Older adults who have chronic kidney disease (CKD) or cerebrovascular disease affecting blood flow to the brain, such as stroke, are at twice the risk for neuroinvasive illness, which can cause paralysis and death, after West Nile infection, researchers at the University of California–Los Angeles (UCLA) and Johns Hopkins suggest in JAMA Network Open.

The analysis used electronic medical record data to identify risk factors for neuroinvasive illness and death after West Nile infection among 3,064 adults from65 health care organizations across the United States from 2013 to 2024.

Less than 1% develop neuroinvasive illness

No vaccines or treatments are available for West Nile infection, which causes about 80% of all mosquito-borne infections—roughly three-fourths asymptomatic and one-fourth mild—each year in the United States. 

A very small proportion of infected people (less than 1%) develop neuroinvasive illness, which involves the brain, spinal cord, and meninges (membranes that line the skull and enclose the brain and spinal cord). This form of the disease kills about 10% of those affected.

“West Nile virus (WNV) is the leading mosquito-borne infection in the US, causing West Nile fever or West Nile neuroinvasive disease (WNND) with substantial morbidity and mortality,” the study authors wrote. “Contemporary analyses are needed to identify at-risk populations and to target interventions.”

The researchers noted that a priority of the 2024 Centers for Disease Control and Prevention (CDC) National Public Health Strategy to Prevent and Control Vector-Borne Diseases in People is to reduce the yearly number of WNND cases to below 500 by 2035.

Aging population could lead to greater risk of severe disease

Of all 3,064 patients diagnosed as having WNV infection, 53% had West Nile fever (average age, 55 years; 50% women), and 47% had WNND (average age, 59 years; 61% men). The distribution by geographic region included 35% in the West, 35% in the South, 15% in the Midwest, and 13% in the Northeast. 

Risk factors for WNND were older age (11% higher risk per decade increase in age), male sex (29%), CKD (21%), cerebrovascular disease (CEVD; 22%), blood cancer (38%), immune suppressant use (43%), high blood pressure (18%), alcohol-related disorders (54%), and multiple sclerosis (over double the risk).