Simian
Virus 40 & Its Possible Link with Mesothelioma
The relationship between mesothelioma and
Simian virus 40 (SV40) is one that continues to be investigated because there
are mixed reviews within the medical community concerning a causal
relationship. Some investigators have found a causative role while others have
not. The conflicting evidence suggests the need for more research to better
understand the interactions between SV40 and mesothelioma.
Asbestos cancer is a rare disease that
usually develops in the lining of the lungs and its primary cause is asbestos
exposure. SV40 is a DNA virus that contaminated polio vaccines between the
1950s and ’60s. Although the SV40-contaminated vaccines were successful in
reducing the incidence of polio, questions regarding its safety began to emerge
shortly after its use.
Background
on Simian Virus 40
Simian virus 40 is a virus that was initially
discovered in 1960 in rhesus monkey kidney cells. At the time, Dr. Jonas Salk’s
and Dr. Albert Sabin’s polio vaccines were made from these cells and the mass
immunization program for polio had already begun. From 1955 to 1963, an
estimated 10 to 30 million people in the United States were inadvertently
exposed to SV40-contaminated polio vaccinations. The exposure was so significant
that researchers believe 80 to 90 percent of all U.S. children under the age of
20 had received at least one polio vaccination potentially contaminated with
SV40.
When 1961 reports indicated the injection of
SV40 into hamsters could cause tumors, the U.S. government instituted a
screening program requiring all new polio vaccinations to be free of SV40.
However, vaccinations made prior to this regulation were not withdrawn from the
mass immunization program and children continued to be exposed until 1963.
Conflicting
Evidence about SV40
The amount of research that supports a causal
relationship between mesothelioma and SV40 is significant enough to warrant
concern. In animal testing, 100 percent of those injected with SV40 into the
pleural space developed mesothelioma within three to six months. There is also
a general consensus that 50 to 60 percent of malignant mesothelioma cases in
the United States contain SV40.
Quick
Fact
In the early 1960s, researchers found SV40
had the ability to transform hamster and human cells and induce tumor
formation. After the discovery of SV40-like DNA sequences in mesothelioma
samples in 1994, investigators have continued to test the causal relationship
between SV40 and malignant pleural mesothelioma.
Tumors
In some studies, researchers have found human
mesothelial cells to be uniquely susceptible to SV40, and have suggested that
the virus and asbestos may act as co-carcinogens. SV40 has been detected in
human tumors in more than 40 different laboratories and many of these tumors
were mesothelioma. Although there is evidence of SV40 acting as a causative
agent in the development of asbestos cancer, there is an equal amount of
evidence arguing against this causal relationship.
SV40
Working Group
With the intent of resolving the conflicting
relationship between mesothelioma and SV40, the International SV40 Working
Group was formed in 1997. The group included the majority of laboratories
studying SV40 in human tissues. In a study funded by the National Cancer
Institute, nine laboratories agreed to participate in the study, which tested
tissue samples for traces of SV40. Each laboratory was given a set of 25 human
mesothelioma samples and 25 normal lung tissue samples. The samples were not
labeled and each laboratory used their own method for detecting SV40. Results
showed that none of the mesothelioma samples were consistently positive for
SV40, suggesting a lack of evidence for a causal relationship between
mesothelioma and SV40.
Vaccinations
Other studies showed similar results. In one
evaluation of children born during the years that SV40-contaminated
vaccinations were used, an overall difference in cancer incidence was not
reported when comparing the group to children who received uncontaminated
vaccinations. But for every study that shows a lack of evidence for a causal
relationship between cancer and SV40, another study exists that show a possible
correlation. Considering the conflicting evidence, the medical community has
yet to accept or reject a causal relationship between mesothelioma and SV40.
Researchers continue to investigate the issue and they hope to reach a
conclusion through future studies.