WASHINGTON (AP) – Federal health officials are investigating whether a more potent strain of E. coli is behind an outbreak linked to fresh spinach that has sickened at least 131 people, half of whom have been hospitalized.

The Centers for Disease Control and Prevention said Tuesday that fully 50 percent of those reported sick in the outbreak were hospitalized. That’s more than the 25 percent to 30 percent seen in other outbreaks, said Dr. David Acheson of the Food and Drug Administration’s Center for Food Safety and Applied Nutrition.

“We’re running higher than that,” Acheson told reporters in a conference call. “One possibility is this is a virulent strain.”

Also unexpected was the 15 percent of food poisoning victims who developed a type of kidney failure called hemolytic-uremic syndrome. Five percent is more typical, Acheson said.

He cautioned that the numbers could be skewed by underreporting of less-severe cases of illness: “It’s too early to say at this point,” he added.

Reports of illness continued to trickle into the CDC – the tally was up from Monday’s 114 sickened, though the death toll remained at one, a 77-year-old woman from Wisconsin. Officials said that the cases appeared to have occurred earlier but were only now being reported and that consumers were no longer being exposed to contaminated spinach.

No one appears to have fallen ill since Sept. 5, according to the CDC.

Still, the FDA continued to warn people not to eat raw spinach.

Natural Selection Foods LLC, a San Juan Bautista, Calif., company whose multiple brands many people reported eating before falling sick, has recalled spinach products distributed throughout the United States. The company also distributed spinach to Canada, Mexico and Taiwan.

River Ranch Fresh Foods of Salinas, Calif., recalled spring salad mixes containing spinach purchased from Natural Selection.

On Tuesday, a third company said it was voluntarily recalling salad mixes that may contain spinach supplied by Natural Selection. RLB Food Distributors LP, based in West Caldwell, N.J., said the recall included various salad mixes sold under the Balducci’s and FreshPro brands and distributed on the East Coast.

Meanwhile, FDA inspectors visited nine California farms Tuesday, seeking signs of past flooding or cases where contaminated surface areas had come into contact with crops, said Robert Brackett, director of the FDA’s Center for Food Safety and Applied Nutrition.

They were on the lookout for animal droppings in the fields; checking on sanitary conditions inside the plants where produce is processed; and taking samples from produce itself, as well as from common areas in the processing plants that could harbor bacteria.

“They will look for any obvious or even suspected places where this organism could gain access to the produce,” said Brackett, while acknowledging it was unlikely they would pinpoint the exact source of the contamination.

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On the Net:

Centers for Disease Control and Prevention: http://www.cdc.gov

Natural Selection Farms: http://www.ebfarm.com

AP-ES-09-19-06 1949EDT