West Nile Virus

West Nile virus is a Flavivirus spread by certain species of mosquitoes. It is the most predominant mosquito borne disease in California, and is endemic in the state, which means it’s always present in the environment as it cycles between mosquitoes and a wide variety of bird species. Humans and other animals like horses are also susceptible but do not contribute to the disease cycle.

May contain: toy
CDC/ P.E. Rollin

How is the virus spread locally to humans?

Only certain species of mosquitoes can transmit the virus. Unfortunately, the most efficient vectors of West Nile virus are common in Los Angeles County. The southern house mosquito (Culex quinquefasciatus) is the region's most common vector of West Nile virus. 

Female mosquitoes pick up the virus when they feed on infected birds. The virus penetrates the mosquito’s gut where it replicates and moves into the salivary glands. It is during subsequent blood-feeding by infected female mosquitoes that transmission of the virus can occur.

May contain: animal, bird, text, mammal, and horse

What are the symptoms of West Nile virus?

Primary care physicians are usually the professionals who order West Nile virus tests for their patients. Four out of five people will not show symptoms. People who do show symptoms will experience fever, nausea, head and body ache. Unfortunately, there are uncommon situations where people experience more severe neurological symptoms as a result of inflammation in the brain and spinal cord. These symptoms may result in paralysis, coma and even death. 

Who is at risk?

Anyone living in an area where West Nile-infected mosquitoes exist is at risk of infection. Those most susceptible to symptoms and severe illness are the very young, elderly, chronically ill, and those with a suppressed immune system.

How can I protect myself from getting this virus?

No human vaccine exists for West Nile virus. The best prevention is to wear insect repellent applied directly to all exposed skin. Repellents should contain one of the following active ingredients: DEET (N,N-diethyl-meta-toluamide), IR3535, Oil of Lemon Eucalyptus, or Picaridin/KBR 3023.  Reduce the mosquito population by removing standing water around the home and eliminating containers that might catch and hold water (even a bottle cap). Report standing water that cannot be removed and/or adult mosquitoes to vector control.

How did West Nile virus get here?

West Nile virus was first recognized in the West Nile District of Uganda in 1937. It first appeared in the United States in New York in 1999 – most likely carried by an infected stowaway mosquito in a plane or cargo ship. Within four years the virus had swept across the country with the seasonal movement of infected birds and arrived in the Los Angeles area in late 2003.

Current Status of West Nile:

May contain: mosquito, animal, insect, and invertebrate
Culex Mosquito, female

Human cases occur in Los Angeles every year. West Nile virus is now a permanent part of the ecology of local bird and mosquito populations and will never go away. 

For more information on West Nile Virus please visit:

Center for Disease and Control Prevention-West Nile Virus

West Nile California