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Last Updated

16 Apr 2003

Source: New York Times, April 15, 2003

How One Person Can Fuel an Epidemic


A child in China so infectious that he is nicknamed "the poison emperor." A Chinese doctor who infects 12 fellow guests in his Hong Kong hotel, who then fly to Singapore, Vietnam and Canada. An elderly Canadian woman who infects three generations of her family.

Watching as the mysterious illness called severe acute respiratory syndrome hopped around the world and exploded in new outbreaks, epidemiologists began to ask themselves an unsettling question: is it carried by "superspreaders"?

The notion that some people are hyperinfective, spewing germs out like teakettles while others simmer quietly like stew pots, has been around for at least a century, ever since Typhoid Mary became notorious in 1907.

For some diseases, including tuberculosis, smallpox and staphylococcus infections, superspreaders definitely exist. They have been variously called "superinfectors," "supershedders" and even "cloud cases" for the mist of invisible droplets trailing them.

But while there are anecdotal case studies of individuals behind some outbreaks, there is little concentrated research in the field. "There hasn't been enough time, thinking and probing" to hazard more than a guess as to why superspreaders are responsible for so much of the spread of SARS, said Dr. Donald A. Henderson, the epidemiologist who led the global eradication of smallpox.

Dr. Joshua Lederberg, emeritus professor of microbiology at Rockefeller University and a Nobel laureate in medicine, said there were many hypotheses for instance, that superspreaders are partly resistant to the disease they spread. But he added, "It's epidemiological conjecture."

As several experts pointed out, it is hard to describe how a disease spreads when its cause has not even been nailed down; in the case of SARS, a coronavirus is still just a prime suspect. Moreover, no one knows the answers to basic questions about disease transmission why, for example, AIDS is transmitted by blood but not by coughing, while tuberculosis is usually the opposite.

Several SARS patients have infected more than 30 people, according to the World Health Organization. The biggest reported superspreader is a 26-year-old airport worker admitted to Prince of Wales Hospital in Hong Kong in early March. He infected 112 people, including every doctor and nurse who treated him.

Doctors suspect the cause was a jet nebulizer that sprayed medicated mist deep into his phlegm-filled lungs four times a day for seven days. The mist expanded his lungs and was itself exhaled.

"You put someone with a viral infection in their lungs on a nebulizer well, yeah, you're going to spread the disease," said Dr. Susan C. Baker, a professor of microbiology at Loyola University of Chicago. "The air that goes in has to come out."

That, experts said, is a good example of a leading theory about superspreaders that their infective powers are not genetic, but are due simply to unhappy coincidences. They have shedding sores in the throat that make their coughs extra deadly. They have no symptoms and feel well enough to go out. They have an occupation like flight attendant, doctor or prostitute that involves close contact with many strangers. Or they get sick while in a group of people with low resistance.

In many outbreaks, said Dr. Jack M. Gwaltney Jr., an expert in the common cold at the University of Virginia, children are the spreaders.

Referring to a well-known study of a cold outbreak at the Eagle Heights Apartments in Madison, Wis., and to an early theory that the outbreak of more than 300 SARS cases in the Amoy Gardens apartment complex in Hong Kong was spread by cockroaches, he said: "Don't blame the cockroaches. In Wisconsin, it wasn't the cockroaches, it was the kids."

But neither children nor cockroaches are suspects at Amoy Gardens now. The leading theory is that leaking sewage contaminated sidewalk puddles. Residents walked through them, then took off their shoes and picked up the disease by touching their faces or eating without washing their hands. Many viruses are shed in feces, famously including polio, which can spread to diaper-changing parents.

Whoever put SARS in the Amoy Gardens sewage pipes and one regular visitor was a dialysis patient at the Prince of Wales Hospital while the airport worker was on the nebulizer would be a superspreader, with the help of rusty pipes.

Some people become superspreaders because they contact many others in the hours before symptoms develop. A famous case of the superspreading of smallpox was described in 1913: a man who took two trains across England, and was said to have infected nearly 100 people en route. No one in his compartment noticed any rash on his face.

Another theory is that some people have more contagious strains. Flu viruses mutating between animals and humans can become more or less infectious, said Dr. Megan Murray, a professor of epidemiology at the Harvard School of Public Health. The Norwalk virus, found on cruise ships, is highly infectious.

But in tuberculosis outbreaks, for example, what matters is not the strain but whether the carrier has a throat infection or, more commonly, lung cavities. The oxygenated bacteria grow faster, "so they cough up huge amounts," said Dr. James Plorde, an infectious disease expert with the University of Washington.

A famous tuberculosis superspreader, described in The New England Journal of Medicine in November 1999, was a 9-year-old boy in rural North Dakota, an immigrant from the Marshall Islands, who in 1997 and 1998 infected his family and 56 schoolmates. The boy had deep cavities in his lungs, while his twin brother, who was two inches taller and 11 pounds heavier, had a mild case and was not infectious.

Some populations are genetically more susceptible, so the first carrier to get it often becomes a superspreader. For example, Dr. Plorde said, "people of European descent handle TB much better than American Indians presumably because their genetic stock survived more epidemics of TB."

Also, a second infection can turn someone with a mild primary illness into a superspreader.

In 1996, the journal Annals of Internal Medicine described an experiment conducted after an outbreak of antibiotic-resistant staphylococcus in a hospital's surgical intensive-care unit. Of 64 people tested, one medical student was found to have staph germs in his nose that matched those infecting eight patients. He had a mild cold during the week the patients were infected, he said.

Since he was healthy again, his dispersal of staph germs was tested, and was unremarkable. Then, with his permission, he was given another cold. Three days later, his sneezes were tested, and he was spraying out 40 times as much bacteria.

Secondary infections are thought to be part of the rapid spread of AIDS in Africa. The virus spreads much more rapidly in populations where untreated genital sores are common.

Gaetan Dugas, the gay airline attendant blamed for much of the early spread of AIDS in North America who was dubbed Patient Zero in Randy Shilts's book "And the Band Played On," would be considered a superspreader like Typhoid Mary because he willfully infected others. The book says he even taunted some men he had slept with by pointing to the sores on his arm and saying, "gay cancer maybe you'll get it."

Mary Mallon, or Typhoid Mary, infected as many people as she did because she never got sick enough to stop working, and she refused to quit her chosen occupation: cook.

"If she'd been an epidemiologist or a reporter," observed Dr. James Curran, dean of Emory University's school of public health, "she wouldn't have been Typhoid Mary."