The Melanoma Mimic: Zoonotic Anchoring in Neurodiagnostic Failure

Background

Multiple intracranial lesions carry a broad differential neoplastic, infectious, inflammatory, and vascular causes. When a patient presents with a zoonotic exposure history alongside multifocal brain imaging, physicians are at risk of anchoring on infection and rashly closing the diagnostic process. This case matters, it demonstrates how a exposure narrative can change clinical reasoning, delay diagnosis, and expose patients to unnecessary therapy. We present an immunocompetent man whose hemorrhagic, multifocal melanoma brain metastases initially mock a disseminated CNS infection, emphasizing the diagnostic expertise needed when imaging findings are radiographically nonspecific.

Case Presentation

A 78-year-old man with HNT, DM2, BPH and GERD presented as a stroke alert after his wife found him with word-finding difficulty and nonsense speech following an afternoon nap; he was last well that morning. He reported several days of progressive dysphagia and word substitution, calling chili "Caesar salad." Examination revealed a mild right facial droop, inappropriate responses, slurred speech, and antalgic gait (NIHSS 3). Blood pressure was 177/101mmHg and WBC count was 12.3. Non-contrast CT and CT angiography demonstrated multiple hemorrhagic, hyperdense masses with extensive vasogenic edema; CT perfusion showed no ischemic penumbra, arguing against acute infarction. A history of eating raw sausage from the freezer and exposure to rodent droppings prompted concern for toxoplasmosis, leptospirosis, hantavirus, and disseminated fungal disease, and empiric vancomycin, cefepime, and voriconazole was initiated.

Findings

A comprehensive infectious workup was unrevealing: procalcitonin 0.07; negative blood and fungal cultures; and negative cryptococcal antigen, Aspergillus galactomannan, Fungitell, Histoplasma, and Toxoplasma serologies; urinalysis and CXR were normal. Contrast-enhanced MRI revealed numerous enhancing lesions characteristic of metastatic melanoma. CT of the chest, abdomen, and pelvis identified no primary or extracranial metastatic site. Empiric antimicrobials were discontinued. Neurosurgery deferred biopsy; oncology recommended whole-brain radiotherapy. The patient was discharged to outpatient radiation therapy with memantine for cognitive support. The diagnosis rested on characteristic imaging, as no biopsy was obtained for histologic confirmation.

Discussion & Conclusion

This case illustrates how a zoonotic exposure can change clinical reasoning. Multiple hemorrhagic, edematous lesions are radiographically nonspecific, and melanoma is among the primary tumors most prone to intratumoral hemorrhage, frequently producing vasogenic edema that mimics abscess or septic emboli. In immunocompetent patients, toxoplasmosis and primary CNS lymphoma are less likely than in immunocompromised hosts, and the patient's intact immune status should have lowered the infectious differential sooner. Anchoring on the zoonotic narrative and premature closure delayed recognition and exposed the patient to unnecessary therapy. The central lesson is diagnostic discipline: contrast-enhanced MRI, not CT, is the decisive study for multifocal enhancing lesions. A thorough exposure history should broaden, not narrow, the differential, and deliberately revisiting the working diagnosis when confirmatory data are lacking is the most effective safeguard against anchoring and premature closure. A key limitation is the absence of tissue confirmation.