epidemic typhus

Pathophysiology: Rickettsia prowazekii is an obligate intracellular, gram-negative bacterium transmitted by the body louse. Lice become infected after feeding on a bacteremic human, and the organism multiplies in the louse gut; transmission to humans occurs not via the bite itself but through louse feces that contaminate the bite wound or mucous membranes (scratching facilitates entry). R. prowazekii invades vascular endothelial cells, causing vasculitis, which underlies the characteristic rash and systemic manifestations (thrombosis, vascular leakage, organ dysfunction).

Clinical features: Abrupt onset of high fever, severe headache, myalgia, and malaise after an incubation period of about 1-2 weeks. A maculopapular rash typically begins on the trunk (axilla/upper torso) and spreads centrifugally to the extremities, classically sparing the palms and soles (this centrifugal spread differs from Rocky Mountain spotted fever, which spreads centripetally from wrists/ankles inward). Severe cases can progress to petechiae, confusion, stupor, and multiorgan involvement; mortality can be high if untreated, especially in epidemic settings tied to poor sanitation, overcrowding, and cold weather (war, refugee crises, natural disasters).

Brill-Zinsser disease: A recrudescent, milder form of epidemic typhus that can occur years after the initial infection, as R. prowazekii can remain latent in lymphatic tissue and reactivate, especially under stress or immunosuppression.

Diagnosis: Primarily clinical and epidemiologic (exposure to lice, crowded conditions); confirmed via serology (indirect immunofluorescence assay) or PCR. Weil-Felix test (older, less specific) can show cross-reacting antibodies to Proteus OX19.

Treatment: Doxycycline is first-line and highly effective, even in children and pregnant women when risk outweighs benefit, due to the severity of untreated disease. Chloramphenicol is an alternative.

Differentiation: Distinguish from murine (endemic) typhus (R. typhi, transmitted by fleas, milder disease) and Rocky Mountain spotted fever (R. rickettsii, transmitted by ticks, rash starts peripherally and moves centrally).

High-yield board associations: lice vector, R. prowazekii, flying squirrels as a reservoir in the US (sporadic cases), centrifugal rash sparing palms/soles, Brill-Zinsser disease as reactivation, association with war/poverty/crowding, and vasculitis as key pathologic mechanism.