Rickettsial Infections: Scrub Typhus & Q Fever

OVERVIEW OF RICKETTSIAL INFECTIONS

I. SCRUB TYPHUS

Etiology

• Caused by Orientia tsutsugamushi (formerly Rickettsia tsutsugamushi)
• Differs substantially from Rickettsia species genetically and in cell-wall composition — it lacks lipopolysaccharide
• Obligate intracellular pathogen; infects endothelial cells and macrophages
• Remarkable antigenic diversity — numerous strains with loss of cross-protective immunity as quickly as 1 month after infection

Epidemiology

• Vector: trombiculid mites (chiggers) — only the larval stage feeds on a host
• Transmission: infected larval mites inoculate organisms into skin at bite site
• Reservoir: maintained by transovarial transmission in mites
• Endemic and re-emerging in eastern and southern Asia, northern Australia, western Pacific and Indian Ocean islands
• In some endemic areas, >3% of the population is infected or reinfected each month
• Immunity to homologous strain wanes over 1–3 years
• Emerging cases reported in Chile; serologic evidence in South/Central America and Africa
• Orientia chuto discovered in southwestern Asia — challenging classic epidemiology
• High-risk exposure: areas of heavy scrub vegetation during the wet season

Pathogenesis

• Chigger bite → inoculation of O. tsutsugamushi → local replication in skin → eschar formation
• Organisms spread via lymphatics and bloodstream → systemic endothelial infection
• Target: endothelial cells and macrophages → vasculitis (perivasculitis)
• Vascular injury → tissue edema, organ dysfunction
• Interstitial pneumonia, myocarditis, meningoencephalitis from endothelial involvement
• Vigorous immune response: lymphocyte-mediated macrophage activation

Signs and Symptoms

• Incubation period: 6–21 days
• Onset: fever, headache, myalgia, cough, gastrointestinal symptoms
• Classic triad (often incomplete in indigenous/endemic populations):
  1. Eschar at chigger feeding site (painless, black-crusted ulcer with erythematous halo) — present in <50% of returning travelers; far less common in endemic populations
  2. Regional lymphadenopathy (± generalized lymphadenopathy)
  3. Maculopapular rash — appears on day 4–6, trunk-predominant; present in <40%
• Pulmonary involvement: interstitial pneumonitis, nonproductive cough, bibasilar rales — prominent in ~35%
• CNS: confusion, stupor, meningoencephalitis (severe cases)
• Relative bradycardia (Faget sign) may be present
• Splenomegaly, hepatomegaly
• Myocarditis, acute respiratory distress syndrome (ARDS) in severe cases
• Untreated illness: average 12 days (range 9–18 days); can be fatal
• Case-fatality rate without treatment: can reach 30% depending on strain; with treatment, <1%

Diagnosis

1. Clinical diagnosis based on epidemiologic exposure + classic features (eschar + fever in endemic area is highly suggestive)
2. Serology (standard):
   • Indirect immunofluorescence assay (IFA) — gold standard; requires acute and convalescent serum (4-fold titer rise confirmatory)
   • Weil-Felix reaction (OX-K agglutination) — historically used, low sensitivity/specificity
3. PCR of blood, eschar swab, or biopsy — most useful early in illness; eschar swab is highest yield
4. Immunohistochemistry of eschar biopsy
5. Culture — rarely performed; biosafety level 3 required
6. Lab: thrombocytopenia, elevated LFTs, hyponatremia, leukopenia early/leukocytosis late

Differential Diagnosis

• Other rickettsial infections (typhus group, spotted fever group)
• Leptospirosis (also causes eschar-like lesion in some cases, jaundice more prominent)
• Dengue (similar rash, thrombocytopenia; no eschar)
• Typhoid fever (rose spots vs. maculopapular rash; slower pulse; no eschar)
• Malaria
• Meningococcemia
• Viral hemorrhagic fevers
• Infectious mononucleosis
• Enteroviral infections

Treatment

• Doxycycline 100 mg PO/IV twice daily × 7 days — first-line; should be started empirically without waiting for confirmatory serology
• Azithromycin — effective alternative, especially in pregnancy and children (500 mg daily × 3 days)
• Chloramphenicol — effective but more toxic; option in settings without doxycycline
• Rifampicin — used in doxycycline-resistant strains (reported in northern Thailand and Pescadores Islands)
• Tetracycline — alternative; less preferred than doxycycline
• Response to doxycycline is usually rapid (defervescence within 24–36 hours) — lack of response suggests alternative diagnosis or resistance

Prevention

• Avoid chigger habitat (scrub vegetation in endemic areas); wear protective clothing
• Insect repellents (DEET) on skin and permethrin on clothing
• Routine inspection for chiggers after field exposure
• No licensed vaccine available
• Weekly prophylactic doxycycline has been used (200 mg/week) for travelers; evidence is limited
• Control: acaricides in high-risk areas; rodent control (reservoir reduction)

II. Q FEVER

Etiology

• Caused by Coxiella burnetii
• Gram-negative bacterium; unique among rickettsiae — no arthropod vector required for human infection; transmitted primarily by aerosol
• Obligate intracellular pathogen — multiplies in acidic phagolysosomal vacuoles of macrophages and monocytes
• Phase variation:
  • Phase I (virulent): expresses full-length LPS; found in natural infections
  • Phase II (avirulent): LPS-deleted mutant; develops in cell culture; more reactive in acute infection serology
• C. burnetii is genetically heterogeneous — strains vary in clinical severity
• Extremely hardy: survives for weeks to months in the environment; resistant to heat, desiccation, UV light
• Inhalation of a single organism can cause infection
• Classified as a CDC Category B bioterrorism agent

Epidemiology

• Worldwide zoonosis — found on every continent except New Zealand and Antarctica
• Principal reservoir: sheep, goats, cattle (ruminants); also cats, dogs, rabbits, birds, and ticks
• Shedding: infected animals shed C. burnetii in birth products (highest concentration), feces, urine, and milk
• Transmission routes:
  1. Inhalation of contaminated aerosols — primary route (parturient ruminants, contaminated soil/dust)
  2. Ingestion of unpasteurized dairy products
  3. Rarely: sexual intercourse, perinatal transmission, blood transfusion
• Ticks play a role in animal-to-animal spread but are not the primary vector for human infection
• High-risk groups: farmers, veterinarians, abattoir workers, laboratory workers
• Seasonal: peaks at lambing/calving season
• Large outbreak: Netherlands 2007–2010 (>4,000 cases) — linked to infected dairy goat farms
• Males have more severe disease; middle-aged individuals more frequently hospitalized
• Many U.S. soldiers infected in Iraq (Operation Iraqi Freedom)

Pathogenesis

• C. burnetii inhaled → phagocytosed by alveolar macrophages → survives and replicates within acidic phagolysosome (adapted to low pH environment, unlike most intracellular pathogens)
• Disseminates hematogenously to liver, spleen, bone marrow, cardiac valves, vascular endothelium
• Acute Q fever: formation of characteristic "doughnut" granulomas (fibrin-ring granulomas) in liver and other tissues — pathognomonic
• Chronic Q fever: C. burnetii persists despite high antibody titers → multiplies within macrophages → causes chronic endovascular infection; requires T-cell-mediated immunity for control
• Risk factors for chronicity: pre-existing valvular heart disease, vascular prostheses/aneurysms, immunocompromised state (including pregnancy, hematologic malignancy, HIV)

Signs and Symptoms

After infection:

• ~60% seroconvert without apparent disease
• ~38% experience self-limited acute illness
• Only ~2% require medical evaluation
• 0.2–0.5% develop chronic infection months to years later

Acute Q Fever (incubation 2–3 weeks):

• Fever (high, abrupt onset) ± rigors — the most constant feature; isolated prolonged fever in 15%
• Severe headache (retroorbital, frontal)
• Athralgias and myalgias
• Fatigue, anorexia, weight loss
• Hepatitis — elevated LFTs, jaundice (granulomatous hepatitis); may be the sole manifestation
• Pneumonia — dry cough, pleuritic chest pain, patchy infiltrates on CXR; occurs in 30–50%; sometimes severe
• Skin rash — uncommon (~10%); maculopapular; no eschar (key distinguishing feature from spotted fevers and scrub typhus)
• No eschar — C. burnetii does not produce an eschar
• Pericarditis, myocarditis (rare in acute phase)
• Aseptic meningitis, encephalitis (rare)
• Splenomegaly

Chronic Q Fever (>6 months after acute illness):

• Endocarditis — most important and life-threatening manifestation; occurs in ~60–70% of chronic cases
  • Culture-negative endocarditis on abnormal native or prosthetic valves
  • Most common: aortic and mitral valves
  • Features: progressive valvular destruction, embolic phenomena, hepatosplenomegaly, purpuric rash, clubbing, low-grade fever
• Vascular infection — infected vascular grafts or aneurysms (second most common manifestation)
• Osteomyelitis, chronic hepatitis, chronic fatigue syndrome ("post-Q fever fatigue syndrome")

Diagnosis

Serology (gold standard):

• Indirect immunofluorescence assay (IFA):
  • Acute Q fever: Phase II IgG ≥200, IgM ≥50
  • Chronic Q fever: Phase I IgG ≥800 (high Phase I titers distinguish chronic from acute)
  • Phase II antibody rises first and higher in acute; Phase I antibody predominates in chronic
• Enzyme immunoassay (EIA) available
• Complement fixation — older method; less sensitive

Other methods:

• PCR (blood, tissue) — most useful in first 2 weeks of acute illness, before antibody rise
• Culture — C. burnetii can be cultured in BSL-3 facilities; shell vial culture on HEL cells; impractical clinically
• Histopathology: fibrin-ring ("doughnut") granulomas in liver biopsy — characteristic but not pathognomonic
• Echocardiography — mandatory in chronic Q fever to assess valvular disease; TEE preferred

Lab findings:

• Elevated ALT/AST (hepatitis), mildly elevated bilirubin
• Thrombocytopenia, leukopenia or normal WBC
• Elevated CRP, ESR
• CXR: patchy infiltrates (round, segmental); pleural effusion possible

Differential Diagnosis

• Acute Q fever:
  • Community-acquired pneumonia (atypical: Mycoplasma, Legionella, Chlamydophila)
  • Viral hepatitis (A, B, C, EBV, CMV)
  • Typhoid fever
  • Brucellosis (also from livestock, similar granulomatous hepatitis)
  • Leptospirosis
  • Malaria
  • Other rickettsial diseases (but Q fever lacks eschar)
  • Sarcoidosis (doughnut granulomas also in sarcoidosis)
• Chronic Q fever/endocarditis:
  • Culture-negative endocarditis (Bartonella, Brucella, HACEK organisms, fungi)
  • Marantic endocarditis
  • Autoimmune disease (SLE, antiphospholipid syndrome)

Treatment

Acute Q fever:

• Doxycycline 100 mg PO twice daily × 14 days — first-line
• Alternatives: macrolides (azithromycin), fluoroquinolones (less preferred)
• Most acute cases are self-limited; treatment shortens duration and prevents chronicity

Chronic Q fever/Endocarditis:

• Doxycycline + hydroxychloroquine (combination) for ≥18 months (native valve) or ≥24 months (prosthetic valve)
  • Hydroxychloroquine alkalinizes the phagolysosome → reduces MIC of doxycycline against C. burnetii significantly
  • Monitor hydroxychloroquine levels (ophthalmic toxicity); serology-guided duration
• Surgical valve replacement may be required
• Phase I IgG titer monitored to guide duration; treatment continued until Phase I IgG <200

Special populations:

• Pregnancy: doxycycline avoided → cotrimoxazole (TMP-SMX) throughout pregnancy; high risk of obstetric complications if untreated
• Post-Q fever fatigue syndrome: supportive; limited evidence for prolonged doxycycline

Prevention

• Vaccination: Q-VAX (phase I whole-cell vaccine) — licensed in Australia; highly effective; requires pre-vaccination screening (skin test + serology) to avoid severe local reactions in previously sensitized individuals
• Pasteurization of milk and dairy products
• Hygienic handling of birth products from ruminants; proper disposal of placenta, birth fluids
• Personal protective equipment for high-risk occupations (farmers, vets, abattoir workers)
• Quarantine of infected herds; testing of livestock in endemic areas
• Notification of Q fever is mandatory in many countries

COMPARATIVE SUMMARY TABLE