Bunyaviruses are a family of arthropod or rodent borne viruses characterized by an enveloped tri-segmented negative sense RNA genome. This family includes various encephalitis-causing viruses including Rift Valley Fever virus and La Crosse virus. We describe a case of encephalitis from a novel Bunyavirus.

A 60-year-old male presented to the emergency department with confusion, headaches, and fevers that were slowly progressive for one week. He also developed trouble with memory and performance of complex motor movements two days prior to presentation. Pertinent past medical history included common variable immunodeficiency (CVID) treated with monthly intravenous immunoglobulin (IVIG) infusions. Physical examination revealed no neurological deficits except for altered sensorium with inability to recall his location and various life events. MRI Brain with and without contrast performed twice within a span of three days showed no acute abnormalities. Continuous electroencephalographic recording demonstrated mild background slowing with no evidence of epileptiform activity. Lumbar puncture revealed lymphocytic pleocytosis (40 WBCs) in CSF, elevated protein (92 mg/dl), and negative meningitis/encephalitis panel by PCR. CSF was sent for metagenomic sequencing which revealed a previously undescribed novel Bunyavirus, closely related to Lone Star virus (84 to 95 percent homologous). The method of transmission was thought to be tick-borne, given the patient’s avid hiking history and the similarity to Lone Star virus. Repeat MRI of the brain obtained three weeks post-admission, given no clinical improvement, demonstrated bilateral basal ganglia FLAIR hyperintensities without contrast enhancement. Despite continued supportive therapy, the patient deteriorated clinically, became comatose, and passed away within four weeks.

Lone Star virus has been previously isolated in ticks but previously unreported to cause pathology in humans. Our case report describes the first reported human case of encephalitis caused by a novel Bunyavirus highly homologous to Lone Star virus, educating clinicians of this possibility.