February 24, 2004
Phone: (504) 865-5714
Did you know that along with the chicken sandwich you had for lunch today, you ate potentially cancer-causing avian viruses? Don't worry--as long as the chicken was properly cooked, the virus was dead and therefore harmless.
Still, such viruses are the "most potent cancer-causing viruses in nature," according to Eric Johnson, associate professor of epidemiology. No one is sure if these viruses cause cancer in humans, but he has found evidence that they might. Johnson conducted two retrospective studies of poultry workers in Baltimore and Missouri and found elevated rates of cancer in both groups.
The Baltimore study found that poultry workers had four times the expected rate of esophageal cancer, while workers in Missouri had higher rates of lung, kidney, pancreas, blood and lymphatic cancers. There are two types of exogenous retroviruses that cause cancer in chickens--avian leukemia and sarcoma viruses (ALSV) and reticuloendotheliosis viruses (REV).
These viruses can kill a chicken within a week of infection. All chickens that make it into the commercial food supply have a sort of inactive, endogenous form of these viruses in their genomes. But the inactive viruses can sometimes escape from the genome and behave like the exogenous form, with the potential to cause cancer in other species.
The viruses are present in the chicken's blood, organs and eggs. The endogenous forms have recently been discovered in vaccines for measles, mumps and yellow fever, which are grown in chicken embryo cells.
Johnson figured that poultry workers would have the most exposure to these viruses, and looked at cause of death of about 2,000 people who worked in Baltimore chicken processing plants between 1950 and 1979. Only about 14 percent had died at the time of the last follow-up, which was done in 1989.
In Missouri, Johnson followed a group of 7,700 younger poultry workers. Only 6 percent of this group had died at the time of the last follow-up in 1990. He hopes to do another follow-up on both groups soon. Another way of finding out if these viruses are active in humans is to look for viral antibodies. Early studies were inconclusive.
"But we did a study while I was at the NIH using more modern techniques, and we were able to find antibodies to both ALSV and REV not only in poultry workers but in the general population as well," said Johnson. "In our view, that's evidence that humans are exposed to this virus."
Scientists in a lab in France found antibodies to ALSV in humans, although a study by the Centers for Disease Control and Prevention did not. However, Johnson and others have shown these viruses can infect human cells in the laboratory.
"Now the evidence that we're looking for is whether we can find these viruses in the genomes of people who have died of cancer," Johnson said.
Viruses are known to cause about 15 percent of human cancers. Human papilloma virus causes cervical cancer and hepatitis B virus causes liver cancer. The causes of most human cancers are as yet unknown, but researchers are investigating whether viruses might contribute to other forms of cancer, such as breast cancer. However, even if avian viruses are found to cause cancer in humans, thorough cooking effectively kills them.
Care should be taken when handling raw chicken, which should not come in contact with any cuts or scrapes that the handler might have. Johnson also suggests that measures should be taken to protect poultry workers from the viruses.
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