West Nile Virus

What Is West Nile Virus?

West Nile Virus (WNV) can produce a potentially serious illness.  This important mosquito-borne virus can infect humans, horses (and other mammals), and birds primarily in urban and suburban environments.  WNV is similar to another mosquito-borne disease - St. Louis Encephalitis (SLE).  Historically, WNV typically occurs in Africa, Eastern Europe, Asia and the Middle East.  In 1999, WNV became a new disease in the U.S. starting in New York City and then spreading across most of the country during the next few years.  DHEC confirmed WNV-infected birds for the first time in Beaufort County in September 2003.  During ideal environmental conditions, experts anticipate WNV to be a seasonal health risk that will provide challenges for mosquito control and public health personnel during the summer and fall, although the mosquito season in Beaufort County is usually April - November.

How Does WNV Spread?

The principle route of human infection with WNV is through the bite of an infected mosquito.  A few types of mosquitoes become carriers of WNV when these blood-sucking insects feed on infected birds.  After a developmental period of 10 to 14 days for the virus, infected mosquitoes may spread WNV to humans and other animals during subsequent biting activities.  Most mosquito bites will not lead to a WNV infection.

Fifty-seven different types of mosquitoes occur in Beaufort County (see Mosquitoes of Beaufort County, South Carolina).  However, only one type of mosquito (the "southern house" mosquito - Culex quinquefasciatus) serves as the primary carrier of WNV in Beaufort County.  This mosquito typically is active (and biting) during mid- to late-evening hours.

Additional routes of WNV infection represent a very small proportion of human cases and signify neglible risks.  These routes include:  blood transfusions, organ transplants, mother-to-child (ingestion of breast milk and transplacental), and occupational (laboratory accidents).

WNV is not spread through casual contract, such as touching or kissing a person infected with the virus.

What Is the Risk of Becoming Sick From WNV?

All persons (within WNV-infected areas) are at risk of acquiring WNV illness.

Persons at least 50 years of age show the highest health risk to develop serious symptoms of WNV if they become sick, and should take special care to avoid mosquito bites.

Persons who work or play outdoors for extended periods of time are at an increased risk of acquiring WNV because of the possible prolonged exposure to infected mosquitoes.  Avoiding or protecting against mosquito bites is essential during WNV activity.

Currently, health personnel evaluate all donated blood for WNV before use.  The risk of acquiring WNV through blood transfusions and organ transplants is very small, and should not dissuade individuals who need surgery.  If you have concerns, talk to your medical care provider.

Pregnancy and nursing do not increase the health risk of acquiring WNV; however, these risks are still under investigation.  If you have concerns, talk to your medical care provider.

The health risk status to immunocompromised (weakened immune system) persons for WNV is unknown.

Complacency about taking precautions and preventive measures for humans and horses may occur in some areas where WNV is prevalent during multiple years.  Since 1999, WNV does not cause widespread human mortality.  This disease can inflict unpleasant flu-like and sometimes long-lasting symptoms in people.  Even though two equine vaccines are currently available, about one-third of unvaccinated horses die after infection with WNV.

How Soon Do Infected Persons Become Sick With WNV?

Persons may develop symptoms from 3 to 15 days after a WNV-infected mosquito bite.

What Are the Symptoms of WNV Infection?

No Symptoms in Most People:  about 80%  of people infected with WNV will not exhibit any symptoms.

Milder Symptoms in Some People:  up to 20% of the people infected with WNV will develop "West Nile fever" (influenza-like illness).  Symptoms include:  fever, headache, body aches, nausea, vomiting, sometimes swollen lymph glands, and/or sometimes a skin rash on the chest, stomach and back.  Symptoms can last 2 to 10 days.

Serious Symptoms in a Few People:  about 1 in 150 (< 1%) people infected with WNV will develop severe illness or "neuroinvasive disease".  Specific types of "neuroinvasive disease" include: 

  • "West Nile encephalitis" (inflammation of the brain)
  • "West Nile meningitis" (inflammation of the membrane around the brain and spinal cord)
  • "West Nile meningoencephalitis" (inflammation of the brain and the membrane surrounding the brain)
  • "West Nile poliomyelitis" (inflammation of the spinal cord).

Serious symptoms include:  high fever, severe headache, neck stiffness, unconsciousness, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and/or paralysis.  These symptoms may last several weeks, and neurological effects may be permanent.

From 1999-2012, forty-eight states have reported WNV activity for a total of 37,088 human cases with 1,549 fatalities (4.2% mortality rate). The 2012 outbreak represented the highest number of WNV cases reported since 2003. From 2002-2012, 66 human cases of WNV, including 5 deaths (7.6%) were confirmed in SC by DHEC.

How Is WNV Infection Treated?

Specific medical treatment does not exist for WNV infection; however, medical providers can treat the symptoms and complications of this disease.  Currently, a vaccine is not available for humans; nevertheless, research studies continue for a Food & Drug Administration (FDA)-approved vaccine (Note:  effective vaccines are available to protect horses against WNV).  In cases with milder symptoms, people experience fever and aches temporarily.  In more severe cases, people should visit a hospital where they can receive intensive supportive therapy, including intravenous fluids, airway management, respiratory support (ventilator), prevention of secondary infections (pneumonia, urinary tract infection, etc.), and nursing care.  Most WNV-infected people recover fully and appear to develop a long-lasting immunity.

What Should I Do If I Suspect a WNV Infection?

Milder WNV illness typically improves in a brief time, and people do not necessarily need medical support for this infection.  If you develop symptoms of serious WNV illness, such as severe headaches or confusion, seek medical attention immediately.  Severe WNV illness usually requires hospitalization.  Pregnant women and nursing mothers should talk to their medical care provider if they develop symptoms that suggest WNV infection.

What Is BCMC Doing About WNV?

We consider WNV as a serious public health threat, especially with virus activity in nearby Savannah, GA and Charleston, SC during recent years.  DHEC, Clemson Veterinary Diagnostic Clinic, and BCMC collectively monitor this mosquito-borne disease among mosquitoes, birds, horses, and humans.  Subsequently, BCMC will intensify surveillance and initiate mosquito abatement efforts if WNV is detected anywhere in Beaufort County.  Some of these methods include:

  • Increase the collection of adult mosquitoes throughout Beaufort County for the testing of WNV
  • Encourage residents and visitors to help with our surveillance program (see Dead Bird Surveillance for WNV)
  • Monitor WNV activity in adjacent counties, such as Chatham Co., GA; Jasper Co., SC; Hampton Co., SC; and Colleton Co., SC
  • Provide additional training for all BCMC employees
  • Participate in the state-wide WNV surveillance program for mosquitoes and dead birds
  • Coordinate our response plan with other government agencies
  • Educate the residents and visitors of Beaufort County about "mosquito hygiene" (source reduction of mosquito breeding sites and avoidance of biting mosquitoes), which are key to reducing the risk of acquiring WNV

What Are Methods to Avoid Mosquito Bites?

  • When outdoors, wear long-sleeved shirts, long-legged pants, socks, shoes, and other protective clothing to stop mosquito bites
  • When outdoors, avoid places where large number of mosquitoes occur (such as salt marsh and heavily wooded areas)
  • When outdoors, use insect repellents to prevent mosquito bites (Note: you should carefully follow the label instructions)
  • Avoid being outdoors during peak mosquito biting periods (such as at dawn and dusk, and especially during 1 hour before to 1 hour after sunset)
  • Exclude mosquitoes from your home with tight fitting screens on doors and windows
  • Eliminate all sources of standing water on your property that can support mosquito breeding

What Are Methods to Stop Mosquito Breeding?

  • Empty, destroy, recycle, or cover any containers or items that may hold water (tires, cans, buckets, rain barrels, cemetery urns, etc.)
  • Change water in pet bowls and bird baths at least once per week
  • Remove or empty dishes under potted plants
  • Cover unused pools and boats
  • Clean clogged rain gutter

What Does CDC Recommend to Reduce the Risk of Mosquito-Borne Diseases?

The Centers for Disease Control offer 3 recommendations for reducing the risk of acquiring West Nile Virus or Eastern Equine Encephalitis Virus:

  1. Avoid Mosquito Bites - Throughout the mosquito season, residents and visitors should protect themselves from mosquito bites.
  2. Clothing Can Help Reduce Mosquito Bites - Individuals should wear long-sleeved shirts, long pants, and socks as well as use insect repellents containing the ingredient DEET on exposed skin.  Repellents should also be applied to clothing because mosquitoes can bite through thin clothing.  Individuals must read and follow the manufacturer's directions (as printed on the product).
  3. Be Aware of Mosquito Peak Hours - The hours during early morning (dawn) and mid to late evening (dusk) are peak mosquito biting periods for many types of mosquitoes. Individuals should take extra care to use repellent and protective clothing during early morning and evening, or consider avoiding outdoor activities during these intervals.

* Sources of Information:  primarily from Centers for Disease Control and related websites.