Portrait Of a Virus

The virus responsible for last month's vomiting epidemic on campus is a well-known mystery.

While no one is sure exactly how the Norwalk agent causes vomiting and diarrhea, epidemiologists have identified this virus family and are familiar with its handiwork.

Big trouble comes in a little package. The virus, which varies in size from 26 to 34 nanometers, has caused illnesses from Hawaii to Japan to England. It was named after Norwalk, Ohio, where an outbreak struck an elementary school in 1968.

About 10 percent of gastroenteritis cases--or illness involving vomiting and diarrhea--are caused by Norwalk agents, according to Dr. Jonathan Freeman, assistant professor in the School of Public Health's epidemiology department.

But the Harvard epidemic was rare in its size and virulence.

"It is not usually an epidemic agent," Freeman said.

The virus is typically transmitted orally or through fecal matter. According to the text The Principles and Practice of Infectious Disease, the air may be responsible for carrying the virus in some cases.

The same book notes that foodborne spreading is rare. But Harvard, in completing its investigation of the epidemic, said the Norwalk virus spread via the salad bar in the Freshman Union.

Damage in Intestines

Through contact ranging from a handshake to a kiss, the virus enters the intestines, where it seems to cause the damage.

Under the affect of Norwalk agent, "villi are blunted," says the text. Villi, normally finger-like protrudances which absorb food particles, become squashed like mushrooms.

Microvilli also becomes shortened and spaces between the cells on the intestinal wall widen.

Diarrhea, one of the symptoms caused by the Norwalk agent, "is associated with the transient malabsorption of D-xylose and fat," according to the text. After the damage of the Norwalk agent, it may take two weeks for the absorption levels in the body to return to normal.

"Diarrhea washes out the enzymes which are used for digestion," Freeman said. "So that, in general, diarrhea breeds diarrhea."

"In this epidemic, diarrhea was a minor feature," Freeman said. The absorption levels probably returned to normal quickly, he said.

Freeman said the existing knowledge about Norwalk is based largely on an "experiment" on some high school seniors.

"High school kids...went through the illness as a public service in the spring of their senior year," Freeman said. "They went through the illness several times."

No one knows, however, exactly how the agent triggers negative reactions in the body. But the results are clear: two to three days of illness, including stomach cramps, headaches, myalgias, low-grade fever, malaise, diarrhea and vomiting.

On rare occasions, according to the text, intravenous medicine must be used to combat severe diarrhea and vomiting.

When the epidemic began one night last month, doctors at the University Health Services said they had to hook up approximately three dozen students up to IVs. That fact suggests that Harvard students got a very large dose of Norwalk agent.

"The virulence of a case depends on the dose of the pathogen," Freeman said.

Epidemiologists from the U.S. Centers for Disease Control (CDC), however, have still not determined what food originally transmitted the virus. Five foods from the salad bar are suspected.

Students who got the primary dose spread the virus.

"People in the infirmary on the first night who were sick with the primary common source case gave the nurses on duty secondary cases," Freeman said. "Secondary cases could be more or less depending on the dose, but people tend to get less sick on successive occasions."

Experts said some students could still be getting sick with the same Norwalk agent that caused the December epidemic.

Luckily, a victim of the virus is unlikely to get sick again from the same virus for two to three years after the first attack, according to the text.

This resistance results from the build-up of antibodies. In order to find conclusive evidence that the Norwalk virus was responsible for the outbreak, epidemiologists from the CDC will test blood samples from victims of the epidemic for antibodies specific to the agent.

Only the "first wave of evidence makes the Norwalk agent seem most likely," Freeman said. The virus is very hard to identify because the symptoms are indistinguishable from many other gastroenteritis cases.

If the CDC tests do not find antibodies for Norwalk, the cause of the epidemic may remain a mystery.

Statistically, bacterial food poisoning is a much more likely cause than the Norwalk agent.

According to a statement from the CDC, about 92 percent of diagnosed food poisoning cases in 1987 were caused by bacteria. In the same year, 17 percent of food poisoning cases were caused by viral agents such as Norwalk