WEDNESDAY, Nov. 2 (HealthDay News) -- The death toll in the listeria outbreak traced to cantaloupes produced at a Colorado farm has reached 29, with another 139 people sickened in 28 states, U.S. officials said late Wednesday.

Officials said recently that the outbreak appeared to have been caused by unsanitary conditions at the farm.

Inspections on Sept. 22 and 23 by federal and state authorities at the Jensen Farms packing facility in Granada found "unsanitary conditions where the [fruit] may have become adulterated," Sherri McGarry, senior advisor at the U.S. Food and Drug Administration's CORE Network, said during a news conference last week.

Inspectors said the layout of the farm's packing facility allowed water to pool on the floor, making it hard to clean the floor and the equipment used to pack the melons -- and that could have served as a conduit for the germ to latch onto the fruit.

In addition, Jensen Farms did not cool its cantaloupes before placing them in cold storage, which may have caused condensation promoting the growth of listeria, McGarry said.

"We have no reason to believe, at this time, that these practices are indicative of practices throughout the industry," she said.

The plant, which was registered with the FDA in 2010, had never been inspected and was not due to be inspected for five to seven years, she added.

"The tragic deaths and illness from this outbreak have again demonstrated the need to continually address and improve food-safety practices," U.S. Food and Drug Administration Commissioner Dr. Margaret Hamburg said at the news conference.

The agency said that even though the cantaloupes in question were recalled in mid-September, more cases might still emerge since Listeria monocytogenes infection has a long lag time between diagnosis and laboratory confirmation "and also because up to two months can elapse between eating contaminated food and developing listerosis."

The listeriosis-linked deaths have occurred in Colorado (8), Indiana (1), Kansas (3), Louisiana (2), Maryland (1), Missouri (2), Nebraska (1), New Mexico (5), New York (2), Oklahoma (1), Texas (2) and Wyoming (1). The people who have died ranged in age from 48 to 96, the CDC said.

One pregnant woman who contracted the illness had a miscarriage, the CDC said.

Back on Sept. 14, the agency announced that Jensen Farms had voluntarily recalled its Rocky Ford-brand cantaloupes and the produce was "now off store shelves." Consumers -- especially older adults, people with weakened immune systems and pregnant women -- should discard this brand of cantaloupe if it is in their refrigerator, the agency said. Other brands of cantaloupe are safe to consume, however.

At a recent news conference, CDC Director Dr. Thomas R. Frieden called the cantaloupe-linked outbreak "the deadliest outbreak of a foodborne disease that we've identified in more than a decade."

Unlike other bacteria, listeria can flourish in colder temperatures. So, "if you've got a contaminated cantaloupe in your refrigerator, the listeria will continue to grow," Frieden said. "That's one of the reasons why we may see continued cases from cantaloupe already in people's refrigerators in the days and weeks ahead."

Although listeria tends to infect fewer people, it is typically deadlier than other foodborne germs and disproportionately affects the elderly, newborns, pregnant women and anyone with a weakened immune system. People can develop meningitis from the organism, but many people only experience milder diarrhea.

According to the CDC, some 1,600 cases are reported annually in the United States, resulting in 260 deaths.

The bacterium tends to grow in soil and water. But animals can also carry the germ and pass it on to humans through meats, dairy products and other foods of animal origins. Most listeria outbreaks are from animal products, not produce, the CDC said.

More information

The U.S. Centers for Disease Control and Prevention has more on listeria.