Dec. 10, 2003 (New York) -- You can't "catch" cancer like a cold, but new research now shows that infection with any of a number of different viruses may play an important role in the growth and development of cancer.
More than a dozen different viruses have been linked to various forms of cancer. In most cases, the presence of a particular virus increases the risk of developing cancer or speeds its progression. For example, HIV and other viruses that affect the immune system make infected individuals prone to a variety of cancers by weakening the body's natural defenses.
But in other cases, there is now compelling evidence that certain viruses may also play a critical role in causing cancer. Proving that a particular virus causes cancer, however, is complicated.
"One of the things that is complicated about this is that there is a long time between exposure to the virus and development of the cancer," says Bruce Johnson, MD, of the Dana-Farber Cancer Institute in Boston. "Number two is that it's not a one-to-one thing. There are a lot more people who are exposed [to a virus] than have evidence of a tumor."
Johnson moderated a briefing today in New York City on the virus-cancer link sponsored by the American Society of Clinical Oncology.
Currently, Johnson says the scientific evidence is strongest for the link between cancer and the human papillomavirus (HPV) and the hepatitis B and C viruses, but many other viruses are also under investigation for their ties to cancer.
HPV Linked to More Than Cervical Cancer
HPV has long been associated with an increased risk of cervical cancer, but new research suggests that the virus may also cause head, neck, and anal cancers. HPV is the most common sexually transmitted virus, and about 5.5 million people in the U.S. become infected with HPV each year.
In many people, HPV infection causes no symptoms and resolves on its own without causing illness. But about 18 of the 120 different types of HPV are closely associated with cancer and are known as high-risk types.
"Over the last three years there has been an explosion of data on HPV," says Maura Gillison, MD, PhD, assistant professor of oncology at the Johns Hopkins School of Medicine in Baltimore, who also spoke at the briefing.
Gillison says HPV has now been implicated in 99% of cervical cancer cases. But new studies show that the virus is also responsible for the majority of tonsillar and anal cancers, which helps explain a dramatic rise in these types of cancer in recent years.
Gillison says up to 15% of the cancers reported each year worldwide are caused by HPV infection, and the virus accounts for one-third of all cancers attributable to infectious agents.
Hepatitis Infections Behind Rise in Liver Cancer
Researchers say viruses are also to blame for a dramatic 71% increase in the incidence of liver cancer from 1976-1995. Hepatitis B and C viruses are the major risk factors for developing liver cancer, and more than 80% of liver cancer cases worldwide are linked to hepatitis B infection in areas where the virus is prevalent, such as developing countries.
But experts say hepatitis C may actually account for the rise in liver cancer in developed countries. Hepatitis C is transmitted primarily through intravenous (IV) drug use, blood transfusions with infected blood, and sexual contact.
Although hepatitis C infection is less common than hepatitis B, affecting about 100 million vs. 300 million worldwide, researchers say hepatitis C is more dangerous.
Many people with hepatitis B never develop symptoms and the virus resolves on its own. But more than 80% of those infected with hepatitis C develop a chronic infection that can lead to cirrhosis of the liver and liver cancer.
But liver cancer does not develop overnight after hepatitis infection and often takes years to develop.
"If you can block infection or [successfully] treat chronic hepatitis infection, you don't get sick," says Andrew Zhu, MD, PhD, assistant physician at Massachusetts General Hospital in Boston, who also spoke at the briefing.
In the U.S., the number of liver cancer cases and related deaths has doubled between 1975 and 1998. Although this increase has affected all ethnic groups, the largest increase has been among white men ages 45-54. Zhu says those trends are likely to continue over the next 10 to 20 years.
As researchers learn more about the role viruses play in cancer, they can develop better, more targeted treatments to reduce the risk or slow the progression of cancer in infected individuals.
But experts say the greatest promise lies in the development of vaccines that protect people at risk from infection and prevent cancer from occurring.
One example of such a vaccine already in development is an HPV-16 vaccine that targets a form of the HPV virus closely associated with cervical cancer.
A clinical trial of the vaccine published last year in TheNew England Journal of Medicine showed that young women given three doses of the vaccine developed antibodies against HPV-16 and were protected against both persistent HPV infection and cervical cancer.
In areas where hepatitis B is prevalent, the primary mode of transmission is from mother to child during birth, but the virus may also be transmitted through sexual intercourse, IV drug use, or blood transfusions.
A vaccine that protects against hepatitis B has been in use for many years. The American Academy of Pediatrics recommends the hepatitis B vaccine for all children 18 years and younger. In addition, adults at increased risk of hepatitis B, such as health-care workers, should also get the vaccine.
A nationwide vaccination program in Taiwan that started in 1984 has now been credited with reducing the incidence of liver cancer in children by 50%.