| Summary: | Brown recluse spiders, also known as Loxosceles reclusa, are endemic to the Southwest and Central Midwestern United States. A bite from this spider can cause a range of clinical manifestations, anywhere from a painless papular lesion to life-threatening reactions. We report a possible spider bite presenting as leukostasis initially suspected to be acute leukemia. A 22-year-old female patient presented to the emergency department with confusion and right upper arm pain, redness, and swelling after a suspected spider bite. Initial labs showed WBC count of 103.5x10e3/µL, hemoglobin of 3.3 g/dL, positive Direct Coombs’ test, creatinine of 1.8 mg/dL, transaminitis, and lactic acid of 20 mmol/L. Acute leukemia with leukostasis was suspected. She was started emergently on hydroxyurea in conjunction with prophylaxis for tumor lysis syndrome. However, peripheral smear showed left-shifted granulocytosis with lymphocytosis, monocytosis, and no blast cells or evidence of myelodysplasia. Bone marrow aspirate showed mildly hypercellular marrow with myeloid hyperplasia and no myelodysplasia. Flow cytometry analysis confirmed a left-shifted myeloid maturation pattern with 0.3% myeloblasts. BCR-ABL1 and JAK2 testing was negative. Hence, she had no evidence of leukemia but rather had leukostasis from a spider bite. Hydroxyurea was stopped and follow-up labs normalized. Sphingomyelinase D in the brown recluse spider venom is unique to Loxosceles and Sicarius and may be responsible for the unique clinical presentation of loxoscelism. The presentation of hyperleukocytosis complicated by shock with an unclear history poses a diagnostic challenge. In diagnostic uncertainty, consider delaying chemotherapy until a diagnosis can be confirmed to avoid potential harm.
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