Dry pet food contaminated with salmonella sickened at least 79 people over three years, U.S. health officials said Thursday while warning that pet food may be an under-recognized source of infections in humans, especially young children.

"This outbreak is the first documented outbreak to associate human salmonella infections with contaminated dry dog food and to trace human illness to a contaminated pet food plant," the U.S. Centers for Disease Control said in its weekly report on illnesses and deaths.

No one is known to have died of the rare strain, called Salmonella enterica serotype Schwarzengrund. The illnesses occurred between Jan. 1, 2006, and Oct. 31, 2008.

Among the seven cases where age information was available, six were two years old or younger.

In five cases where people visited a health-care professional, two had bloody diarrhea and one had to be hospitalized. No information on symptoms was available for the other cases.

"Dry pet food has a one-year shelf life," the CDC said.

"Contaminated products identified in recalls might still be in the homes of purchasers and could cause illness. Persons who have these products should not use them to feed their pets but should discard them or return them to the store."

No illness was reported in any pet.

On Sept. 12, Mars Petcare US announced a voluntary recall of more than 100 products in the U.S., including Special Kitty, Pedigree and Member's Mark, among others.

On Oct. 1, the company announced its factory in Everson, Pa., would be closed permanently, the CDC said.

Wash hands after handling pet foods

Since 2006, at least 13 recall announcements involving 135 pet products such as dry dog food and cat food, pet treats, raw diets and pet supplements have been issued because of salmonella contamination, according to the U.S. Food and Drug Administration.

The last reported case in the outbreak was tested on Sept. 18, the CDC said.

In March, the Canadian Food Inspection Agency announced a recall of Roll Over Pork Tenders Premium Dog Treat, made by Alberta-based Rollover Premium Pet Food, because the pet food may contain salmonella bacteria.

Consumers and health departments should be aware that all dry pet food, pet treats and pet supplements might be contaminated with pathogens such as salmonella, the CDC said. Canned pet food is unlikely to be contaminated because the manufacturing process should eliminate bacterial contamination, the agency added.

To prevent infection, people should wash their hands for at least 20 seconds with warm water and soap immediately after handling dry pet foods, especially before preparing and eating food for humans, health officials in the U.S. and Canada recommend.

Infants should be kept away from pet feeding areas, and children under five should not be allowed to touch or eat any types of dry pet food, treats or supplements, the CDC advised.