The federal advisory that warned patients on certain cancer and autoimmune infusion drugs about mosquito and tick bites contains a second problem that has drawn far less attention. In those same patients, the blood test most commonly used to diagnose West Nile virus can come back negative even when the infection is present.
That matters most right now. West Nile transmission peaks in late summer and early fall, and the 2026 season has been running well ahead of recent years. A family caring for someone on one of these medications may be heading into the riskiest weeks of the year with a diagnostic tool that does not reliably work for their relative.
