Rickettsiales, Family Rickettsiaceae
Organism is an obligate intracellular, Gram negative bacteria. It is maintained
in nature by transovarial transmission in larval trombiculid mites (chiggers)
or harbored in rodent reservoirs. Humans can be exposed to the bacteria through
chigger bites. Chigger bites often result in a pruritic rash. The bacteria
invades host cells by induced phagocytosis and escaping into the cytoplasm of
the host cell, but specific virulence factors related to phagosomal membrane
lysis have not been identified. It is also resistant to B-lactam antibiotics
due to lack of peptidoglycan in its cell wall.
Manifestations: O. tsutsugamushi
causes scrub typhus. Primary disease symptoms include acute febrile illness,
headache, myalgia, sweats, conjunctival injection, lymphadenopathy, and mental
changes. Eschar develops at the site of the bite.
- Early phase of infection: Leukopenia,
thrombocytopenia, and abnormal liver tests
- Late phase of infection: Pneumonitis, endocarditis, encephalitis,
hemorrhaging, and intravascular coagulation
typhus is endemic across extensive parts of Asia, South Asia, Australia and the
Pacific, and disease prevalence is stable in those areas. US experiences are
related to exposures of military personnel during WWII, the Korean War, and the
presentation of the disease and serologic tests to detect antibodies are mainly
used in diagnosis. However, detection of antibodies is only effective in
non-endemic areas because many unaffected individuals will have antibodies in
endemic areas. The organism can be isolated in cell culture, and
immunofluorescence and immunoperoxidase staining can also be used to
demonstrate organisms in tissue.
treatment with tetracyclines (e.g. doxycycline) or chloramphenicol. Azythromycin
is used in pregnant women and when resistance is suspected. Some evidence of
antibiotic resistance is being described in South Asia.
best prevention measure is to avoid ticks. In mouse models, riboflavin and
light have been seen to reduce infectivity.