The Washington area is experiencing its worst rabies outbreak in recent memory, and national health officials are warning residents that the disease will be around for the foreseeable future.

The Atlanta-based Centers for Disease Control have launched an investigation into the causes and potential impact of this sudden outbreak, which has been confirmed in 149 animals in Fairfax County, 242 animals in Loudoun County, two animals in Alexandria, and six in Montgomery County.

"They've had rabies in the Southeast for two decades. Once it is established, it remains for a long period of time," said Dr. Gregory Parham, a CDC veterinarian.

The center is trying to determine what percentage of rabid raccoons can actually transmit the disease. Experts also are studying the migration patterns of the rabid raccoons, and bracing for an eventual onslaught into densely populated urban areas.

"This is an unusual outbreak," said CDC veterinarian Dr. William G. Winkler. "It is unique in its intensity, the rapidity with which it is appearing and spreading, and its potential for exposing more people than any other rabies outbreak in the last 20 years.

"It will be in the District this year. It will slow down a little this winter. But you will have rabid raccoons in the District before you have cold weather."

The CDC, the National Zoo and the U.S. Park Service will begin trapping raccoons in Rock Creek Park, vaccinating them, marking them and then releasing them, in anticipation of the disease spreading into the nation's capital, Winkler said.

Rabies is transmitted in saliva, either through bites or contact with openings in a person's skin. If left untreated, it is almost always fatal. And once its symptoms show up, there is no known cure. But the symptoms can be prevented if the victim is immediately treated with an initial vaccine followed by a series of five injections. Those five shots, administered into a muscle in the arm, are considerably less painful than the old rabies cure, a series of shots through the stomach of a powerful vaccine that often left severe side-effects.

Twenty persons have received rabies treatments this year in Loudoun County, the area jurisdiction with the highest incidence of animal rabies, according to Loudoun County health director Earl Virts. The victims include 10 people who came in contact with a rabid house cat, a woman bitten by a rabid skunk and a man bitten on the ear by a raccoon near the Fairfax County/Loudoun County border.

In Fairfax County, 43 persons have been treated for rabies this year, including four persons who were bitten by rabid raccoons and another 34 who came in contact with rabid raccoons, according to Joseph C. Muzyka, information officer for the health department.

Those persons treated for raccoon bites in Fairfax include a Reston woman who was bitten by a rabid raccoon while getting out of her car, and an Oakton woman who was severely bitten and mauled by a vicious rabid raccoon in her attic.

"The introduction of this rabies in the area leaves us baffled," Muzyka said. "Good Lord! Of the 149 rabid animals, 137 were raccoons. Why? That's the question. Something happened. But it's like asking who started V.D.--you could start with the mummies and go all the way back."

"We were essentially a rabies-free county for 14 years," said Fairfax County health director Dr. Garth Dettinger. "Now Virginia is number two behind Texas in having the most proven rabies cases this year, and 90 percent of it comes from Loudoun, Fairfax and Fauquier counties."

Texas reported a total of 568 rabid animals between Jan. 1 and Sept. 25 of this year. Virginia reported 441 rabid animals. California was third, reporting 387 rabid animals. Those figures were compiled in the CDC's Morbidity and Mortality Weekly Report, dated Oct. 1.

Dettinger said the CDC has asked for the saliva glands of 100 rabid animals for tests. "We'll be shipping a group of heads to them shortly," he said.

The disease has spread rapidly across the Potomac. Maryland officials had already confirmed 53 rabies cases this year through the end of September, mostly in the raccoon populations in Allegany and Frederick counties in the West. Some skunks, a few bats, one cow and several dogs and cats were also confirmed to have rabies in the state.

The disease began spreading west into Montgomery County in June, and the county now has recorded six confirmed rabid raccoons. On Sunday, the state recorded its first human victim when a 5-year-old Wheaton boy was bitten by a rabid raccoon on the C&O canal towpath. Three other persons who handled that raccoon were forced to begin the rabies treatment.

Tuesday night, a Chevy Chase woman began the rabies treatment at Suburban Hospital after she was bitten on the leg by an animal that scurried out of a sewer drain. A physician later identified the teeth marks as belonging to a raccoon.

Dr. Lewis Holder, director of Montgomery County's department of health, predicted rabies will remain in the area at least for two to three more years. "It will probably get worse before it gets better," he said.

Officials in all affected areas have essentially adopted the same program to combat the disease. Area county health officials have launched major public awareness campaigns, in schools and through the media, warning persons to avoid all wild animals, particularly those spotted in the daytime and which appear unusually lethargic or friendly.

All the affected counties are sponsoring vaccination clinics for dogs and cats, although only Montgomery is offering the vaccinations free of charge.

The Humane Society of the United States meanwhile reissued its earlier warning that the disease is spreading into the mid-Atlantic states and asking all pet owners to have their dogs and cats vaccinated, according to spokeswoman Janet M. Huling.

Montgomery has a law requiring dog vaccinations, but officials estimate there are at least 20,000 unvaccinated dogs in the county. Last Sunday, the first day of the free vaccination clinics, only 170 dogs showed up.

Virginia has no animal vaccination requirements, but the affected counties have sponsored clinics along with the local humane societies. Fairfax County held a dozen clinics in May and June, and vaccinated 2,963 pets. "Quite a few cats came in," Muzyka said.

Health officials are particularly concerned about the disease spreading to cats. Huling of the humane society said the incidence of rabies "is worse of a problem in cats" since cat owners, thinking their pets are house animals, are less likely to have them vaccinated. Also, cats are more likely to find raccoons in attics, walls and rafters. Huling quoted CDC statistics saying the incidence of rabies was 30 percent higher in cats.

In Montgomery, County Council members said they would immediately consider a new emergency law requiring that cats be vaccinated.

Montgomery is offering free vaccinations to dogs again next Sunday in the parking lot at the corner of Monroe Street and Fleet Street in Rockville.