One of the largest-ever US recalls grew to more than half a billion chicken eggs Saturday when a second producer pulled eggs potentially contaminated with salmonella bacteria, an industry official said.

There are no known salmonella deaths due to the eggs, but the US Food and Drug Administration is investigating the origins of a salmonella outbreak that has jumped to 1,953 cases between May and July - more than twice the normal number of cases for the period.

"Hillandale Farms has recalled 170 million eggs" that had been distributed across 14 US states, Krista Eberle, director of food safety programs at the industry's Egg Safety Center, told AFP.

"This is certainly one of the largest recalls the industry has seen," Eberle added.

Some 380 million eggs were voluntarily recalled one week earlier by Wright County Egg, in Galt, Iowa, officials said.

The FDA and the Centers for Disease Control and Prevention (CDC) were seeking to confirm just how many people across at least 14 states, from California to Arkansas, have actually been sickened by the eggs.

FDA spokeswoman Pat El-Hinnawy said there were on-site investigators at both Hillandale Farms and Wright County Egg. "The recalls are related," El-Hinnawy said.

The CDC has warned that the outbreak could actually be larger than reported, because most people who get sick from salmonella do not seek medical treatment.

"Far more people likely have been impacted," said CDC media relations director Glen Nowak.

The FDA said infected rodents many have spread the salmonella.

The companies have decided to pasteurize all their fresh stocks of eggs to kill off any salmonella bacteria. Eberle said some of the recalled eggs could be pasteurized and then used in commercial egg products like liquid egg whites and pancake mix.

Salmonella is spread most often by the consumption of food contaminated by animal fecal matter, according to health experts.

The microbe usually flourishes within the intestinal tracts of fowl and mammals.

An estimated 400,000 people are infected with food-borne salmonella each year in the United States, according to the CDC.