Health Advisory

Summary:

• The Centers for Disease Control and Prevention (CDC) released a Health Advisory on August 11, 2026, describing the increased risk of severe neuroinvasive arboviral disease in patients receiving B cell-depleting or modulating medications, particularly anti-CD20 monoclonal antibodies (mAbs).
• Anti-CD20 mAbs currently approved in the United States include rituximab and biosimilars, ocrelizumab, ofatumumab, ublituximab, obinutuzumab, and ibritumomab tiuxetan. Among published case reports of arboviral neuroinvasive disease in patients on anti-CD20 mAbs, overall mortality was approximately 40%, and most survivors experienced long-term neurologic sequelae. West Nile virus (WNV) was the most common infecting arbovirus in these case reports.
• WNV is endemic in the Fairfax Health District (FHD). Mosquitoes that carry WNV are active from late spring through early fall.

Suggested Actions:

• When prescribing any immunosuppressive medications or caring for patients taking any immunosuppressive medication, particularly B cell-depleting mAbs, educate patients about the risk of severe arboviral disease and advise them to take measures to prevent mosquito and tick bites. Recommendations include:
  • Using an insect repellent that contains DEET, picaridin, IR3535, Oil of Lemon Eucalyptus (OLE), para-menthane-diol (PMD) or 2-undecanone.
  • Wearing light-colored long pants and long sleeves to cover exposed skin.
  • Avoiding areas where mosquitoes are especially common during peak biting times, particularly at dawn and dusk. Avoiding contact with ticks by staying away from wooded and brushy areas with high grass and leaf litter.
  • Taking steps to control mosquitoes around their yards by “tipping and tossing” items that hold water such as tires, buckets, flowerpots, corrugated downspout extensions, and other containers; and using larvicides, such as Mosquito Dunks, in water that cannot be eliminated.
  • Keeping doors, windows, and screens in good repair.
  • Checking clothing and their body for ticks after spending time outdoors.
• Be aware that patients with an arboviral disease who are immunocompromised can have a prolonged incubation period or an atypical course and might not present with illness during the usual months when mosquitoes and ticks are active (i.e., May through November in the United States).
• When an arboviral infection is suspected in a patient receiving B cell-depleting mAbs, preferentially order molecular testing (i.e., RT-PCR or mNGS) as decreased B-cell function makes serology unreliable.
• For questions about testing or reporting arboviral diseases, contact the Fairfax County Health Department by calling 703-246-2433 or by emailing HDCD@fairfaxcounty.gov.

Additional Information:

• WNV is the most common arboviral infection in Virginia and the United States. Other arboviruses in the US include eastern equine encephalitis virus, Powassan virus, Jamestown Canyon virus, La Crosse virus, Cache Valley virus, and Potosi virus. For more information on arboviruses in the US, please visit CDC Webpage.
• Over the last ten years (2016–2025), 31 cases of WNV have been reported to FCHD, an average of 3 cases reported every year. Of these cases, 65% had neuroinvasive disease and 2 died. The cases were geographically distributed throughout the entire Health District. In 2026 YTD, three cases of WNV have been reported in the FHD. The United States had an early and strong start to this year's WNV season, and many weeks of the arbovirus transmission season remain.
• Most persons with WNV infection are asymptomatic (70–80%) or have only mild illness with fever, headache and muscle aches that can last days to weeks. Such cases are rarely diagnosed or reported. Fewer than 1% develop neuroinvasive disease (meningitis, encephalitis, acute flaccid paralysis).
• FCHD’s Division of Environmental Health routinely traps and tests mosquitoes for WNV throughout the FHD every mosquito season. FHD residents may request a mosquito inspection of their yard from FCHD’s Division of Environmental Health at 703-246-2201, TTY 711. This inspection will result in recommendations to decrease mosquito populations on residents’ property.

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