A 78-year-old woman is hospitalized with a 2-day history of lethargy, headache, and confusion in September. She is an avid gardener living in Connecticut. Medical history is unremarkable, and she takes no medications.
On physical examination, temperature is 38.9 °C (102 °F) and pulse rate is 110/min. She is lethargic and ori-ented only to name. She resists passive flexion of the neck and the ocular examination. No rash is present, and the remainder of the examination is normal.
Laboratory studies show a normal complete blood count and liver chemistry tests. Cerebrospinal fluid shows a leukocyte count of 94/uL (94 x 10%/L), with 88% lymphocytes, 11% monocytes, and 1% polymorphonuclear cells.
Serology for Borrelia burgdorferi is negative.
Which of the following is the most likely diagnosis?
(A) Anaplasmosis
(B) Babesiosis
(C) Lyme disease
(D) Powassan virus infection
Given the patient's presentation with lethargy, headache, confusion, fever, and neck stiffness, along with cerebrospinal fluid (CSF) analysis showing a lymphocytic pleocytosis, a viral encephalitis is strongly suggested. Additionally, the patient lives in Connecticut and is an avid gardener, which increases her exposure to tick-borne diseases.
Powassan virus is a tick-borne flavivirus that can cause encephalitis and is transmitted by the same ticks that spread Lyme disease (Ixodes scapularis). This is particularly relevant given her geographic location in Connecticut, where Powassan virus is endemic.
**Anaplasmosis (A) ** and **Babesiosis
(B) ** are also tick-borne diseases, but they typically present with different symptoms and blood abnormalities, such as leukopenia and thrombocytopenia (anaplasmosis) or hemolytic anemia (babesiosis), neither of which are described in this patient.
**Lyme disease (C) ** can cause neuroborreliosis, but it is less likely given the negative serology for Borrelia burgdorferi.
Thus, the most likely diagnosis is:
**D. Powassan virus infection**