West Nile fever

West Nile fever is an infection caused by the West Nile virus (WNV).

This virus belongs to the genus Flavivirus within the family Flaviviridae. It is a significant concern for public health because it can affect the human nervous system. While many people do not realize they are infected, the virus can cause serious illness in others. Understanding how the virus moves through nature helps scientists track its impact on humans and animals.

The virus functions through a complex biological process once it enters a host.

The WNV virion is a tiny particle, about 45 to 50 nanometers in size. It has a relatively smooth protein surface and contains a single strand of positive-sense RNA. This genetic material is between 11,000 and 12,000 nucleotides long. Once the virus binds to and enters a cell, its envelope fuses with the cellular membrane. This allows the RNA genome to be released into the cytoplasm. The genome then acts as messenger RNA (mRNA) to create viral proteins and serves as a template for replication. New virus particles are eventually packaged and transported to the cell membrane to spread.

Transmission of the virus follows a specific cycle between mosquitoes and birds.

Mosquitoes in the genus Culex act as the primary vectors, or carriers. These mosquitoes become infected when they feed on birds that carry the virus. Common hosts include members of the Corvidae and thrush families, such as the American crow and the American robin. Some bird species develop high enough viral levels to pass the infection back to biting mosquitoes. Humans and horses are considered "dead-end hosts." This is because the virus does not multiply well enough in these mammals to infect the next mosquito that bites them.

Infection typically manifests in three distinct levels of severity. About 80% of infected people show no symptoms at all. Roughly 20% develop West Nile fever (WNF), which is a febrile syndrome. This causes flu-like symptoms such as high fever, headache, chills, and fatigue. These symptoms usually last between 3 and 6 days. Less than 1% of cases progress to West Nile neuroinvasive disease (WNND). This occurs when the virus attacks the central nervous system. WNND can include West Nile virus encephalitis (WNE), which is inflammation of the brain, or West Nile meningitis (WNM), which is inflammation of the membranes surrounding the brain. Some patients may even experience West Nile poliomyelitis (WNP), a syndrome characterized by sudden limb weakness or paralysis.

The history of the virus shows how it has spread globally. Scientists first discovered the West Nile virus in Uganda in 1937. It was not detected in North America until 1999. Today, the virus is found across Africa, Asia, Europe, Australia, and North America.

In the United States, thousands of cases are reported annually. Most of these cases occur during the late summer months of August and September. Because the virus can be spread through rare instances like blood transfusions or organ transplants, blood banks now routinely screen donors to ensure safety.

The significance of the disease is most visible in its impact on vulnerable populations. While most people recover, those over 60 years old or those with weak immune systems face higher risks. For individuals who develop neuroinvasive disease, the risk of death is approximately 10%. Severe cases can lead to permanent brain damage or respiratory failure. Even non-neurologic complications can occur, such as inflammation of the heart or liver. Because there is no human vaccine, prevention relies on controlling the mosquito population. Reducing standing water in items like old tires, buckets, and gutters helps prevent mosquitoes from breeding.

West Nile virus research connects to broader studies of ecology and climate. Scientists have observed that the virus replicates faster and spreads more easily to birds at higher temperatures. This connection suggests that climate change could influence how the disease spreads in the future. Additionally, studying the virus in birds serves as an early warning system. By monitoring bird populations, health officials can detect potential outbreaks before they reach humans. Protecting ourselves through mosquito nets, screens, and repellent remains the most effective defense against this widespread virus.