Image caption Markets around the US are warning customers about the egg recall

Half a billion eggs in the US have been recalled in an investigation into a salmonella outbreak, which now includes two farms in the state of Iowa.

Iowa's Hillandale Farms announced it was recalling 170 million eggs after tests confirmed salmonella.

The news follows a recall of 380 million eggs by another Iowa farm earlier this week.

A spokeswoman for the Food and Drug Administration (FDA) said the two cases are linked.

Regulators said salmonella cases have increased threefold since May.

About 2,000 salmonella cases were reported between May and July, roughly 1,300 more than usual, the Centers for Disease Control said.

The newest recall from Hillandale Farms applies to eggs sold between April and August.

The salmonella strains causing the illnesses are the same from both farms, according to the Food and Drug Administration.

Hillandale Farms said the eggs were distributed under five different names in 14 US states, while Wright County Egg announced this week the company distributes its product under more than a dozen names.

The recent outbreak could be caused by rodents, shipments of contaminated hens, or tainted feed, says Cornell University microbiology professor Patrick McDonough.

I would anticipate that we will be seeing more illnesses reported likely as a result of this outbreak Dr Christopher Braden, Centers for Disease Control

Mr McDonough told the Associated Press news agency it is not uncommon for outbreaks to have multiple sources because both farms could have rodent problems or have feed that was contaminated.

Increasing illnesses

Centers for Disease Control (CDC) officials warn that salmonella cases may increase.

"I would anticipate that we will be seeing more illnesses reported likely as a result of this outbreak," said Dr Christopher Braden of the CDC.

Cases that occurred after mid-July may not have been reported yet, Dr Braden told reporters.

But he also said that not all of the roughly 2,000 reported cases of the bacteria-borne disease could be linked to the outbreak.

Hillandale Farms eggs are distributed under the names Sunny Farms, Sunny Meadow, Wholesome Farms, West Creek, and Hillandale Farms.

And Wright County Egg eggs are packaged under the names Lucerne, Albertson, Mountain Dairy, Ralph's, Boomsma's, Sunshine, Hillandale, Trafficanda, Farm Fresh, Shoreland, Lund, Dutch Farms and Kemp.

Salmonella poisoning can be deadly for people with compromised immune systems, but no deaths have so far been reported.

Proper cooking can kill the bacteria, but authorities recommend discarding or returning any potentially affected eggs.

The US Egg Safety Center has information on which eggs US consumers should avoid.