Preliminary test results released yesterday still failed to pinpoint the cause of the epidemic that last week caused acute nausea and vomiting in hundreds of students, but the search may be narrowing.
Jonathan M. Freeman, assistant professor in epidemiology at the Harvard School of Public Health, said initials tests show the cause of the epidemic may be a bacterial toxin or a virus. But the cause may ultimately turn out to be unidentifiable, a University statement cautioned.
A survey by the federal Centers for Disease Control and Prevention (CDC) found that Freshman Union patrons who ate the vegetable stir-fry at Sunday's dinner last week or who had salad on that Monday or Tuesday were far more likely to become ill than those who did not. The survey asked questions of 300 sufferers and non-sufferers of the disease.
According to a University statement released yesterday, the same vendor supplied vegetables for the salad bar and for the stir-fry. Harvard declined to identify the vendor.
But an inspection by Harvard's Environmental Health and Safety Office last Monday found the plant to be clean, according to the University statement. Freeman said he finds the preliminary test results "unsatisfactory" because they are not conclusive. He said he hopes for more specific answers in six weeks.
Some medical experts excused the slow pace of the investigation, saying a comprehensive study will take time.
"It takes time to make a diagnosis," Dr. Paul E. Kilgore, a medical epidemiologist for the CDC, said in the University statement.
Lab tests have eliminated live bacteria as a possible cause for the illness, Freeman said. Tests are now being performed to determine if the agent was a bacterial toxin.
Additionally, blood serology and electron microscopy performed by the CDC will be used on stool and vomit samples to try to isolate a virus, according to the University statement.
Samples from the salad bar have been sent to the Food and Drug Administration in Washington, D.C. for analysis, Freeman said.
If the cause is viral, it will fall into the category of about a dozen round-structured viruses called Norwalk-like agents, which are often transmitted to large groups of people simultaneously via food, according to the statement.
These viruses are often passed on to those who come in contact with infected carriers. The Norwalk Virus is responsible for 40 percent of non-bacterial diarrhea yearly, according to the Merck Manual, a medical reference guide to diseases.
A bacterial toxin is released from living bacteria. When the food is cooked, the toxin remains, although the bacteria has been killed.
Unfortunately, the causes of between 30 and 40 percent of gastrointestinal disease outbreaks are "never found," according to the University statement