History of West Nile virus in Timeline

By Popular Timelines Editorial Team · Updated: July 20, 2025

West Nile virus (WNV), a Flavivirus related to Japanese encephalitis, causes West Nile fever. Primarily transmitted by Culex mosquitoes, it cycles mainly between birds and mosquitoes. Humans and horses can contract the virus and exhibit disease symptoms, while other animals rarely do. WNV is a single-stranded RNA virus belonging to the Flaviviridae family, which also includes Zika, dengue, and yellow fever viruses.

1937: First Isolation of West Nile Virus

In 1937, the West Nile virus was first isolated in the West Nile district of Uganda, which is how the virus got its name.

1999: First North American WNV Cases

In 1999, the first North American cases of West Nile Virus were reported. Following this, the virus spread throughout the United States, Canada, Mexico, the Caribbean, and Central America. There have been human and equine cases, and many birds are infected.

2002: Introduction of Equine WNV Vaccines

In 2002, equine West Nile Virus vaccines were introduced in the United States, leading to a significant decline in the number of WNV cases among vaccinated horse populations.

2007: WNV in Killer Whale

In 2007, a fatal case of West Nile Virus in a killer whale in Texas broadened the known host range of West Nile virus to include cetaceans.

2008: Lineage 2 WNV in European Horses

In 2008, lineage 2 of the West Nile virus, previously only seen in horses in sub-Saharan Africa and Madagascar, began to appear in horses in Europe, with the first known outbreak affecting 18 animals in Hungary.

2010: Lineage 1 WNV in South African Horses

In 2010, Lineage 1 West Nile virus was detected in South Africa in a mare and her aborted fetus. Previously, only lineage 2 West Nile virus had been detected in horses and humans in South Africa.