WASHINGTON?Contaminated chicken feed is likely to blame for a salmonella outbreak at two major US egg producers that has already sickened some 1,700 people, federal health officials said Thursday.

"We don't know if the feeding ingredients came to the facility contaminated or if the feed got contaminated at the facility," said Jeff Farrar, the associate commissioner for food protection at the US Food and Drug Administration (FDA).

Farrar told reporters the chicken feed in question had only gone to two Iowa farms ? Wright County Egg and Hillandale Farms ? and not been distributed elsewhere in the country.

The two farms recalled more than half a billion eggs last week ? the industry's biggest such recall in US history ? after learning salmonella from their products sickened consumers. Several dozen egg brands in 22 US states were affected by the move.

"We are looking at all possibilities here of how contamination could have gone into the feed or on to the farm," Farrar said.

"This contamination can come in through numerous routes ? including rodents, shared equipment, workers ? so we are looking into all those possibilities in our investigation."

Health experts say salmonella is spread most often by the consumption of food contaminated by animal fecal matter.

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

Some 1,700 people have fallen ill in the United States from salmonella found in fresh eggs between May 1 and August 25, according to Christopher Braden, acting director of the US Centers for Disease Control and Prevention's food-borne diseases division.

An estimated 400,000 people are infected with food-borne salmonella each year in the United States, according to the CDC. It can be deadly to vulnerable populations such as the young, elderly, or those with compromised immune systems.

Recent years have seen various massive food recalls in the United States ? from salmonella-tainted peanut butter to pistachios to frozen spinach and milk ? amid criticism that America's food regulation regime is under-staffed.