Anatomy of an illness: Canada learns IC lessons

Outbreaks show evidence of contact superspread’

Perhaps the most intriguing aspect of the severe acute respiratory syndrome (SARS) epidemic was its patterns of spread. Sometimes, few people became infected, despite exposure. In other cases, dozens of people became ill.

In most cases they tracked, epidemiologists were able to find links. "SARS can usually be associated with close contact with another known case," says L. Clifford McDonald, MD, a medical epidemiologist and member of the Centers for Disease Control and Prevention SARS task force.

But the "superspreaders" — people who are able to infect many others — present an infection control and employee health challenge.

Consider the cases of "Mr. A" and "Mr. B" in the index hospital in Toronto. The first time Mr. D came into the hospital, he was asymptomatic. After a short stay, he was discharged, and then readmitted with a myocardial infarction on March 13. On the second day of his stay, he developed very small infiltrates on his chest X-ray, but he did not seem to have SARS.

Within four days, other cases of SARS began to emerge. He had infected nine employees, five patients, and three visitors in the coronary care unit. On the adjacent unit, where he had stayed, five employees, four patients, and four visitors developed SARS. One employee, one patient, and one visitor apparently were infected in the area immediately outside the coronary care unit.

On March 15, Mr. B entered the emergency department (ED) in airborne precautions as a suspected case of SARS and was placed in an isolation room. His wife registered him at about 11:30 p.m., went to his room at about midnight, then stayed the remainder of the night.

Five of approximately 38 visitors to the ED were infected, "four of whom left the department between 10:45 p.m. and midnight," explains Allison McGeer, MD, FRCPC, director of infection control at Mount Sinai. Also infected: eight staff members including nurses, security guards, clerks, and housekeepers. "This is not obviously an airborne [transmission]," she says. "Nobody who arrived in the department after she left got sick. She left nothing behind her in the air. She left nothing behind her in the environment that made people sick afterward. But it’s droplet transmission at much longer than expected distances.

"We just got unlucky, and we had three people who shed very large quantities," she says.

McGeer also notes that 25 health care workers became infected with SARS while using personal protective equipment. Only three of those were in the second phase of the outbreak, when staff were well educated in using precautions and alert to the risks of breaks in infection control.

"SARS precautions work. They work very well, if not quite perfectly," she adds. "It’s not recognizing cases that’s going to get you in trouble."

McGeer also advises hospitals to rethink policies that inadvertently may encourage health care workers to come to work sick. "We’ve worked very hard at reducing absenteeism. That is a very bad thing when it comes to spreading viral respiratory disease in hospitals."