Mosquito bites and West Nile virus

Mosquito bites and West Nile virus

Mosquitoes are out with a vengeance this summer. In addition to getting a bad bite, there is the danger of acquiring viruses carried by mosquitoes. It’s important to protect yourself from harm and prevent the spread of such viruses. 

The Shelby County Health Department’s Mosquito Control Program tested pools in Shelby County and found mosquitoes carrying the West Nile virus in zip codes 38115 and 38118.  Mosquitoes carrying the virus are expected to become widespread throughout the county before the end of summer.

As of July 9, cases of human West Nile virus (WNV) infections have been reported in Arizona, Arkansas, Nevada, New Jersey, Oklahoma and Wyoming, according to the Centers for Disease Control and Prevention. It is early in the WNV season so it remains to be seen how many infections there will be this year.

What is West Nile virus?

West Nile virus (WNV) is a virus that humans acquire through the bite of a mosquito that is infected with the virus. Viruses, like WNV, that are spread by mosquitoes are called Arboviruses. The term Arbovirus comes from the combination of the words “ARthropod-BOrne virus”. Arthropod is the scientific name for animals such as insects, spider and crustaceans (e.g. shrimp). There are many other arboviruses, but very few are acquired in the continental United States.

Mosquitoes become infected by feeding on infected birds, which are the primary reservoirs for the virus. The life cycle of the virus is maintained between birds and mosquitoes, particularly Culex species mosquitoes. This occurs because birds can develop a large amount of viruses in their blood, which can then be easily transmitted to other Culex mosquitos when they are bitten again. These mosquitoes also feed on humans and horses, but these animals do not have a large enough amount of viruses in the blood for a long enough time to infect other mosquitoes, so it is only birds that maintain WNV in a particular area. WNV can also, rarely, be transmitted from mother to baby during pregnancy or through blood transfusion or organ donation. All donated blood has been tested for WNV since 2003. Some organ donors are screened for WNV but not all. WNV is not spread from person to person and cannot be acquired by touching a dead animal including dead birds; however, it is always a good idea to wear gloves or use a double plastic bag to dispose of a dead animal.

What are the symptoms of West Nile virus infection? 

Most people (80%) infected with WNV never develop any symptoms at all. We know this because there are people living in areas where WNV circulates who have developed antibodies to the infection but have no history of the illness. For the 20% who do become ill after contracting WNV, most develop symptoms within 3-14 days of infection.

Symptoms include

  • Fever
  • Headache
  • body aches
  • nausea
  • vomiting
  • swollen lymph nodes
  • skin rash

Most people recover within a few days, although symptoms can linger for longer in some people.

More rarely, about one in every 150 people will develop severe “neuroinvasive” disease where the virus infects and affects the central nervous system (brain and/or spinal cord).

Signs of neuroinvasive WNV include the signs and symptoms of inflammation of the brain (encephalitis), inflammation of the membranes around the brain known as the meninges (meningitis), inflammation of the spinal cord and nerves arising from it (myelitis).

These signs and symptoms include:

  • high fever
  • headache
  • neck stiffness
  • lethargy or confusion
  • tremor
  • coma
  • convulsions (seizures)
  • muscle weakness or paralysis
  • numbess or tingling
  • visual changes.

If any of these occur, your child should see a physician immediately.

Although anyone can develop neuroinvasive disease, some people are more likely to suffer this severe form of infection than others; individuals over 60 years of age are at higher risk, as are those with chronic illness such as cancer, diabetes, kidney disease or heart disease. Those whose immune systems are suppressed from disease or medication, such as medications to prevent rejection of an organ transplant, are more likely to develop neuroinvasive disease.

How likely is it that I could get West Nile virus infection? 

The risk of acquiring symptomatic WNV infection in any one individual is very low. The number of cases that occur each year is highly variable. The CDC and state health departments monitor for symptomatic disease in humans and horses (the only domestic mammal that develops symptomatic WNV infection). In addition, they test dead birds for WNV, as well as monitor pools of mosquitoes from traps set out for this purpose. People are at greater risk in areas where there have been more identifications of WNV in birds and mosquitoes.

In 2018, every state (except Alaska and Hawaii) and the District of Columbia, reported human WNV infections. The states with the most cases per 100,000 people were Montana, North Dakota, South Dakota, Nebraska, Iowa and Mississippi. 

In Tennessee, there were 12 cases of human WNV infection in 2018; four in Shelby County and one in Tipton County. The remainder were in Middle and East Tennessee. One cases was reported in nearby Desoto County, Mississippi. 

How do I protect myself and my family from West Nile virus infection?

Because WNV is spread by mosquitoes, the only way to prevent infection is by reducing exposure to mosquitos. Culex mosquitos breed in standing water that collects in any type of container. They are frequently found around homes and like to feed at night.

You can take the following precautions to minimize your exposure to Culex mosquitos and other types of mosquitos that can spread infections

  • Any time you are outside, use a recommended insect repellent. Make sure you reapply the repellent at the recommended time intervals. Do not use insect repellent on babies under 2 months of age and keep away from mouth and eyes. Recommended repellents are those containing one of the following ingredients.
    • DEET
    • Picaridin
    • IR3535
    • Oil of lemon or eucalyptus (OLE) or para-methane-diol (PMD)
    • 2-undecanone
  • Wear long sleeves and long pants to reduce the risk of bites. If you plan to spend a long time outdoors, you can soak your clothing in an insecticide called permethrin, which will provide extra protection
  • Culex mosquitos are most active at dawn and dusk. You can significantly reduce your mosquito exposure by avoiding being outdoors at these times. If you need to be outdoors at these times, make sure you use insect repellent.
  • Keep windows and doors closed to keep mosquitoes out of your house. Make sure you don't have any holes in your screens if you are opening windows.
  • Get rid of mosquito breeding areas by eliminating standing water from around your house. This requires a great deal of vigilance as mosquitoes can breed in tiny pools of water such as the water that collects in a bottle cap. Clear away unnecessary containers. Turn over pots, buckets, kids pools, etc., so that water cannot accumulate.

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