Case of the Week

Background:

• Scrub typhus is an acute febrile illness caused by gram-negative intracellular coccobacillus, Orientia tsutsugamushi. In humans, the disease is caused by the introduction of O tsutsugamushi through the skin by the bite of a larval-stage (chigger) trombiculid mite.
• It is characterized by focal or disseminated vasculitis and perivasculitis, which may affect the lungs, heart, liver, spleen, and central nervous system.
• Nearly all affected patients have central nervous system involvement, according to autopsy studies. Central nervous system involvement typically manifests as meningoencephalitis.

Clinical Presentation:

• The typical case presentation includes an eschar at the site of chigger feeding, maculopapular rash, and regional lymphadenopathy.
• Common neurologic features are headache, altered sensorium, seizure, focal weakness, and neck stiffness.
• CSF examination may reveal lymphocytic pleocytosis and raised protein level.
• Using nested polymerase chain reaction, rickettsial DNA has been isolated in CSF.
• A significant increase in serum antibody titer, as measured by the Weil-Felix agglutination test or indirect immunofluorescence, can be used to make a diagnosis.

Key Diagnostic Features:

• Two-thirds of patients with scrub typhus show features of  meningoencephalitis.
• Cortical and subcortical white matter hyperintensities on T2-weighted and FLAIR images with restricted diffusion have been reported, which pathologically may represent infarcts secondary to pathogen-induced vasculitis.
• Contrast enhancement is usually not seen, but a ring-enhancing lesion in the corpus callosum and multiple nodular parenchymal enhancements may be seen.
• Limbic encephalitis may be seen in some patients and manifests as neuropsychiatric symptoms.

Differential Diagnoses:

• Tubercular meningoencephalitis: usually presents with thick exudates in subarachnoid space predominantly in basal cisterns. Enhancing leptomeningeal tuberculomas may also been seen. Tubercular vasculitis with infarcts may be seen.
• Diffuse leptomeningeal glioneural tumor: These tumors are commonly encountered in children and adolescents. Thick nodular leptomeningeal enhancement along basal cisterns and over the surface of brain and spinal cord is seen. Numerous small sub-pial cysts over the surface of inferior parts of cerebral hemispheres and posterior fossa may be seen.

Treatment:

• Currently, the drug of choice is doxycycline (100 mg orally or intravenously twice daily).
• Both macrolides and chloramphenicol have also been used successfully.
• There is not a vaccine for scrub typhus yet. Therefore, prevention centers primarily on avoiding insect bites.