Targeted proteins called monoclonal antibodies may work to treat West Nile virus, a mosquito-borne disease that came to North America in 1999, researchers reported Sunday.

They found the laboratory-engineered antibodies cured mice infected with the virus, which usually causes only mild fever but can cause deadly brain inflammation in some patients.

“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,” said Dr. Michael Diamond of Washington University in St. Louis, who led the study.

“It also completely protected the mice against death.”

West Nile disease, common in North Africa, parts of Europe and the Middle East, first appeared in New York in 1999 and quickly spread across the continent, affecting Canada and Mexico as well.

It infects birds, horses and people and is spread by mosquitoes. In 2003 it infected a reported 2,300 people and killed 264, according to the U.S. Centers for Disease Control and Prevention. In 2004 it infected 2,470 and killed 88.

“West Nile virus virus has emerged in the United States as a regular seasonal threat, particularly for people over 50,” said Dr. Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases, which funded the study.

“We currently do not have a proven therapy for people with serious West Nile disease, so we will continue to aggressively pursue all promising leads for an effective treatment,” Fauci said in a statement.

The researchers decided to develop monoclonal antibody after finding that antibodies taken from the blood of people who recovered from West Nile fever could cure mice infected with West Nile virus.

Writing in the journal Nature Medicine, the team said it made 46 monoclonal antibodies and screened them until they found the most effective ones against West Nile virus.

Rockville, Maryland-based MacroGenics Inc., made a human-like version of the most effective antibody.

When tested in mice bred to be susceptible to West Nile virus, the antibodies protected them from death even if they got severe cases of the disease.

“Our results are the first successful demonstration of a humanized monoclonal antibody as postexposure therapy against a viral disease and suggest that antibody-based therapeutics may have more broad utility than previously appreciated, especially in the treatment of central nervous system infections,” they wrote.