Mosquito season is just around the corner in the U.S., and scientists may have a new lead against West Nile virus infection.

An experimental new treatment based on human infection-fighting antibodies may help fight the virus.

"To our knowledge, these experiments are the first successful demonstration of the use of humanized antibody as a post-infection therapy against a viral disease," says researcher Michael Diamond, MD, PhD, in a news release.

"We could give this antibody to mice as long as five days after infection, when West Nile virus had entered the brain, and it could still cure them," says Diamond in a news release.

Diamond's study appears in the online edition of Nature Medicine.

Fighting Back Against West Nile

A lab-made version of a critical antibody protected infected mice. However, the researchers say more studies are needed before it can be tested in people.

In 2004, 2,470 mild and severe human cases of West Nile virus infection and 88 related deaths were reported to the CDC.

The virus, first reported in the U.S. in 1999, can be fatal, but it doesn't make everyone sick. Some people bitten by infected mosquitoes never become sick or only get mild symptoms. No one knows exactly why.

In the study, researchers tested antibodies from the blood of people who had recovered from West Nile disease. Lab versions were made of the antibodies showing the best potential against the virus.

The lab-made antibodies were more potent than those taken directly from people, the news release notes.

The antibodies were tested in West Nile-infected mice. Three antibodies were linked to better survival even when given four or five days after infection, when the West Nile virus had reached the mice's brain and nervous system.

One particular antibody stood out. According to the study, when given five days after infection, that antibody completely cleared the virus from 68 percent of the mice's brains within four days. The single injection resulted in an 80-90 percent survival rate.

The studies were performed on 5-week-old mice, which have only a 10 percent chance of survival after infection with the virus.

That particular antibody "may be a viable treatment option against West Nile virus in humans," write Diamond and colleagues.

Other Uses?

If the technique proves successful, it might help screen for and develop antibodies for related viral illnesses, says the news release.

For instance, Diamond is using a similar approach to look for an edge against dengue fever, another potentially fatal viral disease spread by mosquitoes, according to the news release.

The dengue virus is found in tropical and subtropical regions worldwide. Today, it affects most Asian countries and has become a leading cause of death and hospitalization in several of those countries, says the World Health Organization.

Some scientists working on the West Nile study work at Macrogenics, the Maryland company that licensed the antibody used in the mouse tests. Diamond now consults for Macrogenics, says a news release from the Washington University in St. Louis' medical school, where Diamond is based.

How West Nile Spreads

The West Nile virus is usually spread by infected mosquitoes. It can also be transmitted by organ transplantation, blood transfusion, or possibly breast milk, says the CDC.

Some infected persons develop fatal illness while others develop a flu-like illness. Severe cases prompt inflammation of the brain or the membranes surrounding the brain and spinal cord. The most vulnerable people are the elderly and those with weakened immune systems, say Diamond and colleagues.

Tips to Thwart Mosquitoes

Here are the CDC's tips for avoiding mosquito bites:

—Apply insect repellent containing DEET.

—Wear long-sleeved shirts, long pants, and socks outside.

—Consider spraying thin clothes with insect repellent containing DEET or permethrin.

—Be aware of peak mosquito hours — dusk to dawn.

—Drain standing water, where mosquitoes lay their eggs.

—Install or repair screens on doors or windows.

The CDC also encourages reporting dead birds to state or local health departments so the birds can be screened for West Nile virus.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Oliphant, T. Nature Medicine, April 24, 2005; online edition. News release, NIH. CDC: "National Institute for Occupational Safety and Health: West Nile Virus." CDC: "West Nile Virus — Surveillance and Control Case Count of West Nile Virus Disease 2004." World Health Organization, "Fact Sheet on Dengue Fever and Dengue Hemorrhagic Fever." CDC: "Fight The Bite! Avoid Mosquito Bites to Avoid Infection." News release, Washington University in St. Louis School of Medicine.