Mosquito and West Nile Virus

West Nile Virus

Evening thunderstorms and the hot, humid summer days are the perfect setup for mosquitoes to appear. Eggs to adults, it takes about a week for mosquitoes to breed. Birds are a reservoir for the West Nile virus (WNV), and mosquitoes can become carriers of the virus by biting infected birds.

When a mosquito harboring the WNV bites a human, the virus can be transmitted. If the human’s immune system is overwhelmed, then one can suffer the symptoms of encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the spinal cord). West Nile virus infection is potentially life-threatening.

West Nile virus is one of several viruses in the family of arboviral infections. The media has helped the general public become aware of WNV in the last five years.

Arboviral infections may be asymptomatic or may result in illnesses of variable severity, often involving the central nervous system (CNS). When the CNS is affected, clinical syndromes ranging from febrile headache to aseptic meningitis to encephalitis may occur, and these are usually indistinguishable from similar illnesses caused by other viruses. Arboviral meningitis is characterized by fever, headache, stiff neck, and abnormal white blood cells. Arboviral encephalitis is characterized by fever, headache, and altered mental status ranging from confusion to coma with or without additional signs of brain dysfunction (e.g., paresis or paralysis, cranial nerve palsies, sensory deficits, abnormal reflexes, generalized convulsions, and abnormal movements).

In addition to WNV, other “members” of the arbovirus family include St. Louis, Powassan, Eastern equine, Western equine, California (with serogroups Lacrosse, Jamestown Canyon, snowshoe hare, trivittatus, and Keystone).

There are a few other viral infections transmitted by mosquitoes, ticks, or midges that affect the central nervous system. They include the Venezuelan equine and the Cache Valley virus.

As in most areas of health, prevention is your best bet when dealing with West Nile virus or any mosquito-borne virus. Avoiding mosquito bites reduces your risk of getting this disease and other pathogens that mosquitoes can carry.

The chance that any one person is going to become ill from a single mosquito bite remains extremely low. The risk of severe illness and death is highest for people over 50 years old or with compromised immune systems, although people of all ages can become ill.

The Centers for Disease Control (CDC) receives nearly 1,000 reports of encephalitis (transmitted by mosquitoes) annually. These diseases can cause serious health problems or even death in the case of encephalitis.

State and local health departments report any suspected cases of WNV to ArboNet, the national electronic surveillance system established by the CDC to assist states in tracking West Nile virus and other mosquito-borne viruses. In 2004, the entire state of Maryland reported sixteen (16) human cases to the CDC, resulting in zero (0) deaths. The District of Columbia had one (1) human case with no deaths. Nevertheless concern exists by the general public for West Nile Virus. California, Arizona, and Texas were the states where most illnesses and deaths were reported in 2004.

Preventing West Nile

Take the following common-sense steps below to reduce your risk:

  • avoid mosquito bites and illness;
  • clean out the mosquitoes from the places where you work and play;
  • help your community control the breeding sites of disease.

Of the active ingredients for insect repellents registered with the EPA, two have demonstrated a higher degree of efficacy in the peer-reviewed, scientific literature. Products containing these active ingredients typically provide longer-lasting protection than others:

  • DEET (N,N-diethyl-m-toluamide)
  • Picaridin (KBR 3023)

Oil of lemon eucalyptus [p-menthane 3,8-diol (PMD)], a plant based repellent, is also registered with EPA. In two recent scientific publications, when oil of lemon eucalyptus was tested against mosquitoes found in the United States it provided protection similar to repellents with low concentrations of DEET.

Oil of lemon eucalyptus has not been tested against mosquitoes that spread malaria and some other diseases that occur internationally. Travel medicine advice should be sought and adhered to according to destination and travel itinerary.

About DEET

Apply insect repellent containing DEET (N,N-diethyl-meta-toluamide) to exposed skin when you go outdoors. Even a short time being outdoors can be long enough to get a mosquito bite. DEET is designed for direct application to human skin to repel insects, rather than kill them. It was developed by the U.S. Army in 1946 and registered for use by the general public in 1957. After completing a comprehensive re-assessment of DEET in 1998, the EPA concluded that, as long as consumers follow label directions and take proper precautions, insect repellents containing DEET do not present a health concern. Human exposure is expected to be brief, and long-term exposure is not expected. Based on extensive toxicity testing, the EPA believes that the normal use of DEET does not present a health concern to the general population. DEET products can be used safely on children. The American Academy of Pediatrics (AAP) Committee on Environmental Health has recently cited: “Insect repellents containing DEET with a concentration of 10% appear to be as safe as products with a concentration of 30% when used according to the directions on the product labels.”

The AAP and other experts suggest using repellent with low concentrations of DEET on infants over 2 months old. Other guidelines suggest using repellents containing DEET after children are 2 years of age.

Repellent products that do not contain DEET are not likely to offer the same degree of protection from mosquito bites as products containing DEET. Non-DEET repellents have not necessarily been as thoroughly studied as DEET, and may not be safe for use on children.

According to the manufacturer and the CDC, DEET is safe for pregnant and/or lactating females. No adverse effects have been reported.

The percentage of DEET relates to protection time.

  • A product containing 23.8% DEET provided an average of 5 hours of protection from mosquito bites.
  • A product containing 20% DEET provided almost 4 hours of protection.
  • A product with 6.65% DEET provided almost 2 hours of protection.
  • Products with 4.75% DEET and 2% soybean oil were both able to provide roughly 90 minutes of protection.

Clothing can help

When possible, wear long-sleeves, long pants and socks when outdoors. Mosquitoes may bite through thin clothing, so spraying clothes with repellent containing permethrin or DEET will give extra protection. Don’t apply repellents containing permethrin directly to skin. Do not spray repellent containing DEET on the skin under your clothing.

The hours from dusk to dawn are peak mosquito biting times for many species of mosquitoes. Take extra care to use repellent and protective clothing during evening and early morning, or consider avoiding outdoor activities during these times.

Some mosquitoes like to come indoors. Keep them outside by having well-fitting screens on both windows and doors. Offer to help neighbors whose screens might be in bad shape.

Mosquito breeding sites can be anywhere. Neighborhood “clean up” days can be organized by civic or youth organizations to pick up containers from vacant lots and parks, and to encourage people to keep their yards free of standing water. Mosquitoes don’t care about fences, so it’s important to control breeding sites throughout the neighborhood.

The best way to avoid West Nile virus and its health effects are to use insect repellant, wear clothing to cover skin, get rid of mosquito breeding sources and standing water. Enjoy the summer.

More information on West Nile Virus and the efforts of the State of Maryland Department of Health & Mental Hygiene can be found at