Sleuths probe mystery of parasitic infection.

Any mystery buff knows that a crime suspect must have motive, method, and opportunity. A medical sleuth trying to solve an epidemic also asks three questions: Who were the victims? Did their paths cross? Does the timing of the encounter suggest exposure to a microbe?

These are the questions that now perplex medical detectives caught up in The Case of the Apparently Infectious Berries-a remarkably widespread outbreak of intestinal infection that in 2 months has afflicted about 1,000 people in 12 states and Toronto. The culprit, Cyclospora cayetanensis, is a single-celled parasite that thrives in the intestine, causing cramps, diarrhea, and vomiting.

No one has died of the ailment, which can be cured with antibiotics. Just 10 people have been hospitalized. But the breadth of the epidemic has health officials scrambling to understand how Cyclospora has insinuated itself into the nation's food supply. Although the disease initially appeared in people who had eaten strawberries and raspberries, the link with fruit remains tenuous.

"We've only had three previous [Cyclospora] outbreaks in the United States-ever. This is not a disease that we've had a lot of opportunity to study," says Sue Binder of the Centers for Disease Control and Prevention (CDC) in Atlanta. The current outbreak is the largest on record.

First recognized as a cause of disease in 1977, Cyclospora is one of a number of infectious agents, such as HIV and Ebola virus, that have emerged over the past 2 decades. It was identified in the United States in 1990 at a Chicago hospital, where contaminated drinking water infected 21 people.

No one knows where in nature the parasite normally resides or how fruit might have become contaminated. Its life cycle is largely unknown, said Mark Eberhard, also of CDC.

The field investigation is being carried out mainly by state and local health officials in states with reported cases. These case reports answer the first question-Who is affected?-but do not reveal where or when people contracted the parasite.

To answer those questions, researchers have undertaken three types of studies. First, they study disease clusters. Often, groups of people contract the infection at social events, Binder says. "You look at the foods served, how they were prepared, and interview people to see if there was some food that sick people ate and well people didn't."

The second approach is a case-control study, which makes possible investigation of isolated cases rather than clusters. Epidemiologists interview sick people and unaffected neighbors, trying to pin down some factor common to those who contracted the disease.

Finally, the Food and Drug Administration is carrying out a trace-back study to identify where infectious foods came from. Their work is complicated by Cyclospora's week-long incubation period, during which memories fade and food may be discarded, so it can't be tested.

In Houston, afflicted people ate strawberries. In Charleston, S.C., they ate strawberries and raspberries. The CDC reported on June 28 that fresh fruit may carry Cyclospora, and urged people to wash all produce before eating it. Despite an intensive search, however, no infectious produce has been found. - S. Sternberg