Just when you thought it was safe to go back into the garden, venomous, prickly green caterpillars are on the loose in the Washington area.

Health officials are warning the public to avoid handling the saddleback caterpillar because its hair-like spines are poisonous. Even casual contact can result in rashes, swellings and general irritation. Although the caterpillar has not been known to cause serious illness in this country, officials said, it can cause serious swelling.

Lynn Trimmer, poison information specialist for the National Capital Poison Center at Georgetown University Hospital, said yesterday that her office has received 18 reports of unhealthy caterpillar encounters in recent months, far more than in the past.

"You are going to find them all over," Trimmer said. They can be found on trees, shrubs and leafy plants. "We are getting calls from Maryland, Virginia and Washington."

Rose Ann Soloway, the center's education and communications coordinator, said the increase in reported incidents apparently stems from an unusually large caterpillar population this season.

The saddleback is not your regular garden-variety caterpillar. Trimmer said it is easily identified by its bright lime-green back, with a brown spot on the top surrounded by a white ring, giving the appearance of a saddle on a saddle blanket.

"The rest of it is kind of creepy looking," Trimmer said, describing the horn-like tufts on its head and tail, and tiny spines that cover its half-inch to inch-long body.

The saddleback is native to this area and first appears about May, she said. By November it weaves itself into a cocoon to evolve into a nondescript brown moth. Meanwhile, it can be quite a nasty little critter.

Trimmer said that when the spines of a saddleback caterpillar make contact with the skin, poison enters the body, causing pain and irritation.

Officials said the National Capital Poison Center emergency number (625-3333) is answered 24 hours a day by trained nurses.