Tiny turtles, a staple of many school science labs and an appealing family pet for people allergic to cats and dogs, may be responsible for a growing number of salmonellosis outbreaks, a study suggests.

Sales of turtles with shells less than 4 inches long have been banned in the United States since the 1970s because the creatures are known carriers of Salmonella, which can cause fever, abdominal cramps and diarrhea so severe that some patients need hospitalization.

Despite the sales ban and the known risk, salmonellosis outbreaks tied to turtles have increased since 2006, a research team led by scientists from the Centers for Disease Control and Prevention report in the journal Pediatrics.

“All turtles — healthy and sick, big and small — can carry Salmonella,” said lead author Dr. Maroya Walters, an epidemiologist at the CDC in Atlanta. “Because young children have less developed immune systems and are more likely to engage in hand-to-mouth behaviors, turtles of any size are not appropriate pets for households, schools or daycares with children younger than 5 years of age.”

Salmonella is part of the normal gut flora of turtles, and there’s no way to distinguish a healthy turtle from an infected one, researchers note. The bacteria are present in feces as well as surfaces and water that the animals touch, making it easy for infections to spread to kids who touch the turtles or play with the tank or habitat.