For most people, the arrival of warm spring and summer weather ushers in pleasant thoughts of sunny days spent relaxing outdoors with friends and family, maybe at a beach or park or just hanging around at a backyard barbecue or pool party. After a winter’s worth of bleak, gray, cold weather, few things replenish the spirit like a little rest and relaxation in the great outdoors. But as idyllic as all that sounds, there’s a very real danger lurking outside, and it comes from tiny uninvited guests – mosquitoes carrying the highly contagious West Nile virus (WNV).

Every year, about 2,000 cases of West Nile Virus are reported to the Centers for Disease Control and Prevention (CDC), but thousands more may become infected with the disease and not even know it. Up to 80% of those infected with WNV won’t develop any symptoms or will develop symptoms so subtle and relatively brief that the infection may go undiagnosed. In the other 20%, the infection can cause more pronounced symptoms, with a few developing severe and even life-threatening side effects.

Although the chances of developing these more serious side effects is slim, knowing what to look for can help ensure you seek medical attention at the first sign of symptoms, in addition to taking steps to prevent the illness in the first place.

What is West Nile virus?

West Nile virus is a member of the genus Flavivirus, which includes the virus responsible for dengue fever, an illness especially common in Southeast Asia, Africa and Central America. Although both viruses are spread by infected mosquitoes, each one prefers a different species.

The relatively recent appearance of WNV in the U.S. and Europe may seem to suggest that the disease has a modern origin, but the virus has actually caused infections in Africa and the Middle East for centuries. Some scholars suggest that Alexander the Great’s death in 323 B.C.E. may have been caused by an outbreak of the virus.

WNV was first isolated in a Ugandan patient in 1937. By the 1990s, outbreaks had been reported in Europe. In 1999, an outbreak in New York City marked the first known invasion of the virus in the Western hemisphere. Since then, the disease has spread throughout the U.S. and Canada, as well as Central and South America.

How does West Nile virus spread?

The disease cycle of West Nile virus is relatively straightforward: Infected mosquitoes carry the virus and transmit it via their saliva when they bite a host, most commonly birds and mammals. Although humans and other mammals can be infected with a viral load high enough to cause serious side effects, they usually don’t receive with a viral load large enough in their cases to cause reinfection. This means that if a mosquito bites a human or other mammal infected with West Nile virus, the bitten human or animal won’t pick up enough of the virus to carry it to another host. Mammals and most birds are therefore considered “dead end hosts,” as the infection ends with them.

Some birds, most commonly those in the crow and robin families, do develop a high enough viral load to allow for reinfection. The blood levels of the virus are high enough to be “picked up” by a mosquito if it bites the host. If a mosquito bites an infected bird, it can carry the virus to another host – either another bird or a mammal – and potentially cause an infection. Very rarely, West Nile virus can be spread through organ transplants or blood transfusions, or from mother to baby during pregnancy, delivery or breastfeeding. The virus cannot be passed through casual contact between an infected host and an uninfected mammal or bird, nor can it be transmitted by handling a dead bird that’s infected by the virus or by consuming an animal that had been infected.

What are the symptoms of West Nile virus infection?

About 80% of those infected with West Nile virus develop no symptoms or very subtle symptoms that typically are overlooked or ignored. Roughly one out of five individuals, however, will develop flu-like symptoms (called West Nile fever) including fever, fatigue, headache, rash, body and joint pain, and nausea or vomiting. Most of these infections resolve within a week or two, although fatigue can persist for months afterward.

In a few cases, the virus invades the nervous system. This can cause far more severe side effects like encephalitis and meningitis – serious diseases that cause inflammation of the brain or the protective covering of the spinal cord and brain – or West Nile poliomyelitis. In these severe infections, symptoms can include high fever, severe headache, neck stiffness, confusion, seizures, coma or paralysis. About 10% of those who develop a serious infection will die as a result of the disease.

Anyone can become infected with West Nile virus, but serious side effects are most common among people over 60 years of age and those with compromised immune systems, as well as people with chronic illnesses like high blood pressure, cancer, kidney disease and diabetes, and those who have had organ transplants.

West Nile Virus FAQ and Statistics

Organizations like the CDC and WHO (World Health Organization) keep close tabs on the spread of West Nile virus in the U.S. and worldwide.

  • Since 1999, about 44,000 cases of West Nile virus infection have been reported to the CDC; about half of those have been the more severe form of the disease that attacks the central nervous system.
  • In most people, it takes from two days to about a week from the time of the mosquito bite to the onset of symptoms.
  • Most infections occur between June and September.
  • The virus is diagnosed through blood tests or by analyzing the patient’s spinal fluid to look for antibodies produced in response to the infection.
  • Because birds are a primary host for West Nile virus, the CDC recommends reporting dead birds to your local or state health department to enable them to perform testing or other reconnaissance efforts in your area.
  • West Nile virus is only spread by the female mosquito, which needs blood to produce her offspring. Male mosquitoes do not bite.
  • WNV can infect any mammal. In fact, researchers in Texas discovered that a male killer whale housed in a San Antonio marine park died of West Nile virus in 2007.

How to Prevent West Nile Virus Infection

There is currently no West Nile vaccine. The best ways to prevent infection are with personal mosquito repellent products and by controlling mosquito populations. To protect yourself and your family members from becoming infected with West Nile virus:

  • Avoid being outdoors when mosquito activity is at its peak, typically between dusk and dawn.
  • Stay away from standing water or other damp areas likely to host breeding populations of mosquitoes.
  • Cover beds with mosquito netting if you sleep outdoors or in areas with unscreened windows or doors.
  • Wear long pants and tops with long sleeves.
  • Purchase clothing and gear that’s been pretreated with repellent, or apply repellent to your clothing and outdoor gear yourself.
  • Use mosquito repellent products approved and registered by the Environmental Protection Agency (EPA), and always follow application instructions carefully.

Take care with personal mosquito repellents: using them on young children can cause serious side effects, including poisoning. In general, insect repellents should never be used on infants under two months old, and products containing oil of lemon eucalyptus or para-menthane-3,8-diol should not be used on children under three years. Before using any repellent, ask your pediatrician to recommend products safe for children, as well as tips on proper application. Use mosquito netting over cribs and strollers.

You should also eliminate areas on your property where mosquitoes are most likely to breed. That includes any possible reservoir for standing or stagnant water, including upturned flower pots, pool covers, old tarps, buckets, watering cans, or other containers where water can collect. Uncovered or unsealed water barrels and cisterns can also provide a home for breeding populations, as can clogged gutters and discarded tires. Clean pet dishes and birdbaths frequently – not only to destroy mosquito eggs, but also for the health of your pets and your resident bird populations.

If you still have mosquito issues after trying all these methods, wide-scale repellent products and insecticides can help keep the mosquito population under control. Use extreme caution with commercial products – they can cause serious and even deadly side effects to wildlife, as well as pets and humans. Calling in a pest control company with extensive experience in mosquito control can be a wise investment to ensure that these products are applied properly.

West Nile Virus in Horses, Pets and Other Animals

Like humans, other mammals are also subject to infection from West Nile virus. A study published in the journal Emerging Infectious Diseases looked at West Nile virus infection among animals, specifically in dogs and cats. While both species can become readily infected by the virus, dogs and cats seem able to ward off serious signs and symptoms of the disease, and they cannot pass the infection on to humans or other animals. But even though deaths and serious illness from the virus are low among dogs and cats, they do occur from time to time. Any prevention strategies to avoid the West Nile virus should include protecting pets from being bitten by infected mosquitoes.

Horses are more vulnerable to the West Nile Virus. They tend to be much more susceptible to infection and much more likely to develop serious side effects, including death. Statistics from the New Jersey Agricultural Experiment Station of Rutgers University report equine mortality rates of about 34%, with data from Pennsylvania reflecting about a 40% mortality rate from the disease.

In horses, symptoms of West Nile infection are often similar to other types of encephalitis and can include:

  • loss of appetite
  • weakness or paralysis of hind limbs
  • vision problems
  • difficulty swallowing
  • head pressing
  • walking in circles or aimless wandering
  • depression
  • hyperexcitability
  • seizures

As with humans, this virus cannot be spread from one mammal to another. It can only be transmitted by a mosquito that becomes infected from biting a host bird. West Nile Virus is just one of several serious diseases that can be transmitted by mosquitoes.

Although the incidence of serious mosquito-borne infection may be relatively rare, it only takes one infected mosquito to cause a life-threatening disease. Taking precautions to prevent infection should become part of your normal spring and summer routine. Get family members involved, and work as a group to identify and eliminate risks, including containers or areas of standing water and damaged window or door screens or seals, and look into pest control options early in the season. Mosquito control is one activity where devoting a little time to prevention can yield major benefits for you and for the ones you love.