Image caption Listeria thrives in low temperatures and outbreaks are rarely associated with fresh produce

At least 13 deaths and 72 illnesses have been linked to a listeriosis outbreak from fruit in the US state of Colorado, health officials say.

Three more deaths could be related to the contaminated cantaloupe melons.

Eighteen states have reported infections from one of the four strains of listeria involved.

Of 13 confirmed deaths, four were in New Mexico, two in Colorado, two in Texas and one each in Kansas, Nebraska, Maryland, Missouri and Oklahoma.

Listeria infections have been reported from California to Virginia, the Centers for Disease Control and Prevention (CDC) said in an update.

Pregnant women vulnerable

Investigators are trying to establish if three deaths in New Mexico, Kansas and Wyoming are linked to the tainted fruit.

That long incubation period is a real problem Dr Robert Tauxe , Centers for Disease Control and Prevention BBC - Health Listeriosis

The CDC warned the number of incidents was likely to grow, since symptoms can take four weeks or more to appear.

"That long incubation period is a real problem," Dr Robert Tauxe of the CDC said.

"People who ate a contaminated food two weeks ago or even a week ago could still be falling sick weeks later."

The source of the outbreak has been traced to cantaloupes grown at Jensen Farms in Granada, Colorado.

Earlier this month, the US Food and Drug Administration (FDA) confirmed it had found the bacterial strain Listeria monocytogenes in melon samples from Jensen Farms.

In response, the company issued a recall in mid-September of its Rocky Ford-brand cantaloupes.

Jensen Farms distributed the melon brand to at least 17 states between the end of July and September.

The company said in a statement said it was fully co-operating with efforts to contain the outbreak.

The FDA is investigating how the contamination could have happened.

The elderly, pregnant women and people with weakened immune systems are most vulnerable to ill effects from listeria.

Listeria bacteria can grow at room temperature and even refrigerator temperatures.

Officials have advised that all potentially contaminated produce be thrown away immediately, and that any surfaces it may have touched be sanitised.

The toll has surpassed the number of deaths - nine - that was linked to an outbreak of salmonella in peanuts almost three years ago.

In 1988, 21 people died in a listeria outbreak from contaminated hot dogs, while in 1985, 52 deaths were linked to listeria contamination in Mexican-style soft cheese.

The CDC says around 800 listeria cases are reported in the US each year.