The notion that viruses cause cancer may seem bizarre and unintuitive. We usually associate these microscopic disease-causing agents with flu, colds and nasty gastrointestinal illnesses.
However, pioneering researchers began to study the association be-tween viruses and cancer more than 100 years ago. This early work led to a rich history of discoveries that forever changed the way we think about cancer and its causes.
Most prominent among these was Dr. Peyton Rous, an American physician who won the 1966 Nobel Prize in medicine for his discovery of tumour-inducing viruses.
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As an aside, Rous spent time wrangling cattle in Texas at the turn of the century before resuming his medical education in New England and embarking on his research. One can speculate on the influence this hard work and fresh air had on his creativity moving forward.
Rous worked with chickens.
Tumours from chickens were harvested and small pieces were injected into unaffected chickens. These subsequently developed tumours.
Next, he used a tiny filter to remove all cells and bacteria before injecting the resulting filtrate into chickens.
These also developed tumours. Based on these findings, it was concluded that a virus was the most likely cause.
These early discoveries led to the finding that papilloma viruses cause certain cancers in people.
Perhaps it is the strangeness of this concept that led to some of the controversy surrounding the human papilloma virus vaccine.
The vaccine, which significantly reduces the incidence of cervical cancer in women, is truly a life-saver. Rather than treating cancer once it occurs, this vaccine is an example of preventive medicine at its finest: halting the virus that causes cancer in the first place.
Cancer cells “go rogue” by ignoring normal feedback mechanisms. They replicate and grow without regard for the normal rules that govern the body’s tissues.
They also retain more primitive characteristics. For example, skin cancer cells act and look more like the deepest layer of the skin that gives rise to the other superficial layers than the top skin cells that slough off.
Viruses induce cancer by inserting their own DNA into that of their host. This gives some viruses the ability to switch on genes that permit cancer cells to invade tissues and ignore signals that would normally trigger their own death.
Other viruses switch off genes that slow down or stop cell replication, which not only permits uncontrolled propagation but also accumulates increasingly more genetic mutations with each cycle.
However, besides chickens, what about other animals?
There are several examples of these phenomena in wild and domestic species.
Last year, researchers at the University of California discovered a new virus within brain tumours of wild raccoons. Even mice have their own suite of cancer-causing viruses, including one that leads to breast cancers.
Sarcoids are the most common tumour in horses. These disfiguring and notoriously difficult to treat skin cancers may be caused by bovine papilloma viruses.
Some bloodlines and breeds appear to be more often affected by sarcoids, suggesting that a combination of genetic predisposition and virus infection results in tumor formation. Equine papilloma viruses have been associated with certain genital cancers in horses.
Cattle have their own papilloma viruses, which are associated with benign warty-like growths in the mouth, fore-stomach, penis and bladder. Infected cattle are more prone to develop cancer when grazed on bracken fern.
Bovine enzootic leucosis is a disease of cattle that is caused by bovine leukemia virus. Infection induces a form of cancer known as lymphoma. The prevalence of infection is high, especially on dairy farms, but only a small percentage of cattle develop lymphoma. Most often, affected cattle have greatly enlarged lymph nodes.
There are two notable exceptions to this trend.
Naked mole rats, which are grotesque subterranean rodents that live in colonies reminiscent of honeybees, are noted for both their long life expectancy (up to 30 years) and their lack of spontaneous cancers, virus-induced or otherwise.
This species is the subject of intensive research to better understand why this is so.
The other is humans, where less than 20 percent of spontaneous cancers are attributed to viral causes.
We know much about the cancer-causing effects of some viruses, but a great deal remains to be learned, including how to implement this knowledge for prevention and treatment in both animals and people.
