Rickettsial infectionsTyphus

Typhus is the collective term to a group of diseases caused by Rickettsia species.Rickettsia are small intracellular bacteria that are spread to humans by arthropod vectors ,including body lice , fleas , hard ticks , and larval mites.Rickettsiae inhabit the alimentary tract of these arthropods and the disease is spread to the human host by inoculation of their faeces through broken human skin , generally produced by scratching.

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Rickettsiae multiply intracellulary and can enter most mammilian cells , although the main lesion produced is a vasculitis due to invasion of endothelial cells of small blood vessels.Multisystem involvement is usual.

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Clinical featuresTyphus fever group

1.Epidemic typhusThe vector of epidemic typhus is human body louse , and like louse-borne relapsing fever , epidemics are associated with war and refugees.

The incubation period of 1-3 weeks is followed by an abrupt febrile illness associated with profound malaise and generalized myalgia..Headache is severe and there may be conjuctivitis with orbital pain.

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A measles-like eruption appears around the fifth day , the macules increasing in size and eventually becoming purpuric in character.At the end of the first week ,signs of meningo-encephalitis appear and CNS involvement may progress to stupor or coma , sometimes with extrapyramidal involvement.

At the height of the illness , splenomegaly,pneumonia ,myocarditis , and gangrene at the peripheries may be evident.Oliguric renal failure occurs in fulminating disease , which is usually fatal.

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Recovery begins in the third week but is generally slow.The disease may recur many years after the initial attack owing to rickettsiae that lie dormant in lymph nodes.The factors that precipitate recurrence are not cleaerly defined , although other infections may play a role.

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Endemic ( murine ) typhus This is an infection of rodents that is

inadvertently spread to humans by rat fleas.The disease closely resembles epidemic typhus but is much milder and rarely fatal.

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Scrub typhusThis disease is spread by larval trombiculid mites ( chiggers ).An eschar ( a black , crusted , necrotic papule ) can often be found at the site of the bite.The clinical illness is very variable , ranging from a mild illness to a fulminant and potentially fatal disease.Unlike other types of typhus the organism is passed on to subsequent generations of mite ,which susequently act as both reservoir and vector.

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Spotted fever groupA variety of Rickettsia species , collectively known as the spotted fever group rickettsiae , cause the illnesses known as spotted fevers.The vector is a hard tick.Although the causative organism and the name of the illness is vary from place to place the clinical course is common to all.After an incubation period of 4-10 days an eschar may develop at the site of the bite in association with regional lymphadenopathy.

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There is abrupt onset of fever , myalgia and headache, accompanied by a maculo-papular rash which may become petechial.Neurological , haematological and cardiovascular complications may occur as in epidemic typhus , although these are uncommon.

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DiagnosisIs generally made on the basis of the history and clinical course of the illness.It can be confirmed serologically by the indirect fluorescent antibody test ( the most sensitive and specific ) , or the latex agglutination test.The Weil-Felix agglutination test , although previously widely used , is not specific or sensitive.

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Treatment*Doxycycline or tetracycline given for 5-7 days is

the treatment of choice.Ciprofloxacin is also effective