Rickettsiae
• Rickettsiae are obligate intracellular, Gram-negative bacteria that
cause various vector-borne diseases in humans.
• These bacteria are transmitted through arthropods such as ticks, lice,
fleas, and mites.
• The diseases caused by Rickettsia spp. include Rocky Mountain
Spotted Fever (RMSF), Epidemic Typhus, Endemic Typhus, and Scrub
Typhus.
Ecology and Transmission
• Rickettsiae primarily inhabit the gastrointestinal tract and salivary
glands of arthropod vectors (ticks, lice, fleas, mites).
• Infected arthropods serve as both vectors and reservoirs, maintaining
the bacteria in nature.
• Humans are accidental hosts, acquiring infections through arthropod
bites or feces contamination.
Transmission
• Tick-borne (e.g., Rickettsia rickettsii → RMSF)
• Louse-borne (e.g., Rickettsia prowazekii → Epidemic Typhus)
• Flea-borne (e.g., Rickettsia typhi → Endemic Typhus)
• Mite-borne (e.g., Orientia tsutsugamushi → Scrub Typhus)
Epidemiology
• Geographically widespread: Found in North and South America,
Europe, Africa, and Asia.
• Seasonal variation: Tick-borne rickettsioses are more common in
spring and summer.
• High-risk groups: People exposed to outdoor environments (farmers,
hikers, military personnel).
Physiology
• Small, pleomorphic coccobacilli (0.3-0.5 µm x 1-2 µm).
• Gram-negative but weakly staining due to minimal peptidoglycan.
• Obligate intracellular organisms:
• Require host ATP, making them energy parasites.
• Grow in endothelial cells, causing vascular damage.
• Do not grow in artificial media—require tissue culture or
embryonated eggs.
• Survive in arthropods for prolonged periods, but are fragile outside
hosts.
Pathogenesis
General Mechanism
• Transmission via arthropod bite or contaminated feces (rubbing into
skin/eyes).
• Rickettsiae invade endothelial cells, leading to:
• Vasculitis: Increased vascular permeability → edema, hemorrhage, organ
failure.
• Platelet activation: Leads to thrombosis and disseminated intravascular
coagulation (DIC).
• Cytokine release: Fever, rash, organ dysfunction
Major Rickettsial Diseases and Pathogenesis
Disease Pathogen Vector Clinical Features
Fever, petechial rash (wrists
Rocky Mountain Spotted Ticks (Dermacentor, & ankles → trunk),
Rickettsia rickettsii
Fever (RMSF) Rhipicephalus) headache, DIC, multi-organ
failure
High fever, centripetal rash
Lice (Pediculus humanus (trunk → limbs), Brill-
Epidemic Typhus Rickettsia prowazekii
corporis) Zinsser disease
(reactivation)
Mild fever, maculopapular
Endemic (Murine) Typhus Rickettsia typhi Fleas (Xenopsylla cheopis)
rash, headache, myalgia
Eschar at bite site, fever,
maculopapular rash,
Scrub Typhus Orientia tsutsugamushi Mites (Leptotrombidium)
lymphadenopathy, multi-
organ failure
Diagnosis
Clinical Suspicion
• History of arthropod exposure (tick bites, lice infestation, travel history).
• Symptoms: Fever, headache, rash, eschar, multi-organ failure.
Laboratory Tests
• Direct Detection
• Immunohistochemistry (IHC) of skin biopsy: Detects Rickettsia antigens.
• Polymerase Chain Reaction (PCR):
• Highly sensitive and specific.
• Best for early detection before seroconversion.
• Serology (Gold Standard)
• Indirect Immunofluorescence Assay (IFA): Detects IgG & IgM antibodies.
• Significant rise in titers between acute and convalescent sera is diagnostic.
• Weil-Felix Test:
• Agglutination test using Proteus antigens (obsolete but historically used).
Culture (Rarely Performed)
• Grown in embryonated eggs or tissue culture, but hazardous and requires biosafety level 3 (BSL-
3) facilities.
Treatment
First-Line Therapy
• Doxycycline (100 mg PO/IV BID for 7-14 days):
• Drug of choice for all Rickettsial infections, including in children.
• Early treatment reduces mortality—do not wait for laboratory confirmation.
Alternative Antibiotics
• Chloramphenicol (for pregnant women or doxycycline contraindications).
• Azithromycin or Clarithromycin (for Scrub Typhus in children <8 years).
• Supportive Therapy
• IV fluids, oxygen therapy, and vasopressors (for shock).
• Management of DIC (platelets, anticoagulation if necessary).
Control & Prevention
Vector Control
• Tick control: Use of acaricides, tick repellents (DEET, permethrin-
treated clothing).
• Lice and flea control: Personal hygiene, insecticides (pyrethroids,
malathion).
Public Health Measures
• Surveillance and rapid outbreak response (especially for typhus in
refugee camps).
• Education on protective clothing and proper removal of ticks.
• Vaccination
• N/B No effective vaccines available for most rickettsial infections.
