The worst rabies outbreak in Virginia in a decade is moving into Northern Virginia's more urban areas, which have had little contact with the usually fatal disease in the past, health officials report.

Three cases of rabies have been found in raccoons in Fairfax County--one just over the border from Arlington County--within the last six weeks, said Chester Bowman, director of laboratory studies for the county health department. They are the first rabies cases reported in the county in more than three years, he said.

Reported cases of rabies in animals statewide increased almost 600 percent in the first 10 weeks of this year compared to the same period in 1981, state health records show. Of the 66 cases reported in Virginia so far this year, 56 were discovered in Loudoun and Fauquier counties.

"We've been anticipating the spread from the counties to the south and west of us," said Richard Amity, director of Fairfax Animal Control, which handles rabies cases reported in the county.

Two of the infected raccoons found in Fairfax County were picked up in outlying neighborhoods in the western part of the county, but the third was discovered about a quarter of a mile from more densely populated Arlington county, Bowman said.

The rabies outbreak began last spring and has escalated dramatically, sweeping across counties along the northwestern and southern edges of the state, said Tom Sayvetz of the Virginia Bureau of Communicable Diseases.

In 1981, health officials recorded 166 rabies cases statewide, up from 35 cases reported in 1980. In little more than the first two months of this year, however, 66 cases already have been reported.

Health officials said they expect the epidemic to escalate even more in the next few weeks during the raccoons' mating season.

"We don't really know what causes rabies to explode in wild populations like that," said Sayvetz. He said, though, that the outbreak among raccoons is unusual for Virginia as past rabies cases have been confined primarily to bats, skunks and foxes.

Some health officials blame the sudden epidemic on infected raccoons imported into the state from North Carolina by Virginia hunting clubs.

Rabies, which affects the brain and central nervous system, is almost always fatal, health officials say. The disease usually is contracted through a cut, scratch or bite from an infected animal.

Arlington and Alexandria officials said they are worried that the disease may spread into their counties.

The rabies outbreak spurred Alexandria to begin trapping stray cats last December in an attempt to reduce the city's wild cat population.

"The program has been very successful," said Gail Snider, superintendent of animal control in Alexandria. "The public has become so aware of the problem that people are borrowing our traps to the point we don't have any left to use."

"We're concerned," said C.E. Pinkerton of the Arlington Environmental Health Bureau. "There's not much we can do to stop it in wild animal populations. We're just urging citizens to make sure their pets are vaccinated."

Even vaccinations are no guarantee that pets won't contract rabies, however. Rabies was discovered two weeks ago in a 2-year-old dog in Culpeper that had been immunized. That was the first case of rabies found in a dog in Virginia since the outbreak began last spring, health officials said.

Rabies also has been found in several cats and one cow in Virginia within the past year, health records show.

Virginia isn't the only state that has been hit by massive rabies outbreaks. The entire nation is experiencing its worst rabies epidemic since 1954, according to Kenneth Bernard of the National Center for Disease Control in Atlanta.

The number of reported rabies cases in Maryland jumped 25 percent last year, with 50 recorded cases: 34 bats, eight skunks, seven raccoons and one groundhog.

In the first 10 weeks of 1982, Maryland health officials recorded 13 rabies cases, 11 of them in raccoons. The remaining two cases were found in a cow and a bat.

Most of the cases were reported from western Allegheny County. One rabid bat was found in Montgomery County this year and Prince George's has reported no cases this year, although two rabid bats were discovered in the county last year.

District of Columbia health officials said they are concerned that the disease could spread into the city, even though no case of rabies has been reported in the District in 30 years.

"We're still clean," said Ingrid Newkirk of the city public health service. "But we're always worried."