West Nile virus is spreading: Most people do not know they have it, but these symptoms can be dangerous

West Nile virus has become a quiet but persistent threat in temperate climates, including the region, even though most infected people never realize they have contracted it.

The virus moves through a natural cycle involving birds and mosquitoes. Birds serve as the main reservoirs, while mosquitoes become infected when they feed on infected birds and can then transmit the virus to animals and humans. People and horses are considered “dead-end hosts,” meaning they cannot pass the infection onward, but a single bite from an infected mosquito can still cause illness.

After an incubation period of three to 14 days, around 80% of infected people develop no symptoms. The other 20% may develop West Nile fever, which can bring a sudden fever, severe headache, muscle and joint pain, fatigue and general weakness. Nausea, vomiting, swollen lymph nodes and a temporary rash may also occur. Most symptoms last several days, although tiredness can continue for weeks.

Severe disease affects fewer than 1% of those infected, but it can attack the central nervous system. Possible outcomes include encephalitis, an inflammation of the brain, and meningitis, an inflammation of the membranes surrounding the brain and spinal cord. Warning signs include high fever, an intense headache, a stiff neck, disorientation, stupor, tremors, seizures and coma. Acute flaccid paralysis, a sudden muscle weakness resembling polio, is among the most serious manifestations.

The risk rises with age. People over 50 and those with weakened immune systems because of chronic illness, organ transplantation or immunosuppressive treatment face the greatest danger. In Europe, the main carrier is the Culex mosquito, particularly Culex pipiens, which is widespread in urban and rural areas. The virus was first isolated in Uganda in 1937 and gained a foothold in the Western Hemisphere after a major outbreak in New York in 1999.