Cancer Experts Puzzled By Monkey Virus

March 12 -- It is a mystery with enormous implications that has stumped some of the smartest minds in cancer research: How did a cancer-causing monkey virus end up in human tumors?

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.