4/15/03 NYT piece on SARS / "superspreaders"
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How One Person Can Fuel an Epidemic
April 15, 2003
By DONALD G. McNEIL Jr. and LAWRENCE K. ALTMAN
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."
Copyright 2003 The New York Times Company. |
