Physicians don’t always have West Nile virus on their radar in patients at high risk for infectious disease—but they should.
West Nile virus is typically transmitted in the summer and fall months, but patients who have gotten a transfusion or organ transplant can present with disease even after mosquitos are long dead.
“Many people don't think about arboviruses in these patients, but it is starting to be a big problem,” said Capt. Carolyn Gould, MD, MSCR, an infectious disease physician and a medical officer in the Division of Vector-Borne Diseases at the Centers for Disease Control and Prevention (CDC).
Since its detection on U.S. soil in 1999, West Nile virus has become the most common cause of mosquito-borne disease in the contiguous United States.
Most people infected with West Nile virus are asymptomatic. However, roughly 20% of infected people will develop a febrile illness, and about 1% will develop neuroinvasive disease such as encephalitis, meningitis and acute flaccid myelitis.
Among those with neuroinvasive disease, 1 in 10 will die, said AMA infectious-diseases director Erica Kaufman West, MD, during an AMA-CDC webinar addressing the ongoing risks of West Nile virus, including risks during transplantation. The webinar, is available on demand at AMA Ed Hub™.
People who are older and who have immunocompromising conditions are at higher risk for more severe disease and death. Primarily spread by mosquitoes, West Nile virus can also be transmitted by infected blood products and transplanted organs, often resulting in severe disease.
“In 2025, there's been a substantial increase in West Nile virus activity with 41% more severe-disease cases and 32% more deaths than what is typically seen with West Nile disease,” Dr. Kaufman West said.
The CDC is working with multiple state health departments and clinical teams on possible transfusion, transplantation and dialysis-related clusters. During the webinar, Dr. Gould and other experts discussed the virus’ latest epidemiology, and best practices for diagnosis and guidance for immunocompromised populations.
Who’s most at risk
People who are 65 or older are five to 20 times more at risk of West Nile virus neuroinvasive disease, compared with those who are younger. The risk among those 65 and older is about 2% compared to less than 0.5% for younger people.
“That’s likely related to age-related immune defects in the innate and adaptive immune response, as well as a decline in blood-brain barrier integrity,” said Dr. Gould.
Patients with immunocompromising conditions have a 30% to 40% higher risk of neuroinvasive disease and death. Other factors such as immunosuppressive medications, hematologic malignancies, being male, or having diabetes, chronic kidney disease, and hypertension can increase the risk of severe West Nile virus disease.
Long-term symptoms and functional disabilities are very common in patients who have contracted the virus, with up to 40% of hospitalized patients needing long-term care or rehabilitation facilities.
West Nile virus has also been seen in organ transplant recipients. “We are aware of 15 transplant clusters associated with West Nile during 2002 to 2024,” Dr. Gould reported, noting that 13 of them took place in the U.S. Infections in these patients are associated with very high morbidity and mortality.
Most of the organ donors involved in these clusters were not screened for West Nile virus, she said, adding that screening could prevent many of these cases. Two other expert speakers presented several case studies on viral transmission through organ transplants, and how these West Nile virus cases are investigated.
What physicians should be looking for
Physicians treating patients who might have a clinically compatible illness to West Nile Virus should do appropriate arboviral testing to facilitate diagnosis and prevent unnecessary testing and treatments such as antibiotics, experts on the AMA-CDC webinar said. This is even true outside the usual seasonal window of June through October.
It’s also important to request confirmatory testing on any unusual cases. This includes cases that are atypical or might have been spread through organ transplants, or cases that may need specialized testing, said Dr. Gould.
If a patient is on a B-cell-depleting immunosuppressant such as rituximab, and presenting with a neurologic illness, “it could be a chronic viral infection and possibly an arboviral infection if they've been potentially exposed to ticks or mosquitoes,” she said.
Preventing West Nile virus is vital
Despite its presence in the U.S. for two decades, “there are still no treatments available and no human vaccines unfortunately, to prevent West Nile virus,” noted Dr. Gould.
“The main thing is supportive care,” in addressing West Nile virus, she said. “Since we don't have proven treatments, prevention is really key and that involves personal protection measures, as well as community and household level control involving larvicides and adulticides.”
Unfortunately, many people don't use personal protective measures. It’s very important to counsel your patients—especially if they are older or have immunocompromising conditions—on how to prevent infection.
Patients can help reduce the risk of West Nile virus disease by:
- Using mosquito repellent.
- Wearing long-sleeved shirts and pants.
- Avoiding the outdoors during high-mosquito times.
- Using screened windows (or air conditioning with closed windows) to keep mosquitos out.
A CME module based on the webinar is enduring material and designated by the AMA for a maximum of 1 AMA PRA Category 1 Credit™️. Learn more about AMA CME accreditation.
