Test results have identified a Norwalk virus as the cause of the epidemic of acute nausea and vomiting in hundreds of students last month, University officials announced yesterday.

"It's almost a foregone conclusion that this was a food-borne virus," said Dr. David S. Rosenthal '59, director of University Health Services (UHS).

"It was a virus that caused the outbreak," said University spokesperson Joe Wrinn.

Officials said their "best hypothesis" is that the Norwalk virus was spread via the salad bar in the Freshman Union. The overwhelming majority of the 225 students who reported illness had eaten in the Union during the 72 hours before the epidemic.

Norwalk is a family of viruses that can cause outbreaks of food-borne illness, Rosenthal wrote in a December 20 letter that was mailed to the parents of all first-year students. The virus has an incubation period of 24 to 48 hours, Rosenthal wrote.

In an interview last night, the UHS director said that Norwalk is the most common cause of gastroenteritis in the country.

In fact, a Norwalk virus was the source of a recent epidemic on the U.S. Navy ship The Saratoga. That epidemic caused about 1000 people to fall ill.

Norwalk viruses are also responsible for 40 percent of non-bacterial diarrhea yearly, according to the Merck Manual, a medical reference guide to diseases.

Rosenthal said medical experts from the Centers for Disease Control and Prevention (CDC) in Atlanta had found a small, round virus in several students as well as a dining service worker.

Using DNA sequencing, experts from the CDC pinpointed the virus as a Norwalk.

"All the cases at Harvard sequenced the same," Rosenthal said. "It's the same virus in all these cases."

Officials said they are unsure whether the food arrived at Harvard contaminated or whether a student or dining hall worker passed the virus to the food during the course of the day.

Surveys pinpointed six foods as possible candidates for carrying the virus. Most of them were items in the salad bar.

Most of those who fell ill either ate in the Union in the hours before the epidemic or were in close contact with someone else who did, Rosenthal said.

The remaining few cases--a small fraction of the total number, Rosenthal said--are most likely the result of a low-level winter gastroenteritis.

'It's Over'

Rosenthal is confident that the team of investigators has successfully identified the source of the disease.

"It's over," he said of the investigation.

But experts at CDC headquarters in Atlanta will attempt to tie up some loose ends.

UHS will draw blood samples from previously-afflicted students over the next few weeks. Antibody levels in the new samples will be compared to those in blood drawn early last month.

If testing can show that the epidemic spurred a rise in antibody levels, "that puts the case on firmer ground," Rosenthal said.

Officials have yet to determine the specific species of virus in the Norwalk family that caused the outbreak.

The CDC hopes to use further DNA sequencing to identify the particular strain. Rosenthal indicated last night that it might be a new species unique to Harvard.

Finally, CDC investigators are continuing to examine the link between the Harvard epidemic and a similar outbreak that occurred at Dartmouth College in late November.

The CDC is comparing stool specimens from Dartmouth with those at Harvard. That investigation will take between two and six weeks, Rosenthal said.

Dartmouth has refused to release a list of its food vendors to Harvard, according to Director of Dining Services Michael P. Berry.

Dartmouth spokesperson Alex Huppe said there was no further news on the source of the Dartmouth epidemic. Dr. Jack Turco who is leading that investigation, did not return several phone calls.

Inspection and Charges

Berry said that the recent epidemic has prompted a comprehensive investigation into the health standards of Harvard's food.

Already, Harvard has discontinued use with two vendors who supplied food in which bacteria levels were "unsatisfactory," Berry said.

Neither vendor was responsible for the food that may have caused the epidemic, Berry said, although both supply Harvard with produce. Berry declined to name the vendors.

On the other hand, Costa Produce, the vendor that supplied vegetables in the Union, "checked out very, very fine," Berry said.

Officials visiting the lab told Berry that the factory was clean enough to make computer chips, Berry said.

Other changes that will soon take or already have taken effect in Harvard's dining halls include: Thrice-daily sanitization of kitchens, up from the current rate of one per day; a change in the sanitizing agent used in the kitchens; inspections of vendors' plants; periodic sampling of vendors; monthly white glove inspections of all units;

Other measures include: The hiring of a consulting firm to do thrice-yearly check-ups; and the use of test strips rather than thermometers to check for bacteria levels.

"Even though we don't know [the source of the virus], we're going to take these precautions," Berry said.

"I said on the first day that I take responsibility, and I still do. Until someone proves otherwise, I'm treating this as if it was [food poisoning].