The discovery of romaine lettuce contaminated with listeria by U.S. Food and Drug Administration was lucky, according to food safety experts.

Usually, contamination in leafy green vegetables, like lettuce, is discovered once illnesses are reported to health officials. But so far, no illnesses have been linked to the listeria outbreak in lettuce since it was reported in September.

"It's random chance that this was found", Caroline Smith DeWall, food safety director at the Center for Science in the Public Interest told the Wall Street Journal.

According to the WSJ report, the deadly listeria contamination was discovered by the U.S. Food and Drug Administration during a random research effort on the safety conditions of leafy green vegetables.

California farm True Leaf Farms, had voluntarily submitted its lettuce to the FDA for examination when it came out positive for listeria contamination.

The lettuce recall from True Leaf farms was extended to 19 states and Alberta, Canada last week. It included 2,498 cartoons of chopped or shredded romaine lettuce, informed the distributor Church Brothers in a statement on September 29.

The states involved in the recall are Alaska, Alabama, Arizona, California, Colorado, Connecticut, Florida, Illinois, Kentucky, Maryland, Minnesota, Missouri, Nevada, New York, Ohio, Oregon, Pennsylvania, Tennessee and Vermont.

According to Church Brothers' CEO Steve Church, only 90 cartoons went to retail sales while the rest went to institutions such as restaurants and cafeterias, reports the Associated Press.

The lettuce was recalled after a check from the Food and Drug Administration found that a sample from one bag tested positive for listeria, an organism that can cause serious and sometimes fatal infections in young children, frail or elderly people and others with weakened immune systems.

Listeria can also cause miscarriages and stillbirths among pregnant women.

No illnesses related to this finding had been reported as of September 29.