West Nile Virus Infections Reach a New High


West Nile Virus is transmitted to humans through the bite of a mosquito. (Photo credit: Sergejs Nescereckis / Alamy)

West Nile Virus is making headlines all across the country this summer. The number of human infections from West Nile this year is the highest ever reported in the United States. What exactly is West Nile Virus? Where did this mosquito-borne illness come from? And can anything be done to prevent its spread?

Humans contract the West Nile Virus (WNV) from the bite of infected mosquitos. Mosquitoes become infected after they bite infected birds. First identified in Uganda in 1937, the virus spread throughout Africa, the Middle East, and Europe. It was first detected in New York in 1999, and it quickly spread across the United States between 2001 and 2004. Evidence suggests it crossed the Mississippi River sometime between late 2001 and early 2002. By 2005 in the U.S., the virus had completely evolved into a new strain that could spread itself even more efficiently than the original. During this same time, the virus also spread to Mexico, the Caribbean, and South America.

What is West Nile Virus?

West Nile is an RNA virus and is related to yellow fever, dengue fever, and Japanese encephalitis. It is a neurotropic flavovirus, which is a type of virus that infects the nervous system, commonly spread through the bites of infected insects. In humans, WNV replication occurs in the Langerhans dendritic cells of skin. Following replication, the virus migrates to and infects the lymph nodes. Infections of peripheral tissues, including the spleen and kidneys, occur soon after. As soon as one week after being contracted, infection of the tissues of the Central Nervous System (CNS) occurs. In most cases, West Nile Virus is cleared from all tissues in about two to three weeks after initial infection.

According to the Centers for Disease Control (CDC), the majority of West Nile Virus infections are not life threatening in fact, 80 percent of those infected by the virus never actually experience any symptoms. In addition, only 1 in 150 infections result in severe illness. Referred to as neuroinvasive infections, these severe illnesses occur in two forms: West Nile encephalitis (which affects the brain) and West Nile meningitis (which affects the tissues surrounding the brain or spinal cord).

Symptoms of West Nile Virus typically appear three to fourteen days after being bitten by an infected mosquito. These symptoms typically last from three to six days and then clear up. The flu-like symptoms may include:

  • abdominal pain
  • diarrhea
  • fever
  • headache
  • lack of appetite
  • muscle aches
  • nausea
  • rash
  • swollen lymph nodes
  • vomiting

Symptoms of the more-severe neuroinvasive infections include confusion, lack of clear thinking, loss of consciousness, muscle weakness, stiff neck, and weakness of a single arm or leg. Confirmation of a diagnosis of the severe form of WNV infection requires a complete blood count (CBC), head CT or MRI scan, or a lumbar puncture. Those diagnosed with the severe form of WNV infection have an uncertain prognosis. Neuroinvasive infections have a 10 percent mortality rate. For those that survive the severe infection, complications such as permanent muscle weakness or brain damage occur in 50 percent of cases.

Populations at greatest risk for WNV include those who are very young or very old, those with compromised immune systems, and pregnant women. Evidence indicates that the virus can also be spread through blood transfusions, organ transplants, and through a mothers milk to her nursing child.

How does West Nile Virus Spread?

As of August 21, West Nile Virus (whether identified in humans, birds, or mosquitoes) has been reported in 47 states. Of the 1,118 reported cases, 629 (56%) were identified as being neuroinvasive, while 489 (44%) were non-neuroinvasive cases. The number of cases of WNV infections in 2012 was the largest amount ever recorded in the United States. Typically, the number of West Nile Virus infections peaks in the early fall, when mosquitoes carry the highest amounts of the virus. Infection rates begin to drop off soon after as colder temperatures arrive and mosquitoes die off.

Though birds were initially thought to be responsible for the spread of the virus, scientists at Johns Hopkins University found that this was not the case. If birds spread the disease, then the movement of the virus would have followed a north-south leap-frog pattern mimicking the birds migratory routes. This pattern was not found. Instead, genetic analysis of DNA collected from the mosquito species C. tarsalis (a major spreader of West Nile Virus) in 20 locations across the Western United States led scientists to identify three clusters of C. tarsalis populations. These clusters showed extensive gene flow between the populations, indicating widespread movement of the mosquitoes, thus confirming that they were the key to the virus spread across the country. Interestingly, the scientists also found that gene flow between mosquito populations was hampered by three different physical barriers the Sonoran Desert, the Rocky Mountains, and the High Plains Plateau.

Though more than 60 mosquito species and over 300 bird species can be infected with the virus, only one or two mosquito species and just a few bird species are responsible for the majority of the virus spread. Research led by scientists at the University of California-Santa Cruz found that the American robin is a super-spreader of the virus. Apparently mosquitoes prefer feeding on robins over other bird species in an area. And, though we most often hear about human cases of West Nile Virus, the mosquito species that are most responsible for the spread of the virus actually prefer feeding on birds–feeding from humans occurs incidentally.

WNV is transmitted the most in developed and agricultural habitats, because its able to take advantage of species that do well around humans. But humans aren’t the only species affected by the virus. Other mammals, reptiles, and amphibians also can suffer from its effects. The virus has particularly impacted bird populations, with millions of North American birds dying since its introduction. For example, crows are less abundant than they once were and robin populations, which had been experiencing exponential growth, have since flattened.

How Can West Nile Virus Be Controlled?

Some particularly hard-hit communities have instituted community-spraying programs. These programs may be land-based or aerial. In Texas, which accounts for the majority of this years West Nile Virus infections, Dallas County has instituted an aerial spraying program to combat the virus spread. The pesticide being sprayed has been approved by the U.S. Environmental Protection Agency (EPA) for ground and aerial use for outdoor residential and recreational areas. This pesticide is made up of two different insecticides Prallethrin and Sumithrin.

Some concerns have been raised about pesticide spraying programs, however. Issues surrounding mosquito-control programs include pesticide effectiveness, potential effects on human health, and its detrimental effects on beneficial insects such as honeybees and ladybugs. Many mosquito pesticides are highly toxic to honeybees, and this is particularly worrisome as bees are already facing decimated populations due to colony collapse disorder. Cats are also susceptible to the active ingredients in mosquito pesticides, as their livers are unable to efficiently detoxify the insecticide from their systems. In communities that have instituted spraying programs, health officials have warned residents to stay inside during spraying hours, to close windows and shut off window air conditioner vents, and to bring in pets and their water and food dishes.

What can you do to protect yourself from West Nile Virus infection? The CDC recommends using mosquito-repellants that contain DEET and wearing long pants and shirts with long sleeves while outside. It is also important to prevent mosquito breeding grounds. Pools of water, such as those in trash cans or plant saucers, should be dumped. Water in bird baths should be changed on a weekly basis. It is also important to not handle dead birds and instead call public health officials if dead birds are found.

More to Explore
CDC: West Nile Virus
NIH: Understanding West Nile Virus
West Nile Virus Transmission Linked to Land Use Patterns and ‘Super-Spreaders’
West Nile On the Rise Again After a Quite Decade
Scientists Moving Toward Ending Threat of West Nile Virus


  1. Enjoyed reading through this, very good stuff, appreciate it.

  2. Very informative. I did not know that mosquitoes feed off of birds. I had no idea this virus has killed so many birds.

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