Rickettsial Diseases¶
Chapter 192 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Rickettsial diseases are caused by obligate intracellular gram-negative bacteria, primarily transmitted by ticks, mites, fleas, or lice.
- Key diseases include Rocky Mountain Spotted Fever (RMSF), Mediterranean Spotted Fever, Scrub Typhus, Q Fever, and Ehrlichioses/Anaplasmosis.
- Clinical features include fever, rash (macular/maculopapular), eschars, and systemic complications like meningitis, pneumonia, or multiorgan failure.
- Doxycycline is the first-line treatment for most rickettsial infections, with chloramphenicol as an alternative in specific cases.
- Diagnosis relies on serology (IFA, PCR), clinical presentation, and epidemiologic context, with early treatment critical to prevent severe outcomes.
1. DEFINITION & OVERVIEW¶
Rickettsial diseases are caused by obligate intracellular gram-negative bacteria (Rickettsia, Orientia, Ehrlichia, Anaplasma, Neorickettsia, Coxiella) transmitted by arthropod vectors. These pathogens infect endothelial cells, leading to vasculitis and systemic inflammation. Clinical manifestations vary by species but often include fever, rash, and systemic complications.
Table 192-1 Features of Selected Rickettsial Infections¶
| DISEASE | ORGANI SM | TRANSMI SSION | GEOGRA PHIC RANGE | INCUBAT ION PERIOD, DAYS | DURATIO N, DAYS | RASH, % | ESCHAR, % | LYMPHA DENOPA THYa |
|---|---|---|---|---|---|---|---|---|
| Rocky Mountain Spotted Fever (RMSF) | Rickettsia rickettsii | Tick bite: Dermacen tor variabilis, D. anders oni, D. similis | United States, Mexico, C entral/Sou th America | 2–14 | 10–20 | 90 | <1 | + |
| Mediterra nean Spotted Fever (MSF) | Rickettsia conorii | Tick bite: Rhipiceph alus sang uineus, R. pumilio | Southern Europe, Africa, Middle East, Central Asia | 5–7 | 7–14 | 97 | 50 | + |
| DISEASE | ORGANI SM | TRANSMI SSION | GEOGRA PHIC RANGE | INCUBAT ION PERIOD, DAYS | DURATIO N, DAYS | RASH, % | ESCHAR, % | LYMPHA DENOPA THYa |
|---|---|---|---|---|---|---|---|---|
| African Tick-Bite Fever | Rickettsia africae | Tick bite: Amblyom ma hebra eum, A. v ariegatum | Sub-Saha ran Africa, West Indies | 4–10 | 4–19 | 50 | 90 | +++ |
| Scrub Typhus | Orientia ts utsugamu shi | Mite bite: Leptotrom bidium deliense | Asia, Australia, Pacific/Ind ian Ocean islands | 9–18 | 6–21 | 50 | 35 | +++ |
| Q Fever | Coxiella burnetii | Inhalation of aerosols from infected animals | Worldwid e (except New Zealand, Antarctica ) | 3–30 | 5–57 | 4–18% | None | — |
1.1 Taxonomy and Transmission¶
Rickettsiae are classified into genera based on genetic and antigenic differences. Transmission occurs via tick bites (RMSF, Mediterranean fever), mite bites (scrub typhus), flea bites (murine typhus), or louse feces (epidemic typhus). Coxiella burnetii is airborne and transmitted via aerosolized particles from infected animals.
1.2 Pathophysiology¶
Rickettsiae replicate within endothelial cells, causing vasculitis, increased vascular permeability, and systemic inflammation. Thrombocytopenia, leukopenia, and organ dysfunction are common. Severe cases may progress to septic shock, multiorgan failure, or encephalitis.
2. EPIDEMIOLOGY¶
Rickettsial diseases are globally distributed, with geographic and seasonal variation. RMSF is endemic in the U.S., Mexico, and Central/South America. Mediterranean fever occurs in southern Europe, Africa, and Asia. Scrub typhus is prevalent in eastern/southern Asia, northern Australia, and Pacific islands. Q fever is widespread except in New Zealand and Antarctica.
2.1 Risk Factors¶
Outdoor exposure, tick/lice/mite bites, travel to endemic regions, and contact with infected animals (e.g., goats, sheep, rodents). Immunocompromised individuals, elderly, and those with chronic illnesses are at higher risk for severe disease.
2.2 Demographics¶
RMSF is most common in children <10 years and >70 years. Q fever disproportionately affects farmers, veterinarians, and abattoir workers. Scrub typhus is more common in males and children.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Rickettsiae cause endothelial damage, leading to vasculitis, increased vascular permeability, and systemic inflammation. Pathogenesis involves immune dysregulation, cytokine release, and organ-specific complications (e.g., renal failure, CNS involvement).
3.1 Molecular Mechanisms¶
Rickettsiae evade host defenses by inhibiting phagosome maturation and surviving in acidic phagolysosomes. They induce pro-inflammatory cytokines (TNF- α , IL-6) and disrupt endothelial integrity, leading to microvascular injury.
3.2 Antigenic Diversity¶
Rickettsiae exhibit high antigenic variation, limiting cross-protection between species. This contributes to diagnostic challenges and the need for species-specific serologic testing.
4. CLINICAL FEATURES¶
Early symptoms include fever, headache, myalgia, and rash. Severe cases may present with encephalitis, pneumonia, multiorgan failure, or septic shock. Eschars, thrombocytopenia, and leukopenia are common. Neurologic, hepatic, and renal complications are frequent in untreated cases.
4.1 Rash and Eschar¶
Rash appears 2–14 days post-infection, often starting on extremities. Eschars (tache noire) are characteristic of Mediterranean fever and African tick-bite fever. Petechiae and purpura may develop in severe cases.
4.2 Systemic Complications¶
Renal failure, hepatic injury, respiratory distress, and CNS involvement (encephalitis, meningitis) are common. Q fever may present with chronic complications like endocarditis or granulomatous hepatitis.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include viral infections (e.g., influenza, mononucleosis), bacterial sepsis, typhoid fever, leptospirosis, and other tick-borne illnesses (e.g., Lyme disease, babesiosis). Rickettsialpox and viral exanthems (e.g., measles, rubella) must be considered.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis relies on clinical suspicion, serology (IFA, PCR), and imaging. Early testing is critical, as serologic conversion occurs 7–10 days post-infection. PCR and biopsy may confirm diagnosis in equivocal cases.
Table 192-2 Diagnostic Criteria for Chronic Q Fever¶
| PROVEN CHRONIC Q FEVER | PROBABLE CHRONIC Q FEVER | POSSIBLE CHRONIC Q FEVER |
|---|---|---|
| Positive Coxiella PCR in blood/tissue OR IFA ‡1:800 for phase I IgG AND definite endocarditis OR large vessel/prosthetic infection by imaging | IFA ‡1:800 for phase I IgG AND valvulopathy/aneurysm without infection signs OR suspected osteomyelitis/hepatitis | IFA ‡1:800 for phase I IgG WITHOUT meeting criteria for proven/probable chronic Q fever |
| AND symptoms/signs of chronic infection (fever, weight loss, night sweats) | AND immunocompromised state |
6.1 Serologic Tests¶
Indirect fluorescent antibody (IFA) assay is the gold standard. A fourfold rise in phase II IgG titers confirms acute infection. PCR detects rickettsial DNA in blood or tissue samples.
6.2 Imaging and Biopsy¶
Chest X-ray may show pulmonary infiltrates. Skin biopsy with PCR or immunohistochemistry confirms rickettsial infection. FDG-PET/CT is useful for detecting chronic Q fever complications.
7. MANAGEMENT & TREATMENT¶
Doxycycline is the first-line treatment for all rickettsial infections. Early initiation reduces mortality. Supportive care includes fluid resuscitation, oxygen, and monitoring for complications. Chronic Q fever requires prolonged antibiotic therapy and surgical intervention for infected prostheses.
7.1 Antimicrobial Therapy¶
Doxycycline (100 mg bid) is preferred for RMSF, scrub typhus, and Q fever. Chloramphenicol is an alternative in pregnancy or for patients with doxycycline allergy. Rifampin may be used in pregnant women or children.
7.2 Supportive Care¶
Fluid resuscitation, oxygen therapy, and monitoring for septic shock are critical. Mechanical ventilation may be required for respiratory failure. Dialysis is needed for acute renal failure.
8. PROGNOSIS & COMPLICATIONS¶
Mortality is 3–5% for RMSF with prompt treatment but can reach 50% in untreated cases. Chronic Q fever has a 25% mortality rate if untreated. Complications include endocarditis, granulomatous hepatitis, and multiorgan failure. Post-treatment fatigue may persist in 20% of patients.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Doxycycline is preferred in the second/third trimester; chloramphenicol may be used in the first trimester. Pediatrics: Doxycycline is safe in children >8 years. Immunocompromised patients require prolonged therapy and close monitoring for complications.
10. KEY POINTS & CLINICAL PEARLS¶
- Early doxycycline treatment is critical to prevent severe outcomes.
- Serologic testing is unreliable in the first 7–10 days; empirical treatment is recommended in endemic areas.
- Q fever may present with chronic complications (endocarditis, granulomatous hepatitis) requiring long-term antibiotics.
- Differentiate rickettsial infections from viral exanthems, bacterial sepsis, and other tick-borne diseases.
- Use FDG-PET/CT for diagnosing chronic Q fever and detecting intravascular infections.