Virginia health officials are battling the worst outbreak of rabies in the state in more than a decade -- and more than half of the cases reported in the past year have come from Northern Virginia.

Diagnosed cases of rabies in animals have jumped more than 400 percent in the past year alone, state health officials said.

And of the 153 cases of rabies reported in the state in 1981, more than half of them came from Loudoun and Fauquier counties, where raccoon populations have become infested with the fatal virus, health reports show.

Hardest hit has been Loudoun County.

"We had one case last year -- a bat," said Earl Virts, director of the county health department. "This year we had 53 cases." The entire state had only 35 cases reported in 1980.

In Maryland, the incidence of rabies has increased 25 percent in the past year, from 37 to 49 confirmed cases, said Kenneth L. Crawford, director of veterinary medicine for the Maryland Department of Health.

Rabies victims have ranged from a cow to pet cats to skunks, health officials said. In Maryland last summer, a youth was treated for rabies after a rabid skunk wandered into his cottage at a boys' camp.

In Virginia, health officials speculated that a cow in Floyd County in southwestern Virginia contracted rabies when it was bitten by a rabid fox wandering through a pasture.

But the biggest problems have occurred in wild raccoon and skunk populations, said Harry Nottebart, director of the Virginia Bureau of Communicable Diseases.

The sudden outbreak has baffled state and local health authorities, who admit they have no explanation for it. Some blame hunting clubs in southern and western areas of the state that have imported raccoons from North Carolina for hunting dog trials. Other officials say that rabies infestations of wildlife have been moving northward from Florida for the past several years.

Still others say the disease is spilling across the Virginia and Maryland borders from wild animals in West Virginia.

"We're now getting West Virginia's problems," said Crawford.

The entire nation is experiencing the worst outbreak of rabies since 1954, according to Kenneth Bernard of the National Center for Disease Control in Atlanta. The center has recorded nearly 6,800 confirmed cases of rabies in animals this year.

While Virginia has discovered most of its rabies cases in raccoons, Maryland officials say the most common carriers in that state are bats.

District of Columbia health officials haven't recorded a case of rabies in the city in 30 years, according to Ingrid Newkirk of the city public health service.

The more urban locales in Northern Virginia -- Arlington and Fairfax counties and Alexandria -- have found no rabid animals within their boundaries in the past year, but officials in those areas are concerned about a possible spread of the infestation from nearby Loudoun County.

In Fairfax, which has had no reported cases of rabies in 14 years, health officials have an active program of testing suspect wild animals, but to date they have found no cases of rabies.They say, however, that rabies probably does exist in wild animals in the county.

The dramatie and unexplained increase in rabies has alarmed health officials nationwide. The disease, which affects the brain and central nervous system, is almost 100 percent fatal, Bernard said. If treatment is started soon after a person is exposed to a rabid animal, innoculations usually can prevent the person from contracting the disease. Only three humans in the world have been known to survive the disease, he added.

The last case of human rabies in Maryland was reported in 1976 when an Elkton woman was bitten on the finger by a bat that landed on her shoulder. She did not start treatment for several days after being bitten, and she died 44 hours after vaccine treatments began. A human last contracted rabies in Virginia in 1953, said Nottebart.

The infection usually is contracted when saliva from an infected animal enters the blood stream through an open wound, cut or bite.

With the number of confirmed rabies cases escalating rapidly throughout the country, health officials said they are using a revolutionary new vaccine to combat the disease in humans exposed to the virus.

The vaccine, which has been on the market about one year, is made from viruses grown in cultures of human cells and requires five injections for a complete dosage. The old immunization was made with duck eggs and required 23 painful injectionshealth officials said.

The new drug, which Bernard called the greatest breakthrough in rabies research since 1958, when the duck-egg vaccine was developed, is imported from a drug manufacturer in France and is available to physicians only through government agencies, Bernard said. The Food and Drug Administration has not yet approved production of the drug in the United States, he said.

In the past year, 110 humans in Virginia were treated with the vaccine, said Nottebart. He said only about six of them experienced any side effects from the drug, and those were minor.

In an attempt to head off the spread of the disease, Maryland and Virginia officials have been flooding local communities with pleas to immunize pet dogs and cats. Fauquier County veterinarians have vaccinated hundreds of pets in the past several months at special one-day clinics, said John Einarson, health director for Fauquier and four other northwest Virginia counties.

Maryland officials have begun killing some wild raccoons.

"We just have too many raccoons," said Crawford. "We're trying to keep people from giving raccoons food. If we get infections in some of these populations, you can imagine what would happen."

And Alexandria, citing the finding of a rabid cat in Loudoun County, has begun to trap stray cats in an attempt to reduce the city's wild cat population.

Health officials caution residents in all areas against feeding wild animals.

"A rabid animal isn't always vicious," said Einarson. Rabid animals often show no fear of humans, he said. The animals usually act strangely or distressed; only in the most advanced stages do they foam at the mouth and appear vicious.

Wild animals can suppress rabies symptoms for up to seven months, according to health officials, In humans, signs of the disease appear within three days to one week.