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Excerpt from Typhus

Synonyms, Key Words, and Related Terms: epidemic typhus, rickettsemia, louse-borne typhus, classic typhus, Brill-Zinsser disease, recrudescence of epidemic typhus, murine typhus, flea-borne typhus, endemic typhus, scrub typhus, tsutsugamushi fever, body louse, arthropods, Pediculus corporis, Rickettsia prowazekii, Pediculus capitis, Phthirus pubis, Xenopsylla cheopis, Ctenocephalides felis, Leptotrombidium akamushi, Leptotrombidium deliense, Dermacentor andersoni, Dermacentor variabilis, Amblyomma americanum, Glaucomys volans, flying squirrels, multisystem vasculitis, Rocky Mountain spotted fever, RMSF

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Background: Typhus refers to a group of infectious diseases that are caused by rickettsial organisms and result in an acute febrile illness. Arthropod vectors transmit the etiologic agents to humans. The principle diseases of this group are epidemic or louse-borne typhus and its recrudescent form known as Brill-Zinser disease, murine typhus, and scrub typhus.

Pathophysiology: Epidemic typhus is the prototypical infection of the typhus group of diseases, and the pathophysiology of this illness is representative of the entire category. The arthropod vector of epidemic typhus is the body louse (Pediculus corporis). This is the only vector of the typhus group in which humans are the usual host. Rickettsia prowazekii, which is the etiologic agent of typhus, lives in the alimentary tract of the louse. A Rickettsia-harboring louse bites a human to engage in a blood meal and causes a pruritic reaction on the hostís skin. The louse defecates as it eats; when the host scratches the site, the lice are crushed, and the Rickettsia-laden excrement is inoculated into the bite wound. The Rickettsia travel to the bloodstream and rickettsemia develops.

Rickettsia parasitize the endothelial cells of the small venous, arterial, and capillary vessels. The organisms proliferate and cause endothelial cellular enlargement with resultant multiorgan vasculitis. This process may cause thrombosis, and small nodules may develop from the deposition of leukocytes, macrophages, and platelets. Gangrene of the distal portions of the extremities, nose, ear lobes, and genitalia may occur as a result of thrombosis of supplying blood vessels. This vasculitic process may also result in loss of intravascular colloid with subsequent hypovolemia and decreased tissue perfusion and, possibly, organ failure. Loss of electrolytes is common.

Some people may have a recrudescent case typhus (Brill-Zinsser disease). After a patient is treated with antibiotics and the disease has seemingly been cured, Rickettsia may linger in the body tissues. Months, years, or even decades after treatment, organisms may reemerge and cause a recurrence of typhus. How the Rickettsia organisms linger silently in a person and by what mechanism recrudescence is mediated are unknown. The presentation of Brill-Zinsser disease is less severe and mortality is much lower than in epidemic typhus. Risk factors that may predispose a person to recrudescence include improper or incomplete antibiotic therapy and malnutrition.

Murine typhus and scrub typhus share the same pathophysiology as epidemic typhus, although they are somewhat milder. The incubation period is approximately 12 days for the typhus group. Prior infection with Rickettsia typhi provides subsequent and long-lasting immunity to reinfection.