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

Sales of turtles with shells less than 4 inches long have been banned in the U.S. since the 1970s because the creatures are known carriers of Salmonella bacteria, which can cause fever, abdominal cramps and diarrhea so severe that some patients need hospitalization. Young children, the elderly and other people with compromised immune systems are particularly vulnerable to serious complications from these infections.

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 (CDC) 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.

The first reported multistate outbreak of salmonellosis linked to small turtles occurred in 2006 and included four cases, researchers report.

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From 2006 to 2011, four additional outbreaks with a total of 394 cases were investigated, including one outbreak that resulted in the death of a 3.5-week-old infant exposed to a small turtle.

For the current study, researchers examined eight additional salmonellosis outbreaks associated with tiny turtles between 2011 and 2014 that included 473 cases in 41 states, Puerto Rico and the District of Columbia.

Children under 18 years old accounted for 74 percent of the cases, 55 percent of affected patients were under 5 years, and 23 percent were less than 1 year old.

Hispanics accounted for 45 percent of the cases with ethnicity information available.

About 28 percent of patients were hospitalized, typically for around three days, among the cases with data on hospitalization.

Researchers also reviewed data from questionnaires about turtle exposure for 102 cases and found 80 percent of the time, people had turtles at home.

Within this subset of infected patients, nearly two-thirds had direct contact with the turtle or its habitat in the week before symptoms began, as did about one-third of infants and more than half of children under age 5.

Often, turtles feces were disposed of in sinks or bathtubs, which were also common places to keep turtles during habitat cleaning, the study found.

One infant got infected after baby bottles and the turtle habitats were cleaned in the same sink.

A limitation of the study is the difficulty of tracking all turtles back to a point of sale, because they were often distributed illegally, sometimes at street fairs and flea markets and other unregulated locations, the authors note.

Still, the findings underscore the importance of avoiding turtles as family pets when young children are in the household, as well as the necessity for good hygiene, said Dr. Elizabeth Barnett, a pediatrics researcher at Boston University School of Medicine who wasn't involved in the study.

"Families should avoid having direct contact with the animals, should not let the turtle roam freely, should avoid placing animals in sinks, bathtubs or other areas where family members may have contact with them, and should dispose of droppings carefully," Barnett said by email.

Even when turtles aren't in the home, parents should still be on the lookout at daycares or schools, Barnett added.

"If these animals are in schools or child care centers, families may want to call this information to the attention of school authorities as they may be unaware of the risk," Barnett said.