Rickettsiae and similar organIsms

Rickettsiae are small bacteria that are spread to humans by arthropod vectors, namely human body lice, fleas, 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 human skin, generally by irritation and scratching. Rickettsiae multiply intracellularly and can enter most mammalian cells, although the main lesion produced is a vasculitis due to invasion of endothelial cells of small blood vessels. Thus multisystem involvement is usual. The causative organisms and arthropod vectors for rickettsial infections.


Epidemic typhus This is the most important rickettsial infection. Major outbreaks of typhus fever have occurred, mainly during famines and wars. It is found in Africa, Mexico, South America and Asia. The incubation period is 1-3 weeks followed by an abrupt febrile illness associated with profound malaise and generalized myalgia. After 2 or 3 days the fever remains constant at around 40°C. Headache is severe and there may be conjunctivitis with orbital pain. A measleslike 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 meningoencephalitis are evident 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 peripheriesmay be evident. Oliguric renal failure occurs in fulminating disease, which is usually fatal. 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 recrudescence is known as Brill-Zinsser disease. The factors that precipitate recurrence are not clearly defined, although other infections may be important.

Infections due to Rickettsia and Rickettsia-like organisms.
Infections due to Rickettsia and Rickettsia-like organisms.

Endemic (murine) typhus This is a rat infection that is inadvertently spread to humans by a rickettsiae-carrying rat flea. The disease closely resembles epidemic typhus but is much milder and rarely fatal. Rocky Mountain spotted fever and other tick-borne typhus fevers Infected hard ticks transmit this infection to humans, the same arthropod vector being responsible for other tickborne typhus fevers in Africa, India and the Mediterranean (Rickettsia conori causing ‘fievre boutonneuse’), in Central Asia and the Far East (Rickettsia siberica), and in Australia (Rickettsia australis). Rocky Mountain spotted fever is limited to North and South America. As in other tick-borne typhus fevers, many patients will be able to give an account of tick bites or exposure to ticks. Clinical features closely resemble those of epidemic typhus, although the incubation period may be shorter and an eschar (crusted necrotic papule) may develop at the site of the bite in association with regional lymphadenopathy. The typical, generalized maculopapular rash occurs, which includes the palms and soles of the feet. The rash eventually becomes petechial. Neurological, haematological and cardiovascular complications occur as in epidemic typhus.

Scrub typhus Found throughout Asia and the Western Pacific, this disease is spread by larval trombiculid mites (chiggers). Like tick-borne typhus, an eschar can often be found. Again the clinical illness resembles that of epidemic typhus, with an abrupt -onset febrile illness, rash and severe toxaemia. Bronchitis and interstitial pneumonia occur commonly but physical findings in the chest are minimal. The infection may recur despite treatment with antibiotics.
Rickettsial pox and trench fever Rickettsial pox is an urban disease described initially in New York City in 1946. A rodent mite was responsible for the spread of the infection to humans, the epidemic being related to massive expansion of the mouse community in large apartment buildings. The disease is mild, but similar in character to other rickettsial infections. Trench fever is also a relatively mild illness but multiple relapses are common. Although the causative agent of this disease, Rochalimaea quintana, was originally classified with the rickettsiae, it is now known that the organism does not demand an intracellular existence and may be cultured on bacteriological media. Both of these diseases are now rare.


The diagnosis is generally made on the basis of the history and clinical course of the illness. Although the causative organisms can be isolated by inoculation of infected blood into laboratory animals, this is laborious and may  take several weeks. Serodiagnosis using the W eil-Felix Proteus agglutination test, which relies on the fact that Rickettsia and Proteus OX strains have common antigens, has been used for more than 50 years. The Weil-Felix test is now being replaced by complement fixation, indirect fluorescent antibody or indirect haem agglutination tests which are more sensitive and specific. The polymerase chain reaction  peR) is established for the diagnosis of scrub typhus.


Tetracycline 500 mg four times daily for 7 days is given and improvement generally occurs in 48 hours. Ciprofloxacin is also effective. Doxycycline 200 mg weekly protects against scrub typhus; it is reserved for highly endemic areas.


This is achieved by control of vectors, namely lice, fleas, mites and ticks. Lice and fleas can be eradicated from clothing by insecticides (0.5% malathion or DDT). Chemical repellants are also useful. Control of rodents is vital. Bites from ticks and mites should be avoided by wearing protective clothing on exposed areas of the body,especially in high-risk regions. Mites can also be destroyed by chemical spraying from the air.

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