State and federal health officials said Saturday that Pennsylvania's hepatitis A (search) outbreak is winding down, even as the number of those infected climbed over 600. The investigation shifted to how green onions linked to the outbreak became contaminated.

Sen. Arlen Specter (search), R-Pa., met Saturday with Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention (search), and state health officials at a community college near the Beaver County Chi-Chi's restaurant where the outbreak was first confirmed Nov. 3.

"We do feel like this particular outbreak has been successfully ended," Gerberding said, although the number of cases probably will rise over the next few days as lab test results come back.

With 605 infections and three deaths confirmed, health officials have found no secondary cases — meaning all those infected were sickened at the restaurant by the scallions, not through contact with infected people. Most of the green onions were used in a mild salsa and a cheese dip.

Hepatitis A is a virus that attacks the liver and can cause fever, nausea, diarrhea, jaundice, fatigue, abdominal pain and loss of appetite. Hepatitis A usually clears up on its own in about two months.

Nearly 10,000 people have been screened for the virus and more than 9,100 received antibody injections, which prevent hepatitis A from taking hold within the first 14 days of exposure.

Specter, who chairs the Senate subcommittee that controls the CDC's budget, called health officials' work stopping the virus the "gold standard of response."

What remains is pinpointing how the onions were tainted.

The Food and Drug Administration (search) already has identified eight brands of Mexican-grown green onions believed to have caused the Pennsylvania outbreak. Green onions also are suspected of causing outbreaks in Georgia and Tennessee that have sickened more than 330 people, and the strains of hepatitis A found in those states and Pennsylvania are very similar.

But the FDA has yet to determine how and where — from the onion fields to the Chi-Chi's northwest of Pittsburgh and more than a dozen restaurants in the other states — the contamination occurred.

Chi-Chi's chief operating officer Bill Zavertnik flew to Pittsburgh on Saturday to read a short statement saying federal and state health officials found no wrongdoing by the company.

"There is currently no industry-accepted means of testing produce for the hepatitis A virus, and beyond that, there is no possible way to wash hepatitis A off contaminated green onions," he told reporters. Zavertnik, who did not take questions, said the chain did everything possible to prevent the outbreak.

But a food safety expert said grocery stores and restaurants have to do more than just properly handle fresh vegetables — they also need to ensure their suppliers are safe.

"You can do absolutely everything right (at the restaurant) and still suffer contamination and hurt people," said Devon Zagory, senior vice president of Davis Fresh Technologies LLC of Redding, Calif., a food safety consulting firm. "There's no such thing as zero risk."

Because green onions cannot be reliably tested or cleaned to prevent hepatitis A, the FDA in 1997 established voluntary guidelines for fresh fruit and vegetable suppliers.

Among other things, the guidelines call for making sure that water used to grow and wash the vegetables is clean; requiring farms to supply field toilets and make workers wash their hands; and tracking the use of pesticides and fertilizers.

Although the guidelines are voluntary, Zagory said Chi-Chi's "should have done this, and if they haven't done this, I myself would take a rather dim view of their practices."

A Chi-Chi's spokesman didn't immediately respond to an e-mail asking whether Chi-Chi's audits its suppliers' compliance with the FDA standards.

The FDA has ordered green onions traced to the outbreaks in Pennsylvania, Georgia and Tennessee stopped at the Mexican border.

The CDC said the onions could have been contaminated with the virus a number of ways, including contact with impure water or poor hygiene by infected workers.

The Chi-Chi's at the Beaver Valley Mall has been closed since the outbreak was confirmed Nov. 3. Chi-Chi's has also removed green onions from its 99 other restaurants in 17 states, from Minnesota to the mid-Atlantic.