Residents taking certain immunosuppressive drugs face a roughly 40% chance of dying if they contract West Nile virus, the Fairfax County Health Department warned in an advisory posted Monday, Aug. 17.

The alert targets patients on B cell-depleting medications, particularly anti-CD20 monoclonal antibodies such as rituximab, ocrelizumab and ofatumumab. Those drugs treat cancers, autoimmune conditions and transplant rejection. Because they suppress the immune system's ability to form antibodies, patients who get bitten by an infected mosquito have little defense against neuroinvasive disease.

Three West Nile cases have already been confirmed in the Fairfax Health District in 2026. The health department said the nation had an early and strong start to this year's season, with many weeks of arbovirus transmission still ahead.

Why it matters locally

West Nile virus is endemic in the Fairfax Health District. Over the past decade, 31 cases were reported locally, with 65% developing neuroinvasive disease and two deaths. WNV-carrying mosquitoes have been trapped in the McLean area in past seasons, and the insects are active from late spring through early fall, with peak biting at dawn and dusk.

Most people who contract West Nile never know it. Between 70% and 80% show no symptoms, and fewer than 1% develop neuroinvasive disease. But for patients on anti-CD20 drugs, the picture is far grimmer: among published case reports, about 40% died and most survivors suffered long-term neurological damage, according to the CDC's Aug. 11 Health Alert Network advisory.

No treatment exists.

The CDC also described a newer pattern in which at least five rituximab patients infected with rarer arboviruses developed chronic, neurodegenerative encephalitis over months to years. All five died.

What patients should do

The health department recommends immunocompromised patients take these steps:

• Use insect repellent containing DEET, picaridin, IR3535, Oil of Lemon Eucalyptus or 2-undecanone
• Wear light-colored long pants and long sleeves
• Avoid outdoor activity at dawn and dusk, when mosquitoes are most active
• Eliminate standing water around the yard: tip and toss tires, buckets, flowerpots and corrugated downspout extensions
• Use larvicides such as Mosquito Dunks in water that cannot be removed
• Keep doors, windows and screens in good repair
• Check clothing and body for ticks after time outdoors

No vaccine or approved treatment exists for West Nile or other arboviruses found in the United States, making bite prevention the only defense.

Diagnosis challenges

Standard blood tests may not work for these patients. Because anti-CD20 drugs suppress antibody production, routine serology is unreliable. The health department advises providers to order molecular testing, either RT-PCR or metagenomic next-generation sequencing, when arboviral infection is suspected.

Symptoms can also appear outside the typical mosquito season. Immunocompromised patients may have a prolonged incubation period or atypical course, meaning illness could surface well after November.

How to get help

Residents can request a free mosquito yard inspection from the health department's Division of Environmental Health at 703-246-2201 (TTY 711). Providers and residents with questions about testing or reporting arboviral diseases can call 703-246-2433 or email [email protected]. Lucy Caldwell, the health department's director of communications, can be reached at 703-246-8634.