An unusual drug combination that helped an unvaccinated teenager survive rabies has failed to save three other infected children, federal health officials reported Thursday.

It wasn't clear why the treatment succeeded with one child and failed with the others. Factors could include the strain of the virus, the dosing of the drugs and the time between infection and treatment, said Dr. Charles Rupprecht, chief of the rabies program at the U.S. Centers for Disease Control and Prevention.

"We believe speed is of the essence," said Rupprecht, a co-author of a report in this week's issue of the CDC's Morbidity and Mortality Weekly Report.

Rabies is a viral disease most often transmitted through the bite of an infected animal. It attacks the central nervous system and, if untreated, can lead to anxiety, confusion, paralysis, hypersalivation, difficulty swallowing, and fear of water. Death usually occurs within a week of the onset of symptoms.

In October 2004, a 15-year-old Wisconsin girl, Jeanna Giese, was hospitalized a month after she was bitten by a bat in church. She recovered after Milwaukee doctors used drugs to induce a coma and then gave her antiviral medications including ribavirin, ketamine and amantadine.

The success of the so-called Wisconsin protocol was reported in the New England Journal of Medicine.

But treatment based on the same protocol did not succeed in at least three children who developed symptomatic rabies last year. Two of the cases were detailed in the CDC report.

One was a 10-year old Indiana girl who contracted the virus from a bat and died in November. The other was an 11-year-old California boy who apparently got rabies from a dog while he was living in the Philippines and died in December.

A third case from Texas was not included because the report's authors did not receive necessary information before publication, Rupprecht said. That case, reported by the media, involved a Houston teenager bitten by a bat who died last May.

Health officials believe the Indiana girl was infected more than three months before she was treated, and were told the California boy was bitten by a dog about two years before he got sick.

Rabies kills about 55,000 people each year, mostly in Asia and Africa. About 40,000 Americans are exposed each year. Nearly all are successfully treated before symptoms appear, through three doses of vaccine.

The rarity of U.S. cases and other difficulties of constructing a proper study will make examining the Wisconsin protocol difficult, said Dr. David Weber, an infectious diseases specialist at the University of North Carolina.

The emphasis should be on preventing rabies, he added. The Wisconsin protocol involves weeks of specialized attention in a hospital intensive care unit, which is more available in the United States than in many countries where rabies is more common.

"This not a practical treatment for virtually any other country in the world," Weber said.