Encephalitis

A 25-year-old man is brought to the emergency department by his partner following an episode of generalized convulsions. His partner reports that for the past 3 days, the patient has had a high fever and severe headache accompanied by nausea and vomiting. She also mentions that he started complaining of smell and taste changes. A few days ago, they came back from a trip to California in which they hiked and swam in lakes. Temperature is 38.9°C (102°F), pulse is 80/min, and blood pressure is 135/85 mmHg. On physical examination, the patient is confused and unresponsive. Pupils are equal and react sluggishly to light. Brudzinski sign is positive. A lumbar puncture is performed, and cerebrospinal fluid (CSF) analysis shows the following:  

 

Cerebrospinal fluid
Opening pressure	300 mm H2O
Protein	130 mg/dL
Glucose	20 mg/dL
RBCs	Numerous
WBC count	1500/µL with polymorphonuclear predominance
Neutrophils	80%

Gram stain of the CSF is negative. Head imaging results are nonspecific. Which of the following is the most likely causative organism?