Poultry workers may be at particularly high risk of developing several forms of cancer, according to a new study that points to viruses carried by birds as a possible cause.

The findings come from an ongoing effort by researchers to identify job-related illnesses in the nation's 250,000 poultry processing workers. It found higher than expected rates of cancers of the sinuses, mouth and blood, as well as other forms of the disease, in poultry plant employees.

The researchers said cancer-causing viruses transmitted during the handling and slaughter of chickens and turkeys, as well as environmental factors such as exposure to fumes generated during the wrapping, smoking and cooking of meat, along with other aspects of production, may be to blame for the increased rates of illness.

Some of the viruses present in birds are found in the egg supply. And because many vaccines are made using chicken eggs as incubators, the viruses have also been found in the vaccine stock - in particular, the shots against measles, mumps, and yellow fever, according to the researchers. However, scientists have not found evidence that the presence of the viruses is harmful to humans.

Still, These observations have serious public health implications and reiterate the urgent need for studies to be conducted in subjects that have high exposure to the (cancer-causing) viruses of poultry, such as workers in poultry slaughtering and processing plants, they wrote in the journal Cancer Causes & Control.

Study leader Eric Johnson, an epidemiologist at the University of North Texas Health Science Center, in Fort Worth, said the viruses pose no risk to consumers who eat properly cooked poultry products, including eggs. But eating raw or undercooked eggs and poultry or handling raw meat may be hazardous, he told Reuters Health.

The study compared cancer deaths in 2,580 members of the Baltimore meat cutter's union who worked exclusively in six Maryland poultry plants between 1954 and 1979. By 2003, 790 of those workers had died, and the researchers were able to determine the cause of death for 756.

Of the 756 total deaths, 187 were from cancer. Although the overall death rate from cancer was not unusually high, the death rate was much greater-ranging from 3.5 to nearly 9 times higher-for several forms of the disease, including cancer of the tonsils, nasal cavity and sinuses, and a blood cancer called myelofibrosis.

Death rates from certain cancers-including of the tongue, liver and esophagus-also were higher than normal among various groups of workers even when the overall rates were what would be expected in unremarkable, the researchers said.

For certain cancers, the risks were greater depending on the workers' sex or race. Those differences may reflect divisions of labor in poultry plants, where women and men, and whites and blacks, historically were assigned different duties, the researchers noted.

Richard Lobb, a spokesman for the National Chicken Council, an industry group, dismissed the study as an astounding piece of flawed research. Lobb said the scientists did not adequately account for the known cancer risks of tobacco and alcohol use in the workers.

In addition, the suggestion that raw chickens carry viruses that cause cancer in humans is pure supposition unsupported by any scientific facts or studies, he told Reuters Health.

Johnson acknowledged that the study's inability to account for the effects of tobacco and alcohol use was a limitation. But he said other research has shown that poultry and meat industry workers have higher-than-normal odds of developing cancer, even after controlling for smoking.

What's more, smoking and alcohol are not linked to the blood cancers that seem to be more common among poultry workers in the new research, he said.

Moreover, Johnson said, the link between avian viruses and cancer in animals is well established.  

We've known that for years. It's just that we've never had the human evidence, he said. That's what we are providing for the first time.

SOURCE: Cancer Causes & Control, Published Online October 22, 2009.