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Typhus fever

• Typhus refers to a group of zoonotic diseases caused by bacteria that are spread to humans by fleas, lice, and chiggers.
• Typhus fevers include scrub typhus, murine typhus, and epidemic typhus.
• The most common symptoms are fever, headaches, and sometimes rash.

Pathophysiology

Typhus fever is a zoonotic disease, Humans could be infected by bites from ticks, lice, inhalation of the bacteria, and direct contact of bacteria with skin wounds or mucous membranes. Following transmission, white blood cells phagocyte the pathogen and transports it via hematologic or lymphatic route to different organs, specially to those of the reticuloendothelial system. The pathophysiology of typhus fever can be described in the following steps.

Transmission

• Rickettsial pathogens are harboured by parasites such as fleas, lice, mites, and ticks.
• Organisms are transmitted by the bites from these parasites or by the inoculation of infectious fluids or feces from the parasites into the skin.
• Inhaling or inoculating conjunctiva with infectious material also causes infection. {| class="wikitable" !Disease !Etiological agent !Vector |- |Epidemic typhus  |Rickettsia prowazekii |Human body louse |- |Murine typhus |Rickettsia typhi |Infected fleas |- |Scrub typhus |Orientia tsutsugamushi  |Larval mites |}

Dissemination

• Scratching a louse-bite site allows the rickettsia-laden excrement to be inoculated into the bite wound.
• Following transmission, rickettsia are ingested by macrophages and polymorphonuclear cells. On ingestion, they replicate intracellularly inside the lysed cells and disseminate systemically.

Incubation

Incubation period of Typhus fever varies from one to two weeks.

Pathogensis

• The major pathology is caused by a vasculitis and its complications.
• On transmission, Rickettsia is actively phagocytosed by the endothelial cells of the small venous, arterial, and capillary vessels.
• It is followed by systemic hematogenous spread resulting in multiple localizing vasculitis.
• This process may cause result in occlusion of blood vessels and initiates inflammatory response (aggregation of leukocytes, macrophages, and platelets) resulting in small nodules.
• Occlusion of supplying blood vessels may cause gangrene of the distal portions of the extremities, nose, ear lobes, and genitalia.
• This vasculitic process also results in loss of intravascular volume with subsequent hypovolemia and decreased tissue perfusion and, possibly, organ failure.

Natural history

fatal in 20%–60% of untreated cases

Symptoms

Most common symptoms

• Fever
• Headache
• Malaise
• Maculopapular, vesicular, or petechial rash
• Eschar
• Nausea and vomiting.

Less common symptoms

• Abdominal pain
• Cough