"Evidence is mounting that simian virus 40 (SV40) infects humans and is associated with certain types of human tumors," said Dr. Janet Butel, head of the division of molecular virology at Baylor College of Medicine in Houston.

"This association with human disease might lead to new approaches for diagnosing and treating certain types of cancers and other illnesses, but further studies are needed to determine whether SV40 plays a causative role in the tumors," she said.

Butel and Dr. John Lednicky, also a Baylor molecular virologist, presented an overview of what is known about SV40 in the Jan. 20 issue of the Journal of the National Cancer Institute.

SV40, which originated in the rhesus monkey, was discovered in 1960 as a contaminant of polio vaccines given to millions of people from 1955 through early 1963. Monkey kidney cells that were used to grow the polio vaccine were unknowingly contaminated with SV40.

Although no acute illnesses resulting from SV40 were reported in people who were exposed to the contaminated vaccines, some scientists are now  concerned that the virus might pose a cancer risk to humans. Antibodies to SV40 have been detected in up to 10 percent of  adults born after 1962, so possible exposure to the contaminated polio vaccines would not explain how these people were infected with the monkey virus, Butel said.

 "There must be an alternate source of human infection by SV40," she said.

Advanced genetic technology has made it possible to detect DNA from SV40 in several types of human tumors, including brain tumors in children, bone tumors, and tumors associated with exposure to asbestos. Investigators in the United States and Europe have confirmed that this DNA is from authentic SV40 and was not caused by contamination of specimen samples in the laboratory,

Butel said. Genetic analysis has also shown that contrary to previous belief, there are multiple strains, or variations, of SV40.

"This raises the possibility that some strains of the virus are more likely to cause disease or induce certain types of tumors in humans, but that needs to be investigated in further studies," Butel said.

SV40 induces tumors in mice and hamsters and transforms many types of cells, including human cells, in laboratory studies. Healthy rhesus monkeys infected with the virus don't develop obvious signs of disease. But in monkeys infected with the AIDS virus, SV40 has been detected in their brains and other organs.

"This suggests that humans with compromised immune systems might be at greater risk of SV40 infection," Butel said.

Because the original source of SV40 strains now present in humans is unknown, Butel advocates the need for more research on how the virus can be transmitted.

"It is of no small irony that SV40, once having been found as an unrecognized contaminant of a widely heralded viral vaccine, might itself one day become a candidate for vaccine development," she said.

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