Mosquitoes feeding on a person

Mosquitoes conferencing on some poor human’s skin.

I wrote earlier this summer about the allure that some humans have for mosquitoes, but I didn’t delve into any of the disease risks that these bloodsuckers pose. In the United States, the chief health hazard is West Nile virus (WNV), which is a relatively new infectious disease in North America. This blog is a primer on WNV.

As of July 24, 39 cases of West Nile virus infections in humans had been reported nationwide, according to the U.S. Centers for Disease Control and Prevention. Just last week, Virginia reported its first case of a WNV human infection, so the total is now at least 40.

The only fatality thus far occurred in July in southeastern North Carolina. The N.C. Dept. of Health and Human Services has reported only that the person was an adult.

Since the West Nile virus emerged in North America in 1999, human cases of infection have occurred in the 48 contiguous states; it has not spread to Alaska or Hawaii. The first cases were in New York City.

The virus is transmitted by mosquitoes that have become infected by feeding on WNV-infected birds. According to the CDC, more than 300 species of birds have been found to be infected with WNV. Except for crows and jays, however, most birds of these species survive the infection.

The bird is an “amplifier host,” which, the CDC says, is an infected organism in which the virus multiplies and reaches high levels in the bloodstream. Another name for this type of host is reservoir.

The mosquito is a “vector,” and, as such, it transmits the disease-causing agent (WNV) from one host organism (the bird reservoir) to another (human). We humans who are bitten by the vector are “dead-end hosts,” meaning, once we are infected, an uninfected mosquito cannot bite us and then transmit the virus to another susceptible host organism. We’re the end of the line.


WNV was first isolated from a febrile patient from the West Nile district of Northern Uganda in 1937. Although it is still unknown how the virus migrated to North America, it is clear that the source of the WNV strain in New York City originated in the Middle East.

The “epidemiology and ecology” of WNV were first characterized in detail during several outbreaks in the Mediterranean basin in the early 1950s and 1960s, according to a 2003 historical overview of WNV by Dr. James J. Sejvar of the CDC, which was published in The Ochsner Journal. It was not until a 1957 outbreak of WNV in Israel, Dr. Sejvar writes, that “severe neurologic manifestations” of the infection were reported. During subsequent outbreaks—in France in 1962 and in South Africa in 1974—WNV-infected patients with encephalitis and meningitis were recognized.

Before this recognition, WNV was considered to be self-limiting: It may cause a fever, but it generally was a mild illness that resolved on its own.

Indeed, 80 percent of people infected with West Nile virus experience no symptoms or only very mild symptoms, according to the CDC. The other 20 percent develop a fever with symptoms such as headache, body aches, joint pain, vomiting, diarrhea, and a rash. Those who have symptoms recover completely, but may feel fatigue and weakness for weeks or even months.

The CDC classifies WNV-infection cases as either neuroinvasive, meaning the virus has invaded the central nervous system (the brain and the spinal cord), or non-neuroinvasive, meaning it has not. When the virus affects the CNS, encephalitis or meningitis can develop and threaten a person’s life, especially older people.

There currently is no curative treatment for a West Nile virus infection. Medication use is strictly symptomatic—aspirin for fever, for example, or over-the-counter pain relievers for aches. People who are severely ill may need to be hospitalized for “supportive” therapy such as intravenous fluids or pain medication.

The Mayo Clinic reports that interferon therapy, which is a type of immune-cell therapy, is currently being studied for treatment of the encephalitis caused by WNV. Interferons are proteins made and released by your body’s cells in response to invasions by pathogens, such as viruses and bacteria, and to the growth of tumors.


According to the CDC, severe neuroinvasive WNV infection can occur in people of any age, but those over 60 are at greater risk. Not only does the human immune system decline with age, but older people are more likely to have medical conditions, such as cancer, that compromise their immune systems.

Symptoms of the CNS infection include those often associated with a brain or spinal cord inflammation or other neurological disorder: high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis.

Less than 1 percent of all people infected with WNV develop the severe illness, and only about 10 percent of them die from it, the CDC says. It is accurate to say that WNV is a rare, but potentially fatal infection.

People who develop the severe illness may take weeks or even months to recover from it, and they may have permanent CNS damage because of it.

Your physician can confirm the presence of the West Nile virus in your body by testing your blood or analyzing the fluid that surrounds your brain and spinal cord.

If you’re infected, your blood would contain a rising level of antibodies to WNV.

To obtain your cerebrospinal fluid to see if you have an elevated white-blood-cell (WBC) count—WBCs fight infection—your physician would perform a spinal tap, also called a lumbar puncture, by inserting a needle between the vertebrae in the lumbar region of your spine. This is commonly done to diagnose meningitis, which is an inflammation of the meninges (protective layers) of the brain and the spinal cord. An MRI scan can help to detect brain inflammation (encephalitis).

Half of the cases reported to the CDC this year come from two states: California and Lousiana. Texas has reported half as many as each of them: five cases to their 10.

Besides North Carolina, Virginia, California, Louisiana, and Texas, WNV cases have occurred in Alabama, Illinois, Maryland, Mississippi, Nebraska, New Mexico, North Dakota, Ohio, and Oklahoma.

The best “treatment” for a West Nile virus infection is obviously prevention. To protect yourself, wear light-colored clothing, long-sleeved shirts, long pants, and boots and other protective footwear when you’re in mosquito territory; apply insect repellant that has DEET in it, and take other precautions, such as repairing holes in window screens, if you open windows, and emptying out standing water in receptacles such as birdbaths, flowerpots, and trash containers, and cleaning them.

Mosquitoes lay their eggs in and near water, and those eggs incubate faster—and thus produce new mosquitoes faster—in extremely hot weather, like we’ve been experiencing in recent years.

The prime time for the mosquitoes that carry West Nile virus is from dusk until dawn.

Ann, 8/8/18

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