection, guiding immediate doxycycline therapy.
Core Concepts
Rickettsiae are pleomorphic coccobacilli (0.3–0.6 µm × 0.8–2 µm) with a trilaminar cytoplasmic membrane. They are Gram-negative but do not take the stain well; they stain deep red with Machiavello and Gimenez stains and bluish-purple with Giemsa and Castaneda stains.
Being obligate intracellular parasites, they cannot be grown on cell-free media. They generally grow in the cytoplasm of infected cells (spotted fever rickettsiae also grow in the nucleus). Optimum growth temperature is 32–35°C; they can be cultivated in the yolk sac of 5–6 day embryonated eggs, on HeLa/Hep-2/mouse fibroblast cell lines, or by primary isolation in mice/guinea pigs.
Rickettsiae possess three types of antigens: a group-specific soluble antigen (surface protein), a species-specific antigen (acts as adhesin), and an alkali-stable polysaccharide shared with some non-motile Proteus strains (OX19, OX2, OXK). This antigen sharing forms the basis of the Weil-Felix reaction, a non-specific agglutination test used for diagnosis.
| Disease | OX19 | OX2 | OXK |
|---|---|---|---|
| Epidemic typhus | ++++ | ± | − |
| Endemic typhus | +++ | ± | − |
| Spotted fever group | ++ | ++ | − |
| Scrub typhus | − | − | +++ |
| Rickettsial pox | − | − | − |
Epidemic typhus — caused by R. prowazekii, transmitted by human body/head louse; humans are the only natural vertebrate host. Fever, chills, severe headache, and a maculopapular rash beginning on the trunk and spreading to limbs (sparing face, palms, soles); case fatality ~40%.
Brill-Zinsser disease — recrudescent typhus from latent lymphoid tissue infection; milder, shorter duration, lower fatality, but patient remains infectious for lice, risking new outbreaks.
Endemic typhus — caused by R. typhi, reservoir is rat, transmitted by rat flea (Xenopsylla cheopsis); milder disease, case fatality ~1%. Differentiated from epidemic typhus by the Neil-Mooser (tunica) reaction in male guinea pigs (positive for R. typhi, negative for R. prowazekii).
Members include R. rickettsii (Rocky Mountain spotted fever — most serious, mortality 6–70%, transmitted by ticks), R. siberica, R. conori (Indian tick typhus), R. japonica, and R. akari (rickettsial pox, transmitted by mites, mildest form).
Scrub typhus, caused by Orientia tsutsugamushi, is transmitted by trombiculid mite larvae (chiggers); migratory birds may act as reservoirs. Presents with headache, fever, conjunctivitis, deafness and an eschar at the bite site, followed by a maculopapular rash; mortality 10–60%.
Ehrlichia are small, Gram-negative, obligate intracellular bacteria with affinity for blood cells; their intracytoplasmic vacuole inclusions are called morulae. They are tick-borne, causing leucopenia, thrombocytopenia and raised liver enzymes. Three species infect humans: E. sennetsu, E. chaffeensis, E. phagocytophila. Diagnosis is by morula demonstration in Giemsa-stained blood smears, immunofluorescence, or PCR. Doxycycline is the treatment of choice.
Coxiella burnetii, no longer classified in genus Rickettsia, causes Q fever — a worldwide zoonosis transmitted among cattle, sheep and poultry by ticks, and to humans via infected milk, wool, meat, or contaminated soil/straw. It survives holder pasteurization (63°C/30 min) but not flash pasteurization. Presents with fever, chills, headache, pneumonia, endocarditis; spontaneous recovery is usual.
Bartonella bacilliformis causes Oraya fever (Carrion's disease), transmitted by sandfly, with organisms found inside erythrocytes; case fatality 40%. Bartonella quintana causes trench fever, spread by body louse. Bartonella henselae causes cat-scratch disease, transmitted by cat scratch/bite, and can cause bacillary angiomatosis in AIDS patients.
Weil-Felix Reaction & Key Vectors
Diagnostic Panel⚠️ Values summarised from standard microbiology references; always confirm with your laboratory's SOPs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Weil-Felix reactive with OX19 (high titre), OX2 negative | Suggestive of epidemic or endemic typhus | Differentiate using Neil-Mooser reaction and clinical/epidemiological context |
| Weil-Felix reactive with OXK only | Suggestive of scrub typhus (Orientia tsutsugamushi) | Look for eschar and mite exposure history; start doxycycline early |
| Weil-Felix non-reactive across all antigens | Consistent with rickettsial pox or early-phase infection | Repeat testing later in illness or use specific IFA/PCR |
| Morulae seen in Giemsa-stained leukocytes with leucopenia | Suggestive of Ehrlichiosis | Confirm by immunofluorescence or PCR; treat with doxycycline |
Common Errors & How to Avoid Them
Cause: Weil-Felix is non-specific and can give false negatives, especially early in illness or with rickettsial pox.
Prevention: Correlate with clinical/epidemiological features and confirm with specific tests (IFA, ELISA, PCR) where available.
Cause: These are obligate intracellular organisms and will not grow on cell-free media.
Prevention: Use embryonated eggs, cell culture, or animal inoculation for isolation, with appropriate biosafety precautions.
Cause: Coxiella burnetii is highly infectious via aerosol/ingestion and often under-recognised.
Prevention: Take a thorough occupational/animal exposure history in patients with atypical pneumonia or unexplained fever.
Laboratory Tips from the Bench
Always correlate the pattern of Weil-Felix reactivity (OX19/OX2/OXK) with the eschar location and vector exposure history — this combination narrows the rickettsial diagnosis quickly at the bedside.
Remember that rickettsial pox is Weil-Felix negative across the board — a negative Weil-Felix does not rule out rickettsial disease.
Remember OXK for scrub typhus with the mnemonic 'scrub = OXK' — the only rickettsial disease reactive with OXK.
Important Notes
Coxiella burnetii culture from clinical specimens carries a serious laboratory-acquired infection risk and is generally not recommended outside specialised reference laboratories.
Tetracyclines and chloramphenicol are effective across most rickettsial diseases; doxycycline is preferred for Ehrlichia and Coxiella infections.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeFever for 5 days, severe headache, conjunctivitis and a painless dark scab (eschar) on the inner thigh, followed by a maculopapular rash spreading from the trunk.
The combination of an eschar, fever, conjunctivitis, thrombocytopenia and a Weil-Felix reaction reactive specifically with OXK strongly supports a diagnosis of scrub typhus caused by Orientia tsutsugamushi.
- →Eschar formation at the site of chigger bite is a key diagnostic clue for scrub typhus.
- →Weil-Felix reactivity limited to OXK is characteristic of scrub typhus among rickettsial diseases.
- →Doxycycline should be started promptly once scrub typhus is clinically suspected, without waiting for full serologic confirmation.
Frequently Asked Questions
It exploits antigens shared between rickettsiae and certain non-motile Proteus strains, providing an inexpensive, widely-available screening test, though it should always be interpreted alongside clinical findings and confirmed with more specific methods where possible.
Unlike true rickettsiae, Coxiella burnetii is not obligately transmitted by an arthropod vector to humans (though ticks maintain it among animals) and can survive outside host cells for extended periods, explaining its transmission via aerosols, milk and contaminated dust.
Bartonella species belong to a separate family (Bartonellaceae) and, unlike obligate intracellular rickettsiae, can be cultured on enriched blood/chocolate agar, though growth is often slow.
Quick Revision
10-Minute ReviewKey Takeaways
- Rickettsiaceae comprises obligate intracellular, arthropod-transmitted, Gram-negative organisms causing typhus and spotted fever group diseases.
- The Weil-Felix reaction, based on shared antigens with Proteus strains, remains a useful though non-specific diagnostic screening tool.
- Scrub typhus is uniquely OXK-reactive and characteristically presents with an eschar at the bite site.
- Ehrlichia, though related, is diagnosed by demonstrating intracytoplasmic morulae in blood smears.
- Coxiella burnetii (Q fever) and Bartonella species, though reclassified outside genus Rickettsia, share fastidious, vector- or exposure-associated features relevant to clinical microbiology.
- Tetracyclines/doxycycline remain the mainstay of treatment across this group of organisms.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 32: Rickettsiaceae.
- Collee JG, et al. Mackie & McCartney Practical Medical Microbiology, 14th ed.
- Centers for Disease Control and Prevention. Rickettsial Diseases Clinical Overview.