Officials at the U.S. Centers for Disease Control and Prevention are warning that salmonella infections from dry dog food may be an under-recognized source of illness in people after an outbreak of infections in people was been traced to contaminated dry dog food in Pennsylvania.

"This is the first time human illness has been linked to dry dog food," said CDC epidemiologist Dr. Casey Barton Behravesh, co-author of the report on the findings.

The CDC isn't sure how the salmonella bacteria got into the dog food, officials said Thursday.

An estimated 70 people, mostly in the Northeast, were infected in 2006 and 2007 by dog food produced by Mars Petcare at its Pennsylvania plant, CDC said.

About 40 percent of those infections involved infants, according to the May 16 issue of the CDC's Morbidity and Mortality Weekly Report. None of the cases reported were in Maryland; most of them (29) were in Pennsylvania. At least 11 were hospitalized. No deaths were reported, according to the report. No pets became ill.

Salmonella was identified in feces samples from dogs that ate the dry food. It was also found in open bags of the pet food fed to the dogs and in unopened bags of dog food made in the Pennsylvania plant. Mars Petcare voluntarily recalled some bags of the two brands of food involved, but neither of the recalled brands was related to human illness, the CDC said.

"The most important thing is to wash your hands right after you handle any dry dog food, any other pet food, pet treats, even supplements or vitamins," Barton Behravesh said. "In addition, keep infants and other young children away from pet food, because kids tend to want to see what their dogs are eating and grab at the pet food and play with it or even put it in their mouth."

For a report by Reuters on the outbreak, click here.