TUCSON, Ariz. - Residents of a neighborhood next to the University of Arizona say small white rats have been swimming through sewer pipes and into their toilets.

Laura Hagen Fairbanks, spokeswoman for the county's Wastewater Management Department, said she doesn't know where the rodents come from, however they are the kind that researchers use in labs.

University representatives point out that the same type of white rats are sold in pet stores as food for snakes and other animals.

George Humphrey, spokesman for the Arizona Health Sciences Center said university researchers follow strict guidelines for their lab specimens. Lab rats are euthanized, then double-bagged in red biowaste plastic bags before they are taken to Phoenix and cremated.

"There would be no evidence that these are connected to us, and I wouldn't want that to become an urban myth," Humphrey said.

In one sighting, Wastewater Management officials said a woman saw a rat in the toilet, left the toilet seat up and put down sticky trap paper in the bathroom. Then she closed the bathroom door so the rat couldn't get into the rest of the house.

Once the rat got stuck on the paper, she threw it away.

Hagen Fairbanks said no one knows why the rats are found in only one small area of town or why they show their faces only once or twice a year.

Making it from the sewer up the lines into someone's toilet is a difficult trip, Hagen Fairbanks said.

A four-inch pipe, called the house connection sewer, or HCS, runs from the house to a sewer main. And there's no "trap door" or other barrier in place, she said.

But if the lines are running, the rats have to hold their breath and swim uphill in the pipes against the water current.

"If the rat makes it through your HCS, that's a determined rat," she said.

When calls come in, the department can dispose of the rat if the homeowner hasn't done so already. County workers then flush the sewer line as a precaution to keep any others from making their way up.

The Pima County Health Department said it's best not to handle or touch a toilet-surfing rat, although the chance of getting rabies or plague - often associated with rats - is low in this situation.

"Usually if an animal that small has rabies it dies before it can transmit the disease," said Patti Woodcock, a Health Department spokeswoman. And a live flea would be necessary to transmit the plague, she said.