Rickettsia & Scrub Typhus
Medicine · Infectious Disease · lean revision notes
Rickettsia & Scrub Typhus
The rickettsioses are a group of obligate intracellular Gram-negative coccobacilli transmitted by arthropod vectors (mites, ticks, lice, fleas) that infect vascular endothelium and produce a small-vessel vasculitis. For NEET PG, the highest-yield member is scrub typhus (the eschar, Orientia tsutsugamushi, and doxycycline), but spotted fever (RMSF) and the typhus group are repeatedly tested through rash patterns, Weil-Felix reactions, and vectors.
Classification of rickettsial diseases
The family is traditionally split into the spotted fever group, the typhus group, and the scrub typhus group (now reclassified into genus Orientia). A separate cluster includes the Anaplasma/Ehrlichia (ehrlichioses) and Coxiella burnetii (Q fever), which behave differently.
| Group | Organism | Disease | Vector | Reservoir | Weil-Felix |
|---|---|---|---|---|---|
| Spotted fever | Rickettsia rickettsii | Rocky Mountain spotted fever (RMSF) | Hard tick (Dermacentor) | Dogs, rodents | OX-2 & OX-19 |
| Spotted fever | R. conorii | Indian tick typhus / Mediterranean spotted fever | Hard tick (Rhipicephalus) | Dogs | OX-2 & OX-19 |
| Spotted fever | R. akari | Rickettsialpox | Mite | Mice | Negative |
| Typhus | R. prowazekii | Epidemic (louse-borne) typhus | Body louse (Pediculus) | Humans (man) | OX-19 |
| Typhus | R. typhi | Endemic (murine) typhus | Rat flea (Xenopsylla) | Rats | OX-19 |
| Scrub typhus | Orientia tsutsugamushi | Scrub typhus | Larval mite "chigger" (Leptotrombidium) | Rodents | OX-K |
| — | Coxiella burnetii | Q fever | Inhalation (aerosols, no vector needed) | Cattle, sheep, goats | Negative |
High-yield: Scrub typhus is the only rickettsial disease that is Weil-Felix OX-K positive. RMSF and Indian tick typhus are OX-2 and OX-19 positive. Epidemic and endemic typhus are OX-19 positive only. R. akari, Q fever, and ehrlichiosis are Weil-Felix negative.
Etiology & pathophysiology
Rickettsiae are obligate intracellular organisms because they cannot synthesise their own NAD and CoA, so they parasitise host cytoplasm (spotted fever and typhus groups) — except Coxiella, which survives in the phagolysosome. They preferentially invade vascular endothelial cells, multiply, and spread cell-to-cell, producing a widespread small-vessel vasculitis. This vasculitis is the unifying lesion that explains the rash, increased capillary permeability, oedema, hypovolaemia, hypotension, and end-organ damage (encephalitis, pneumonitis, ARDS, acute kidney injury, myocarditis).
In scrub typhus, the larval mite ("chigger") bites and inoculates Orientia tsutsugamushi. A papule forms at the bite site, ulcerates, becomes necrotic, and is covered by a black crust — the eschar, the pathognomonic clinical clue. The organism then spreads via lymphatics and blood to cause generalised lymphadenopathy, hepatosplenomegaly, interstitial pneumonia, and meningoencephalitis. Endothelial injury → vascular leak → ARDS and shock in severe cases.
High-yield: The eschar of scrub typhus is the inoculation site of the mite bite, classically located in moist, occluded, intertriginous areas — axilla, groin, inframammary fold, genitalia, neck, beltline. Always strip the patient and look in skin folds; the eschar is painless and easily missed.
Clinical features
The classic prodrome of any rickettsiosis is an abrupt high fever with severe headache, myalgia, and malaise after an incubation of roughly 1–2 weeks.
Scrub typhus triad worth remembering: fever + eschar + regional/generalised lymphadenopathy, often with a maculopapular rash that begins on the trunk and spreads centrifugally to the limbs (the reverse of RMSF). Conjunctival suffusion, relative bradycardia, and deranged liver enzymes are common. Severe disease presents with ARDS, acute kidney injury, myocarditis, meningoencephalitis, and multi-organ dysfunction — a frequent cause of acute febrile illness with thrombocytopenia and jaundice in monsoon/post-monsoon India, mimicking dengue, leptospirosis, malaria, and enteric fever.
RMSF classically begins with a macular rash on the wrists and ankles that spreads centripetally to the trunk and characteristically involves the palms and soles, later becoming petechial/purpuric. Despite the name, most US cases are not in the Rocky Mountains. The triad of fever + headache + rash is present in a minority early, so empirical doxycycline is started on suspicion.
| Feature | Scrub typhus | RMSF (spotted fever) | Epidemic typhus |
|---|---|---|---|
| Organism | O. tsutsugamushi | R. rickettsii | R. prowazekii |
| Vector | Larval mite (chigger) | Hard tick | Body louse |
| Eschar | Present (pathognomonic) | Usually absent | Absent |
| Rash onset | Trunk → centrifugal | Wrists/ankles → centripetal, palms & soles | Trunk → centrifugal, spares palms/soles |
| Weil-Felix | OX-K | OX-2, OX-19 | OX-19 |
| Notable | Lymphadenopathy, ARDS | Most lethal rickettsiosis | Brill-Zinsser recrudescence |
High-yield: RMSF rash starts peripherally at the wrists/ankles and involves the palms and soles — a feature shared with secondary syphilis and hand-foot-mouth disease. Scrub typhus rash starts centrally on the trunk. This rash-direction contrast is a favourite single-best-answer discriminator.
High-yield: RMSF is the most severe / most fatal rickettsial disease, and delay in starting doxycycline is the strongest predictor of death. Epidemic (louse-borne) typhus is the only one whose reservoir is humans and can recrudesce years later as Brill-Zinsser disease.
Diagnosis & investigation of choice
Diagnosis is primarily clinical and serological; treatment must never wait for confirmation.
Diagnostic approach (stepwise flow):
- Suspect in any acute undifferentiated febrile illness with eschar, rash, or unexplained organ dysfunction in an endemic/monsoon setting. →
- Examine for eschar (strip and inspect all skin folds). →
- Order screening serology — Weil-Felix (cheap, low sensitivity/specificity; OX-K rise ≥1:320 suggests scrub typhus). →
- Confirm with the reference test — Indirect immunofluorescence assay (IFA) for IgM/IgG, the gold standard. →
- ELISA (IgM) is the practical, widely used field/hospital confirmatory test (good sensitivity, easier than IFA). →
- PCR (from eschar, buffy coat, or blood) is the most useful in the early/acute phase before antibodies rise and is the most specific. →
- Start doxycycline empirically — do not delay for results.
| Test | Role | Notes |
|---|---|---|
| IFA (immunofluorescence) | Gold standard | Detects IgM/IgG; needs paired sera (4-fold rise); reference lab |
| ELISA (IgM) | Practical confirmatory | Widely used; correlates well with IFA |
| PCR | Best in early acute phase | Eschar/blood; highly specific; positive before seroconversion |
| Weil-Felix | Screening only | Cheap; Proteus OX-K/OX-2/OX-19 cross-reaction; many false results |
| Giemsa / immunostain of eschar | Adjunct | Demonstrates organism in endothelium |
High-yield: IFA is the gold-standard / confirmatory test for scrub typhus and rickettsioses. Weil-Felix is only a screening test (poor sensitivity and specificity). PCR is the investigation of choice in the early phase before antibodies appear. A single high IgM titre or a four-fold rise in paired sera supports the diagnosis.
The Weil-Felix reaction is a heterophile agglutination test that exploits cross-reacting carbohydrate antigens shared between rickettsiae and certain Proteus strains (OX-2, OX-19, OX-K). It is rarely used in developed settings but still examined heavily in Indian PG exams because of the OX-K = scrub typhus association.
Laboratory clues that accompany rickettsioses: thrombocytopenia, leukopenia or normal counts, transaminitis, hyponatraemia, raised CRP, and elevated LDH. Hyponatraemia and thrombocytopenia in a febrile patient with an eschar are strongly suggestive.
Management / drug of choice
High-yield: Doxycycline is the drug of choice for ALL rickettsial diseases, including scrub typhus, RMSF, and the typhus group — at every age, even in children and pregnancy when the disease is life-threatening, because the benefit outweighs the small theoretical risk of dental staining with a short course.
- Doxycycline 100 mg BD (oral or IV) for 7 days, or for at least 48–72 hours after defervescence. Response is dramatic, with fever typically settling within 48 hours — a near-diagnostic "therapeutic response."
- Azithromycin is the preferred alternative and is the drug of choice in pregnancy and in doxycycline-resistant/poorly responsive scrub typhus (notably some strains). Single-dose regimens are used.
- Chloramphenicol is an older alternative, useful in pregnancy where azithromycin is unavailable; covers RMSF and typhus.
- Rifampicin has been used for doxycycline-resistant scrub typhus (Chiangrai strains) but should not be combined empirically if TB is possible.
High-yield: A rapid clinical response (defervescence within ~48 h) to doxycycline is itself a strong diagnostic pointer toward rickettsial disease and helps differentiate it from enteric fever, dengue, and leptospirosis. Beta-lactams and aminoglycosides are ineffective (intracellular organism).
Supportive care addresses the vasculitic complications: fluid resuscitation for vascular leak/shock, oxygen and ventilation for ARDS, and management of AKI and myocarditis.
Prevention: vector avoidance, protective clothing, permethrin-treated clothing and DEET repellents, clearing scrub vegetation, and rodent control. There is no licensed vaccine for scrub typhus. For epidemic typhus, delousing with insecticides is the key public-health measure. Doxycycline can be used as post-exposure prophylaxis for high-risk scrub typhus exposure (weekly dosing).
Complications
- ARDS / acute respiratory distress — interstitial pneumonitis is a leading cause of death in scrub typhus.
- Meningoencephalitis — CSF shows lymphocytic pleocytosis with normal/slightly low glucose, mimicking viral/TB meningitis.
- Acute kidney injury, myocarditis, hepatitis with jaundice.
- Septic shock and multi-organ dysfunction syndrome (MODS).
- Disseminated intravascular coagulation (DIC) and severe thrombocytopenia.
- RMSF: gangrene of digits (vasculitis/thrombosis), neurological deficits.
- Epidemic typhus: Brill-Zinsser disease (recrudescence years later).
High-yield: In a monsoon-season patient with fever, thrombocytopenia, transaminitis, AKI and ARDS, the differential of scrub typhus, leptospirosis, dengue, and malaria must all be considered; the eschar is the single most useful bedside finding favouring scrub typhus.
Key differentials
Acute undifferentiated febrile illness (AUFI) in India — distinguish by clues:
| Disease | Key distinguishing clue |
|---|---|
| Scrub typhus | Eschar, OX-K positive, lymphadenopathy, doxycycline responds fast |
| Leptospirosis | Conjunctival suffusion, calf myalgia, jaundice + AKI (Weil's), animal/water contact |
| Dengue | Retro-orbital pain, leukopenia, haemoconcentration, NS1/IgM positive, no eschar |
| Malaria | Periodic fever with chills/rigor, peripheral smear/RDT positive |
| Enteric fever | Step-ladder fever, relative bradycardia, rose spots, blood/Widal |
| Meningococcaemia | Rapidly spreading petechiae/purpura, shock |
Other rash differentials for the palm-and-sole rash of RMSF: secondary syphilis, hand-foot-and-mouth disease, and Kawasaki disease.
Mnemonics & eponyms
- Weil-Felix OX strains: "Kiss = scrub" — OX-K = sCrub typhus; "2 and 19 spot the tick" (OX-2 & OX-19 in spotted fever / tick typhus); OX-19 alone in typhus group.
- Rash direction: "Scrub Starts at the Stomach (trunk → out); RMSF Runs from the Rim (wrists/ankles → in, palms & soles)."
- Eschar sites mnemonic ("look where it's dark and damp"): axilla, groin, inframammary, neck, beltline, genitalia.
- Eponyms: Brill-Zinsser disease (recrudescent epidemic typhus), Weil-Felix reaction, Rocky Mountain spotted fever, Q fever ("Query" fever, Coxiella).
Recently asked / exam angle
- Eschar identification image with "diagnosis?" → scrub typhus; "organism?" → Orientia (Rickettsia) tsutsugamushi; "vector?" → larval trombiculid mite (Leptotrombidium, chigger).
- Weil-Felix OX-K positive → scrub typhus (single most repeated one-liner).
- Drug of choice for rickettsial disease / scrub typhus → doxycycline; drug of choice in pregnancy → azithromycin.
- Gold standard for scrub typhus → IFA; best test in early phase → PCR; Weil-Felix = screening only.
- Rash starting on palms and soles / wrists and ankles → RMSF (R. rickettsii).
- Most fatal rickettsiosis → RMSF.
- Reservoir is humans / recrudescence years later (Brill-Zinsser) → epidemic louse-borne typhus (R. prowazekii).
- Rickettsial disease that is Weil-Felix negative / acquired by inhalation without a vector → Q fever (Coxiella burnetii).
- AUFI with thrombocytopenia + eschar in monsoon → scrub typhus over dengue/lepto.
Rapid revision
- Doxycycline = drug of choice for all rickettsioses (incl. children & pregnancy if severe); azithromycin preferred in pregnancy.
- Eschar is pathognomonic of scrub typhus — hunt in axilla, groin, inframammary, neck, beltline.
- Scrub typhus organism = Orientia tsutsugamushi; vector = larval mite (chigger, Leptotrombidium); reservoir = rodents.
- Weil-Felix OX-K positive = scrub typhus; OX-2 + OX-19 = spotted fever/tick typhus; OX-19 = typhus group.
- IFA = gold standard; PCR = best in early acute phase; ELISA-IgM = practical confirmatory; Weil-Felix = screening only.
- RMSF rash starts at wrists/ankles, involves palms & soles, spreads centripetally; it is the most fatal rickettsiosis.
- Scrub typhus rash starts on the trunk and spreads centrifugally (opposite of RMSF).
- Rickettsiae are obligate intracellular, cause small-vessel vasculitis → rash, leak, ARDS, shock.
- Brill-Zinsser = recrudescent epidemic typhus (R. prowazekii); humans are the reservoir; spread by body louse.
- Q fever (Coxiella burnetii) is Weil-Felix negative, acquired by inhalation, no vector needed.
- Defervescence within 48 h of doxycycline is a near-diagnostic therapeutic response.
- Beta-lactams/aminoglycosides are useless (intracellular pathogen); consider scrub typhus in any monsoon AUFI with thrombocytopenia, transaminitis, AKI or ARDS.