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If it is indeed in humans, its role in causing human cancers is unknown.
Scientists say it may play a key part — or possibly no part at all.
The puzzle
began in 1994, when Dr. Michele Carbone, a Loyola University researcher,
found the virus SV40, which had never before been detected in humans, in
half of the human lung tumors he was studying.
SV40 is
known to create tumors in animals, but how it might have gotten into humans
was unclear.
"I
thought there must be something wrong. I must have made a mistake," he
said, remembering the discovery.
Eventually,
60 different lab studies confirmed the results.
"This
finding has been replicated in New Zealand, in China, in Britain, in
France, in Switzerland, in Belgium," Carbone said.
Several labs
did not find any evidence of SV40, and some researchers continue to
question Carbone's findings. Efforts in general to explain the SV40 mystery
have been hampered by unusual acrimony among those studying the problem.
Could
It Have Been Transmitted By Polio Vaccine?
If the
monkey virus SV40 is indeed in humans, there are several possible
explanations for how it got there, says Janet Butel, a virologist at Baylor
College of Medicine and one of America's leading virus researchers.
"One is
that it has always been there in humans, and no one has detected it in the
past," Butel said.
There is
another, much more controversial theory as well, however.
Some
researchers contend SV40 was transmitted to humans through the polio
vaccine, which has saved many lives. The vaccine is made in monkey kidney
cells, and from 1955 to 1963 an estimated 20 million Americans were given
doses contaminated with SV40.
Still, the
virus was not detected in humans until Carbone's 1994 research, possibly
because no one had thought to look for it.
In 1961, the
Food and Drug Administration ordered the vaccine's manufacturers to screen
out the SV40, which they say they did.
But a lawyer
involved in a recent polio case has just published a report claiming
contamination continued.
"In
certain instances, no [SV40] tests were ever performed," the lawyer,
Stan Kops, wrote about one of the vaccine's manufacturers, Lederle.
‘Every
Batch Was Screened,’ Insists Vaccine Maker
Lederle
strongly disputes Kops' claim, telling ABCNEWS in a statement "every
batch of the polio virus used to manufacture vaccine underwent tissue
culture testing for SV40."
If that is
true, it suggests another possible reason SV40 has been found in the brain
tumors of people born after 1963: transmission from mother to child.
"I
think studies need to be done to figure out precisely what the role of the
virus might be in human cancer," said Butel.
Scientists
specializing in SV40 met today in Bethesda, Md., to sort through some of
the controversies.
Some still
question whether SV40 truly exists in humans, but the vast majority of
scientists attending the conference believe the role of SV40 in humans
needs urgent attention.
"We
need to find out what it's doing there," said Butel. "It will be
a great significance if it's proven to have a role in human cancer because
then … it may be possible to block infection and the formation of a
tumor."
There may be
several viruses linked to cancer in humans. 
ABCNEWS'
Nicholas Regush, whose column Second Opinion appears weekly on
ABCNEWS.com, produced this report.
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