Arthropod-Borne Viruses are common infections that typically infect humans via mosquito or tick bites, usually causing asymptomatic infections. They uncommonly manifest as systemic illness and rarely as neuroinvasive disease. We present a case of Encephalopathy with MRI changes secondary to California Virus.

A 69-year-old female presented after being found down by her neighbor.  She was febrile and encephalopathic with continued perseveration. She was ataxic with finger-to-nose dysmetria. She had generalized hyperreflexia with upgoing toes bilaterally. MRI brain demonstrated asymmetric, non-enhancing T1 hyperintensities of bilateral thalami and T2 hyperintensities of bilateral lentiform nuclei, cerebellar vermis and right cerebellar hemisphere.  Lumbar puncture revealed normal opening pressure, elevated 40/cmm WBC with lymphocytic predominance and elevated 147 mg/dL total protein. CSF culture was negative for bacterial, fungal growth and encephalitis/meningitis panel. Serum heavy metal and vitamin levels were within normal limits. There was also initial concern for rabies as patient routinely interacted with wild racoons. Arbovirus and Powassan PCR was subsequently tested and was notable for positive 1:32 titer of California Virus Serogroup IgG. Though she had no recent history of travel outside of Pennsylvania, she was ultimately diagnosed with California Encephalitis and managed with supportive care. Her mentation improved significantly throughout her hospitalization and she was discharged to inpatient rehabilitation.

Arthropod-borne viruses make up a substantial number of viral infections in the United States, with West Nile Virus (WNV) being the leading cause of infection. Our patient was positive for California Virus IgG. Although rare, Jamestown Canyon virus and LaCrosse virus are both reported sources of infection in Pennsylvania. Furthermore, basal ganglia and thalamic lesions have been previously reported in WNV infections however they have never been observed with California Encephalitis. This case highlights the identification and testing of arboviruses (including the California Virus) as an unusual suspect in toxic encephalopathy.