Rabies cases in Fairfax County are up nearly 50 percent so far this year, the highest level in a decade, animal control officers say. And for the fourth consecutive year, Fairfax leads the state in reported cases of the virulent virus.

Fairfax animal control officers have documented 86 cases of rabies, compared with 59 as of this time last year. Both numbers represent a dramatic increase from just a few years ago, when reported cases in the county each year hovered in the twenties or thirties.

Animal wardens say there isn't a single explanation for the surge, noting that rabies occurs in cycles.

"The virus is cyclical, and it's got peaks and valleys," said Barry Lape, a Fairfax animal control officer. "I'd say right now we're at the crest of a peak or very nearly there."

Lape also noted that dense development in suburban areas such as Fairfax brings animals into closer contact with humans, which in turn gives the virus more visibility. In addition, he said, the county's large areas of green space provide habitat for increasing numbers of wild animals that may carry rabies.

Most of the cases documented so far this year (57) involved raccoons. Foxes and skunks accounted for another 20. Lape said a case generally comes to his department's attention when the animal shows up in someone's back yard.

The county's most recent case occurred Wednesday when a raccoon entered a yard in Clifton and started chasing a dog. Fairfax authorities were called to the site after the raccoon began chasing people who live in the house, and officers shot the animal.

Two days earlier, a homeowner in the Merrifield area found a sick fox in the yard. It, too, was rabid.

Statewide, 466 rabies cases have been reported this year, down from 481 as of this time last year, according to the Virginia Department of Health. Loudoun County has recorded the second-highest number of cases this year, 26, followed by Prince William, with 21. Arlington and Alexandria have recorded four each.

Fairfax animal wardens test any wild animal that comes in contact with people or pets. If not already dead, the animal is killed, and its brain tissue examined.

Vaccinations have made rabies rare in dogs, Lape said, but a few cases of rabid cats turn up each year because the vaccination rate is not as high for felines.

In addition to making sure pets get their rabies shots, Lape advises residents to avoid doing things that might attract a wild animal, such as leaving pet food outside.

How will a homeowner know if an animal might be rabid? "One of the symptoms of rabies is aggressiveness and a change of behavior," Lape said. "An animal that is normally shy will climb a six-foot fence to get to your cocker spaniel."