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West Nile, Zika & Other Viruses

West Nile Virus (WNV)

What is WNV?

West Nile virus (WNV) is a mosquito-borne disease that was originally found in Africa. In 1999, it was detected in the eastern United States; since then the virus has spread throughout the United States and is well established in most states, including California.

How do people contract WNV?

  • Infected mosquitoes: Most often, WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers ("vectors") that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite.
  • Transfusions, transplants, and mother-to-child: All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. Transmission during pregnancy from mother to baby or transmission to an infant via breastfeeding is extremely rare.
  • Not through touching: WNV is not spread through casual contact such as touching or kissing a person with the virus, or by breathing in the virus.

How soon do infected people get sick?

People typically develop symptoms from 3 to14 days after they are bitten by an infected mosquito.

What are the symptoms of WNV?

Symptoms vary:

  • Serious Symptoms in a Few People. Less than one percent (about 1 in 150 people) of individuals infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, paralysis, and death. These symptoms may last several weeks, and neurological effects may be permanent.
  • Milder Symptoms in Some People. Up to 20 percent (about 1 in 5) of the people who become infected will display symptoms which can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms generally last for just a few days, although even previously healthy people have been sick for several weeks.
  • No Symptoms in Most People. Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms.

Who is at the greatest risk of getting severely ill from WNV?

  • People over the age of 50 have a higher chance of getting sick and are more likely to develop serious symptoms when infected with WNV.
  • Being outside, especially at dawn or at dusk, increases your risk of being bitten by an infected mosquito. Take precautions to avoid mosquito bites if you spend a lot of time outside, either working or playing.
  • Risk of transmission through medical procedures is very low. All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it.

How is WNV infection treated?

There is no specific treatment for WNV infection. In cases with milder symptoms, people experience fever and aches that pass on their own. In more severe cases, people may need to go to the hospital where they can receive supportive care including intravenous fluids, help with breathing, and nursing care.

If you have had WNV, are you immune to further infections?

It is thought that once a person has recovered from WNV, they are immune for life to future infections with WNV. This immunity may decrease over time or with health conditions that compromise the immune system.

Can animals get sick with WNV?

An infected mosquito can bite any animal, but not all animals will become sick. The disease most often affects birds but may occasionally cause disease in other animals.

  • Wild birds serve as the main source of virus for mosquitoes. Infection has been reported in more than 225 bird species. Although many birds that are infected with WNV will not appear ill, WNV infection can cause serious illness and death in some birds. The most severe illnesses are seen among the corvid birds, which include crows, jays, ravens, and magpies.
  • Squirrels & Rabbits with West Nile virus can develop neurological symptoms such as uncoordinated movement, paralysis, shaking, or circling and may die.
  • Like people, most horses bitten by mosquitoes will not become sick with WNV. However, of those that do, clinical signs may include stumbling, circling, hind leg weakness, inability to stand, muscle tremors, and death. A vaccine to prevent West Nile virus is available for horses and horse-owners should consult with a veterinarian about vaccinations for WNV and other mosquito-borne viruses, such as western equine encephalomyelitis. For more information on West Nile Virus and horses, please visit the California Department of Food and Agriculture.
  • Dogs and cats can be exposed to WNV in the same way as humans. However, these animals are very resistant to WNV and rarely become ill. Concerned pet owners should consult with a veterinarian.

Zika Virus

Zika virus was first reported in the Western hemisphere from Brazil in March 2015.  Zika virus is spread to people primarily through the bite of an infected Aedes aegypti (yellow fever mosquito) or Aedes albopictus (Asian tiger mosquito) mosquito [Can we link this to the section on invasive Aedes?].  It is also possible for a pregnant woman to pass Zika to her fetus during pregnancy or around the time of birth, and a person infected with Zika can pass it to his or her sex partners.  Many people infected with Zika will have no symptoms or mild symptoms that last several days to a week.  However, Zika infection during pregnancy can cause a serious birth defect called microcephaly and other severe fetal brain defects.  Current research suggests that Guillain-Barre syndrome (GBS), an uncommon sickness of the nervous system, is strongly associated with Zika; however, only a small proportion of people with recent Zika virus infection get GBS.  Once someone has been infected with Zika, it is likely he will be protected from future infections.  There is no evidence that past Zika infection poses an increased risk of birth defects in future pregnancies.

For more information, visit

California Department of Public Health Zika Virus page https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/Zika.aspx

CDC Zika Virus page https://www.cdc.gov/zika/index.html

Chikungunya Virus

Chikungunya virus is transmitted to humans from the bite of an infected Aedes aegypti (yellow fever mosquito) or Aedes albopictus (Asian tiger mosquito) mosquito [Can we link this to the section on invasive Aedes?].  Other symptoms may include headache, muscle pain, joint swelling, or rash.  In 2013, chikungunya virus was found for the first time in the Americas on islands in the Caribbean.  Locally-acquired cases were detected in Florida, Puerto Rico, and the US Virgin Islands in 2014.

California Department of Public Health Chikungunya Virus page https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/Chikungunya.aspx

CDC Chikungunya Virus page https://www.cdc.gov/chikungunya/index.html

Dengue Virus

Dengue is a mosquito-borne disease caused by four closely related viruses known as dengue virus-1, -2, -3, and -4.  Dengue virus is transmitted to humans from the bite of an infected Aedes aegypti (yellow fever mosquito) or Aedes albopictus (Asian tiger mosquito) mosquito [Can we link this to the section on invasive Aedes?].  The symptoms of dengue fever are high fever, severe headache, severe pain behind the eyes, joint pain, muscle and bone pain, rash, and mild bleeding (e.g., nose or gums bleed, easy bruising).  Locally acquired dengue is uncommon in most of the United States, but in recent years, locally acquired cases have occurred in Florida, Texas, and Hawaii.  Dengue is endemic in Puerto Rico and the US Virgin Islands, and most of the US-affiliated Pacific Islands.

For more information, visit

California Department of Public Health Dengue Virus page https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/Dengue.aspx

CDC Dengue Virus page https://www.cdc.gov/dengue/index.html

St. Louis Encephalitis Virus

Prior to the introduction of West Nile virus, St. Louis encephalitis (SLE) virus had the greatest medical importance of the mosquito-borne diseases in North America.  SLE virus was first recognized in 1933 in St. Louis, Missouri, and epidemics have occurred sporadically and unpredictably in the subsequent decades.  Symptoms usually appear abruptly, and may include fever, headache, dizziness, nausea, and malaise; more severe symptoms may develop, including stiff neck, confusion, disorientation, dizziness, tremors and unsteadiness.  Like West Nile virus, SLE virus is maintained in a mosquito-bird-mosquito cycle, but no major SLE outbreaks have occurred since WNV arrived in the Americas.

California Department of Public Health St. Louis Encephalitis Virus page https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/SLE.aspx

CDC St. Louis Encephalitis Virus page https://www.cdc.gov/sle/