"The Role of Poultry Viruses in Human Cancers"
The incidence of cancers has been rising for the last half century
and the question is why?
Well, up to 20% of all cancers are caused by infectious agents, chiefly viruses,
something we've known was possible for a century
when a cancer-causing virus was discovered in chickens.
It was such heresy that Dr. Rous wouldn't get his Nobel Prize
for this landmark discovery until 55 years later.
If there are cancer-causing chicken viruses,
what about people who handle or eat poultry?
Concern has been raised about the potential infectivity
of cancer-causing farm animal viruses for decades.
The first question was whether there was any evidence of human exposure?
And indeed, people do have antibodies
to these cancer-causing chicken viruses in their bloodstream,
indicating that the virus is no stranger to our immune systems.
OK, but is there any evidence that the virus itself can get into our blood?
There wasn't until 2001.
There's a cancer-causing herpes virus in poultry.
The question is does it pose a public health hazard?
Researchers used DNA fingerprinting techniques to test the blood
of about 200 people, and 20% had the viral DNA in their bloodstream, 1 in 5.
OK, but that still doesn't mean these viruses necessarily can infect human cells,
but indeed they can.
OK, but do they cause human disease? How are we going to figure that out?
Obviously you can't just inject people.
So researchers looked at poultry workers.
That's how we figured out how other farm animal diseases jumped to humans,
not to mention the discovery of the carcinogenic nature of things
like asbestos and benzene:
you study the workers who are exposed day in and day out.
If they don't have higher cancer rates,
then presumably the viruses are harmless, but unfortunately they do.
Those at high exposure to poultry cancer-causing viruses
do have increased risk of dying from several cancers.
So, the relative ease by which some of the viruses can infect human cells,
and to infect and cause tumors in primates in laboratories,
may be of public health significance, particularly given the increased risk
of cancer among meat workers
and the evidence that we may indeed become infected.
Even if poultry workers are at risk, though,
that doesn't mean people who just eat chicken or eggs are.
Just because those who kill chickens may be 6 times more likely to die
from brain cancer, for example, they've got live birds flapping in their face,
the intensity of exposure to these viruses in the general population
is presumably nowhere near that experienced by poultry workers,
though the general population is nevertheless widely exposed to them
just because we do eat so much chicken and eggs.
This is supported by data showing that it's not only the factory farm workers
that are at higher risk for brain tumors,
but also just butchers or meat cutters who have no exposure to live birds,
especially those who don't wear gloves apparently,
or who frequently have cuts on their hands.
And for other cancers as well.
Those that handle meat for a living also have higher rates of non-cancer mortality,
like increased deaths from heart disease.
Could the viruses be involved there, too?
Some of the poultry viruses don't just cause cancer in chickens,
but also atherosclerosis.
That cancer-causing herpes virus also triggers the buildup of cholesterol crystals.
OK, but that's in chickens. What about in people?
Because chickens infected with Marek disease virus
develop atherosclerotic lesions after infections,
researchers looked for the presence of any herpes virus
in human artery wall tissue, and found it,
though any role viruses play in human heart disease remains speculative.
But here we were all along thinking that the substances present in animal foods
increasing risk of diseases like cancer and heart disease was like heme iron,
saturated fat, cholesterol, dioxins, cooked meat carcinogens,
but we didn't think about the viruses, which is important
not only for supermarket workers,
but also because the general population is exposed as well.
Indeed, that study that found that chicken cancer virus DNA
circulating in people's bloodstreams found about the same rates
in office workers as they did in chicken slaughterhouse workers.