West Nile Virus
Overview of West Nile Virus
Bites from infected mosquitoes cause most West Nile virus infections in people. In rare cases, the virus also has been spread to people through blood transfusions, organ transplants and from mother to baby during pregnancy or breastfeeding. Pregnant or nursing women should see a doctor if they develop symptoms indicative of West Nile virus.
Insect Shield mosquito repellent clothing protects against bites from mosquitoes that may spread West Nile virus. Wearing a long sleeved shirt, pants, socks (tucked into pants) and a hat or scarf treated with our permethrin repellent is a good idea whenever mosquitoes are around!
Approximately 80% of people infected with West Nile virus have no symptoms, so most never realize they have the disease. The unfortunate minority who develop symptoms typically become ill 3 to 14 days after being bitten by an infected mosquito, but they also are unlikely to know they have the virus.
West Nile virus affects the central nervous system. Mild infections cause flu-like symptoms, including fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands or a skin rash. Symptoms can last from a few days to several weeks. People over the age of 50 are more likely to develop symptoms.
Acute symptoms—including severe headaches or confusion, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis— requiring hospitalization occur in less than 1% of those infected. These symptoms may last several weeks, and neurological effects may be permanent. Intravenous fluids and respiratory support are common hospital treatments, but no cure for West Nile virus has been discovered. In 2003, 262 deaths were attributed to West Nile virus in the United States.
The first recorded outbreak of West Nile virus in North America occurred in New York City in 1999. By the summer of 2004, the virus had spread across the US as far as California.
Mosquitoes become carriers of West Nile virus by feeding on the blood of infected birds.
The Centers for Disease Control and Prevention (CDC) website at features up-to-date maps of West Nile virus activity for the United States, including state maps with county-level data.
Detailed Information About West Nile Virus:
What is West Nile virus and how is it transmitted?
The Culex tarsalis mosquito is the primary vector of West Nile virus. In the United States, West Nile virus is transmitted by mosquitoes, mainly members of the Culex species, although other species can spread it as well. The virus goes through a complex life cycle that involves birds as the reservoir of the virus. People are not the usual host and when infected, they become “dead-end hosts” because the virus does not circulate in human blood.
While 317 species of birds have been found to carry the West Nile virus, the majority do not usually show signs of illness and few die, but West Nile virus infection can be fatal in crows and blue jays. It is also fatal for horses in about 40% of cases. Dogs and cats can also become infected but there is no evidence of transmission from these animals to people.
The virus was first isolated from a woman in the West Nile region of Uganda in 1937. It was found to be the cause of equine encephalitis (swelling of the brain and spinal cord in horses) in France and Egypt in the early 1960s. West Nile virus first appeared in the United States in 1999 with reports of encephalitis in horses and people. Since then, it has been reported in all states except Alaska and Hawaii.
How many cases of West Nile virus infection occur in the United States annually?
Since first appearing on the East Coast in 1999, West Nile virus has spread westwards and now is endemic in all states except Alaska and Hawaii.
Cases of West Nile virus reported in the USA in 2000 (above)
Cases of West Nile virus reported in the USA in 2001 (above)
Cases of West Nile virus reported in the USA in 2002 (below)
Cases of West Nile virus reported in the USA in 2003 (below)
Cases of West Nile virus reported in the USA in 2004 (below)
Cases of West Nile virus reported in the USA in 2005 (below)
Cases of West Nile virus reported in the USA in 2006 (below)
How is West Nile virus infection treated?
As with other viral diseases for which there is no vaccine, West Nile virus infection has no specific treatment. Milder cases are usually self-limiting. Patients with encephalitis or meningitis are hospitalized, given intravenous fluids and respiratory support if necessary. These severe forms of West Nile virus infection are often life-threatening and require intensive medical treatment.
Is there a vaccine for West Nile virus?
There is a vaccine available to prevent West Nile virus infection in horses but not people. There are several initiatives in progress at present to develop a vaccine.
Personal protection measures that prevent mosquito bites are key to reducing your risk. Eliminate potential mosquito breeding sites near your home by emptying containers that hold standing water. Your local government may have a mosquito control plan in place to eliminate mosquito breeding sites in public facilities such as storm water drains and reservoirs.