The West Nile virus (WNV) is an RNA virus belonging to the Flaviviridae family—and part of the same broad antigenic group as the Japanese encephalitis virus. Today, it causes one of the most significant mosquito-borne infections across the Americas, Australia, Africa, the Middle East, and Europe—particularly in Mediterranean countries. The primary vectors are common Culex-genus mosquitoes. Birds serve as the natural reservoir for the virus; mosquitoes become infected after feeding on infected birds, while humans are considered incidental and ‘dead-end’ hosts—meaning they do not transmit the virus further to other mosquitoes or people.
Why Public Health Authorities Are Concerned
WNV poses a notable public health challenge due to its pronounced seasonality: cases surge from summer through early autumn, driven by increased viral circulation and mosquito activity. Effective control demands coordinated surveillance and prevention efforts by national and local authorities. Rising temperatures, standing water, favorable ecosystems for mosquito breeding, and migratory bird movements all contribute to the virus’s spread. Greece—alongside Italy, Romania, France, Spain, and others—is among the European countries where WNV has become endemic and continues to cause human cases each year, especially during recent warm seasons.
How Transmission Occurs
Nearly all human infections result from the bite of an infected mosquito. Mosquitoes acquire the virus by feeding on infected birds and then pass it on to humans during subsequent blood meals. Human-to-human transmission via mosquitoes or routine social contact does not occur. However, rare cases have been documented through blood transfusion—now largely prevented by mandatory molecular screening of donated blood—as well as organ, tissue, or cell transplantation. Very few instances of mother-to-child transmission and occupational exposure in laboratory settings have also been reported.
Who Faces the Highest Risk?
Between 60% and 80% of infected individuals remain asymptomatic. Roughly 20–40% develop a mild febrile illness—often called West Nile fever—with symptoms like fever, headache, fatigue, body aches, and occasionally rash or swollen lymph nodes. Less than 1%—approximately 0.5–1%—develop neuroinvasive disease affecting the central nervous system, such as meningitis, encephalitis, or acute flaccid paralysis. Mortality in these severe cases ranges from 5% to 10%, rising to as high as 20% among elderly or immunocompromised patients.
Key risk factors for severe disease include advanced age, underlying chronic conditions—including diabetes, hypertension, kidney disease, hematologic or malignant disorders—and, critically, states of immunosuppression.
Symptoms and Diagnosis
The incubation period typically lasts 2–7 days, though it may be longer in immunocompromised individuals. Mild cases—known as West Nile fever—usually resolve within days to a week. In contrast, neuroinvasive disease may present with high fever, stiff neck, confusion, seizures, muscle weakness, or paralysis. Diagnosis relies on detecting WNV-specific IgM antibodies in cerebrospinal fluid or serum, or through PCR testing in early infection. Clinical evaluation remains essential, especially when neurological symptoms appear during peak transmission months.