Reused water bottles can lead to serious stomach infections if they're not washed properly say Alberta health authorities.

Dr. Gerry Predy, Edmonton's medical officer of health, has sent out a public warning to keep the bottles clean.

"You do get germs from your mouth on it and there's bits of protein on saliva that organisms can thrive and grow on," says Predy.

Predy is responding to a University of Calgary study. Researchers tested the water in children's reused bottles at an elementary school.

Dangerous levels of bacteria were found in nearly three out of four samples.

"If these levels of bacteria were in a public water supply, then the public water supply would be shut down or a boil water advisory would be issued," says Cathy Ryan, one of the researchers.

Many people believe that their bottles are free from dangerous bacteria because they're the only ones using it.

"If I have my individual one, I don't see the point (of washing it)," says Lindsay Parsons, a hockey player. Parsons says his bottle is never washed.

"I never got anything from it (and) I don't let anyone else use it," says Dan Gladwin, Parson's team mate.

Researchers say that's the wrong way to go about it. Ryan says germs grow on the saliva that backwashes into the bottles and that can cause severe stomach infections.

"Most people...feel it's something they themselves have been using (so) they are not too worried about it, but it's just like any other dish or utensil they should be washed every day," says Predy.

Predy says all bottles need to be washed with soap and warm water daily and left to dry.