Rickettsia & Obligate Intracellular Bacteria

A high-yield cluster of zoonotic and arthropod-borne organisms that share two binding themes: they cannot make their own ATP (or are energy parasites) and grow only inside host cells, and almost all of them respond to doxycycline. NEET PG repeatedly tests Weil-Felix reaction, scrub typhus eschar, Chlamydia serovars and Q fever.

Why "obligate intracellular"?

These bacteria cannot be grown on cell-free (artificial) media. They require living host cells — cell lines, embryonated eggs, or laboratory animals. The classic obligate intracellular bacteria are Rickettsia, Orientia, Ehrlichia, Anaplasma, Coxiella, Chlamydia/Chlamydophila and (debatably) Tropheryma whipplei. Rickettsiae have leaky membranes and parasitise host ATP/CoA; Chlamydiae are true "energy parasites" depending on host ATP.

High-yield: The mnemonic for organisms that cannot be Gram stained well / cultured on artificial media — Rickettsia, Chlamydia, Coxiella, Mycoplasma (no cell wall), Treponema, Mycobacterium leprae, Tropheryma. Of these, the obligate intracellular ones are Rickettsia, Chlamydia and Coxiella.

Classification overview

Group	Genus / Species	Vector / Source	Disease	Diagnostic clue
Spotted fever	R. rickettsii	Hard (Dermacentor) tick	Rocky Mountain spotted fever	Centripetal rash (extremities → trunk), palms/soles
Spotted fever	R. conorii	Tick	Indian tick typhus / Mediterranean spotted fever	Tâche noire (eschar)
Typhus	R. prowazekii	Body louse (faeces)	Epidemic typhus; Brill-Zinsser (recrudescence)	Centrifugal rash sparing palms/soles
Typhus	R. typhi (mooseri)	Rat flea	Endemic/murine typhus	Milder
Scrub typhus	Orientia tsutsugamushi	Trombiculid mite (chigger)	Scrub typhus	Eschar + lymphadenopathy
—	Ehrlichia chaffeensis	Tick	Monocytic ehrlichiosis	Morula in monocytes
—	Anaplasma phagocytophilum	Tick (Ixodes)	Granulocytic anaplasmosis	Morula in neutrophils
Q fever	Coxiella burnetii	Aerosol (no vector needed)	Q fever	No rash; pneumonia/hepatitis
Chlamydia	C. trachomatis	Human	Trachoma, LGV, urethritis, neonatal disease	Serovar-specific
Chlamydia	C. pneumoniae (TWAR)	Respiratory	Atypical pneumonia	Young adults
Chlamydia	C. psittaci	Birds	Psittacosis	Bird exposure

Rickettsiae — biology & pathophysiology

Small, pleomorphic, obligate intracellular Gram-negative coccobacilli. They stain poorly with Gram; use Giemsa, Gimenez or Macchiavello stains. The core lesion is a vasculitis: organisms invade and multiply in vascular endothelial cells, causing increased capillary permeability, perivascular cuffing, microthrombi, oedema and a petechial/maculopapular rash. Widespread endothelial injury explains the multisystem features — interstitial pneumonitis, encephalitis, acute kidney injury, myocarditis and, in severe cases, gangrene of digits.

Spotted fever group lives free in cytoplasm and nucleus (actin-based motility); typhus group lyses the cell by bursting it. Transmission: spotted fever and scrub by bite/inoculation of tick or mite saliva; typhus group by rubbing infected faeces of louse/flea into the bite wound or mucosa.

Weil-Felix reaction (must-know)

A heterophile agglutination test exploiting cross-reacting antigens shared between rickettsiae and certain Proteus strains (OX-19, OX-2, OX-K). It is cheap and used where serology/PCR is unavailable, but is non-specific (false positives in Proteus UTI, leptospirosis, severe liver disease) and may be negative in Q fever and rickettsialpox.

Disease	OX-19	OX-2	OX-K
Epidemic / endemic typhus	++++	+	–
Spotted fevers (RMSF, tick typhus)	++	++	–
Scrub typhus	–	–	++++
Q fever / Rickettsialpox	–	–	–

High-yield: Scrub typhus = OX-K positive only. Q fever and rickettsialpox give a negative Weil-Felix (no cross-reacting Proteus antigen). The reference standard is indirect immunofluorescence assay (IFA) for IgM/IgG.

Mnemonic for Weil-Felix: "K for sKrub" (OX-K → scrub typhus).

Scrub typhus (most exam-relevant in India)

• Organism Orientia tsutsugamushi; vector trombiculid mite (chigger) — Leptotrombidium — which is also the reservoir (transovarial transmission). "Tsutsugamushi triangle" — Asia-Pacific including India.
• Eschar ("tâche noire"): painless black necrotic ulcer with erythematous margin at the chigger bite site — pathognomonic but seen in only ~he minority of Indian patients; look in warm, moist, covered areas (axilla, groin, under breast).
• Features: high fever, regional then generalised lymphadenopathy, headache, myalgia, conjunctival suffusion, maculopapular rash, and in severe cases ARDS, meningoencephalitis, AKI and multi-organ dysfunction. A common cause of acute undifferentiated febrile illness and AKI/ARDS in monsoon India.
• Investigation of choice: IgM ELISA (and IFA); PCR on eschar/blood in early disease. Weil-Felix OX-K supportive only.

High-yield: Scrub typhus is a leading cause of acute febrile illness with eschar + thrombocytopenia + transaminitis in India; a doxycycline trial → defervescence within 48 h is both diagnostic and therapeutic.

Epidemic typhus & Brill-Zinsser

R. prowazekii via the human body louse. Associated with war, famine, crowding, refugee camps ("jail fever", "camp fever"). Rash is centrifugal (trunk → periphery), classically sparing palms, soles and face. Brill-Zinsser disease is the late recrudescence years later (latent organisms reactivate), often milder, and such patients can re-seed lice. R. prowazekii also infects flying squirrels (sylvatic reservoir).

Rocky Mountain spotted fever

R. rickettsii, Dermacentor tick. The rash is centripetal (wrists/ankles → trunk) and characteristically involves palms and soles. Triad: fever, headache, rash. May progress to vasculitic shock, AKI and gangrene; most lethal rickettsiosis.

High-yield: Rash direction — RMSF = centripetal, palms+soles INVOLVED; Typhus = centrifugal, palms+soles SPARED. (Mnemonic: "Rocky → Reach the palms.")

Coxiella burnetii — Q fever

A unique rickettsia-like organism, now classed separately.

• No arthropod vector required for human disease — transmission is by inhalation of aerosols from infected parturient fluids, placenta, milk, urine of cattle, sheep, goats. Ticks maintain it among animals.
• Extremely low infectious dose (a single organism) and a spore-like, highly resistant form → a Category B bioterrorism agent.
• Exhibits phase variation: Phase I (virulent, in nature) and Phase II (avirulent, in culture) antigens — key for serology.
• Acute Q fever: self-limited flu-like illness, atypical pneumonia and/or granulomatous hepatitis ("doughnut/fibrin-ring granuloma" on liver biopsy). No rash. Weil-Felix negative.
• Chronic Q fever: culture-negative endocarditis (often on damaged/prosthetic valves), weeks-months later.

High-yield: Serology distinguishes the stage — Acute Q fever → high anti-phase II antibody; Chronic Q fever (endocarditis) → high anti-phase I antibody.

Treatment: acute → doxycycline; chronic endocarditis → doxycycline + hydroxychloroquine for 18 months.

Chlamydiae — biology

True energy parasites; biphasic life cycle between two forms:

1. Elementary body (EB) — small, infectious, metabolically inert, extracellular; attaches and enters by endocytosis. →
2. Inside a vacuole it becomes the Reticulate body (RB) — larger, replicating, non-infectious, metabolically active. →
3. RBs multiply → reorganise back into EBs → cell lysis releases new EBs.

The intracytoplasmic colonies are inclusion bodies (e.g., Halberstaedter-Prowazek bodies in trachoma) stained by Giemsa/iodine (glycogen-containing inclusions of C. trachomatis take iodine, unlike C. psittaci/pneumoniae).

Chlamydia trachomatis serovars — a perennial favourite

Serovars	Disease	Notes
A, B, Ba, C	Trachoma	Chronic kerato-conjunctivitis → blindness; faulty hygiene/flies
D–K	Genital infections, inclusion conjunctivitis, neonatal conjunctivitis & pneumonia, non-gonococcal urethritis, PID, reactive arthritis	STI / vertical
L1, L2, L3	Lymphogranuloma venereum (LGV)	Invasive; inguinal buboes

High-yield: Serovar mnemonic — "ABC → eyes (trachoma); D–K → genitals/babies; L → LGV."

• Trachoma (A, B, Ba, C): the leading infective cause of preventable blindness worldwide. WHO grading MacCallan / FISTO — Follicles, Intense inflammation, Scarring (Arlt's line on tarsal conjunctiva), Trichiasis, corneal Opacity; Herbert's pits (limbal follicle scars) are pathognomonic. SAFE strategy: Surgery, Antibiotics (azithromycin), Facial cleanliness, Environmental improvement.
• Neonatal disease (D–K): acquired during vaginal delivery — inclusion conjunctivitis at days 5–14 (later than gonococcal day 2–5) and afebrile staccato-cough pneumonia at 4–12 weeks with eosinophilia. Treat the baby with oral erythromycin/azithromycin (topical alone insufficient; risk of pneumonia).
• LGV (L1–L3): primary painless papule/ulcer → tender inguinal/femoral buboes with the "groove sign" (nodes above and below the inguinal ligament) → late proctocolitis, fistulae, genital elephantiasis. Frei test is historical; diagnose by NAAT/serology. Treat doxycycline 21 days.

Chlamydia pneumoniae & psittaci

• C. pneumoniae (TWAR): common cause of atypical ("walking") pneumonia and bronchitis in young adults; linked epidemiologically (not causally proven) to atherosclerosis. Iodine-negative inclusions; no animal reservoir.
• C. psittaci: psittacosis (ornithosis) from parrots, pigeons, poultry — atypical pneumonia with hepatosplenomegaly and relative bradycardia (Faget sign). Occupational (bird handlers).

Diagnosis — what to order

Rickettsia/Orientia: clinical + serology is the mainstay.

• IFA = reference standard (4-fold rise IgG, or IgM positivity).
• IgM ELISA widely used for scrub typhus (Indian setting).
• Weil-Felix — cheap, low sensitivity/specificity, supportive only.
• PCR of eschar/buffy coat — best in early disease before antibodies rise.
• Never culture routinely — biosafety level 3 hazard.

Coxiella: phase-specific serology (IFA); PCR; echocardiography for endocarditis. Chlamydia: NAAT (PCR) on urine/swab is investigation of choice; tissue culture (McCoy cells) historical; Giemsa for inclusions; direct fluorescent antibody.

High-yield: NAAT/PCR is the investigation of choice for Chlamydia (replaced culture). For rickettsial disease, IFA is the gold standard but empirical doxycycline must not be delayed for confirmation.

Management — the unifying answer

High-yield: Doxycycline is the drug of choice for essentially ALL of these — Rickettsia, Orientia (scrub typhus), Ehrlichia/Anaplasma, Coxiella, and Chlamydia (alternative). Start it on clinical suspicion; do not wait for serology.

Treatment flow: Suspect rickettsial illness (fever + eschar/rash + endemic exposure + thrombocytopenia/transaminitis) → start doxycycline 100 mg BD empirically → expect defervescence in 48 h (response supports diagnosis) → continue ~7 days / until afebrile.

• Pregnancy & severe scrub typhus: azithromycin is preferred (doxycycline relatively avoided in pregnancy, though a short WHO-endorsed course is acceptable in life-threatening rickettsioses). Chloramphenicol is an alternative in RMSF/typhus.
• Chlamydia genital infection: doxycycline 100 mg BD × 7 days (now preferred over single-dose azithromycin for urethritis/cervicitis); azithromycin 1 g single dose in pregnancy/poor compliance; LGV → doxycycline 21 days.
• Chronic Q fever endocarditis: doxycycline + hydroxychloroquine × 18 months.
• Sulfonamides are contraindicated in rickettsial disease — they worsen it.

Complications

• Scrub typhus: ARDS, meningoencephalitis, AKI, myocarditis, septic shock, DIC.
• RMSF: gangrene, encephalitis, non-cardiogenic pulmonary oedema, death if untreated.
• Epidemic typhus: myocarditis, gangrene, CNS involvement; later Brill-Zinsser.
• Trachoma: entropion, trichiasis, corneal scarring, blindness.
• LGV: strictures, fistulae, elephantiasis (esthiomene).
• Chlamydia D–K: PID, tubal infertility, ectopic pregnancy, Fitz-Hugh-Curtis perihepatitis, reactive arthritis (Reiter: can't see, can't pee, can't climb a tree).

Key differentials

A febrile patient with rash/eschar from the tropics — distinguish rickettsioses from:

Mimic	Discriminator
Leptospirosis	Conjunctival suffusion, myalgia (calf), jaundice + AKI (Weil's disease); can give false-positive Weil-Felix
Dengue	Retro-orbital pain, leucopenia, no eschar, NS1/IgM positive
Enteric fever	Rose spots, relative bradycardia, blood culture / Widal
Malaria	Periodic fever, smear/RDT positive
Meningococcaemia	Rapidly progressive purpura, no eschar

Recently asked / exam angle

• Weil-Felix antigen-disease matching (OX-K = scrub typhus; OX-19 = typhus; negative in Q fever) — repeatedly asked.
• Eschar + lymphadenopathy + AKI/ARDS in monsoon → scrub typhus, vector trombiculid mite/chigger, organism Orientia tsutsugamushi, treat doxycycline/azithromycin.
• Phase variation of Coxiella — anti-phase II = acute, anti-phase I = chronic endocarditis.
• C. trachomatis serovar matching (A–C trachoma, D–K genital/neonatal, L LGV) and groove sign of LGV.
• Rash direction: RMSF centripetal involving palms/soles vs typhus centrifugal sparing them.
• Drug of choice = doxycycline across the board; azithromycin in pregnancy; sulfonamides contraindicated.
• Chlamydia life cycle: EB infectious/inert, RB replicating/non-infectious; inclusion bodies.
• Obligate intracellular list / cannot be Gram-stained image-based questions.

Rapid revision

1. Obligate intracellular bacteria: Rickettsia, Orientia, Coxiella, Chlamydia, Ehrlichia, Anaplasma — no growth on artificial media.
2. Rickettsiae infect endothelial cells → vasculitis; stain with Giemsa/Gimenez, not Gram.
3. Weil-Felix: OX-K = scrub typhus; OX-19 = typhus group; negative in Q fever & rickettsialpox. IFA is gold standard.
4. Scrub typhus — Orientia tsutsugamushi, chigger (trombiculid mite) vector & reservoir; eschar + lymphadenopathy; IgM ELISA.
5. Epidemic typhus — R. prowazekii, body louse; Brill-Zinsser = recrudescence; rash centrifugal, spares palms/soles.
6. RMSF — R. rickettsii, Dermacentor tick; rash centripetal, involves palms/soles; most lethal.
7. Coxiella (Q fever) — aerosol, no vector needed, lowest infectious dose, bioterror agent; no rash, Weil-Felix negative; granulomatous hepatitis.
8. Q fever serology: acute = anti-phase II; chronic endocarditis = anti-phase I (treat doxy + hydroxychloroquine 18 months).
9. C. trachomatis: A–C trachoma, D–K genital/neonatal, L1–L3 LGV; EB infectious, RB replicating.
10. Neonatal chlamydial conjunctivitis day 5–14 + staccato pneumonia at 4–12 weeks → oral erythromycin/azithromycin.
11. LGV — inguinal buboes + groove sign; treat doxycycline 21 days.
12. Doxycycline = DOC for all; azithromycin in pregnancy; sulfonamides worsen rickettsioses.